case study on genetic disorders class 12

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Case Study of a Rare Genetic Disorder: Congenital Insensitivity to Pain With Anhidrosis

Saqib m mughal.

1 Family Medicine, Alpha Medical Practice, Birmingham, GBR

Ayaaz Farhat

2 Family Medicine, Primary Health Care Corporation, London, GBR

A rare autosomal recessive disorder, congenital insensitivity to pain with anhidrosis, is characterised by the congenital lack of pain sensation. Other characteristic symptoms include no sweating, recurrent episodes of hyperpyrexia, retardation of mental abilities and self-mutilating behaviour. Herein, we present a case of a one-year-old male child who initially presented with self-bites on the tongue and then multiple fractures with no report of pain or crying, which initially indicated carelessness of parents. Based on further in-depth assessment indicating a family history of similar weak bones and no pain, the paediatric team conducted investigations along with genetic tests. The child was diagnosed with congenital insensitivity to pain with anhidrosis. Another sibling born later also had the same disorder. Both the children developed eczema, which was difficult to cure due to constant scratching by children as they did not feel any pain. Follow-up studies indicated a slight difficulty in learning abilities and delay in the achievement of milestones. This case report indicates the need for rigorous investigations in such cases to understand the aetiology and appropriate counselling of parents for the utmost care of the child.

Introduction

This is a case report of presumed non-accidental injury that had an underlying medical condition of congenital insensitivity to pain with anhidrosis (CIPA). CIPA is a rare autosomal recessive genetic disease caused by certain gene mutations. CIPA, also known as hereditary sensory and autonomic neuropathy type IV, is a rare genetic condition [ 1 , 2 ]. Very few individuals are affected by this genetical disorder, but cases can be found worldwide. CIPA is clinically characterised by the ability to feel a given stimulus but the inability to perceive pain along with anhidrosis. Insensitivity to pain and thermal sensation, no sweating (anhidrosis) and mental retardation/distress are three major clinical findings associated with CIPA. Various other sensations, like touch and pressure, are maintained. This insensitivity towards pain and thermal sensations could lead to multiple bone fractures, burns and sometimes self-mutilation of fingers, tongue and lips.

Genetic loss-of-function mutation in human tropomyosin receptor kinase A (TRKA gene NTRK1) encoding receptor tyrosine kinase for a nerve growth factor (NGF) was reported to be the cause for CIPA [ 3 ]. Nociceptive sensory neurons and sympathetic autonomic neurons are under the surveillance of NGF and play a role in the activation and homeostasis of other cell types [ 4 ]. Thus, the mutation in NTRK1 causes deficient development of the somatic sensory system, which is located in dorsal root ganglion sensory neurons for pain and temperature [ 5 ]. Development of the autonomic sympathetic nervous system is also affected and results in lost innervation of sweat glands by the sympathetic nervous system [ 6 ]. Central nervous system and bidirectional communication between immune system and nervous system are also affected by NTRK1 mutation [ 7 ]. In addition, functional alterations in NGF affect the normal procedure of fracture consolidation [ 8 ]. Differentiation of osteoblast/osteoprogenitor cells is also hindered. Due to the lack of nociceptive fibres in the skeletal system, metabolism of bones is also affected, and bone fractures are recurrent and common in CIPA patients due to the absence of the trophic role of nociceptive fibres [ 9 ].

Few laboratory tests along with clinical analysis are conducted for diagnosing CIPA based on symptoms like pain insensitivity, anhidrosis, and mental distress. Some tests, like axonal flare test and biopsies, are also performed. In flare test on injection of histamine under the skin, normal flare is not caused at the site of injection in CIPA patients [ 10 , 11 ]. Reduced number of myelinated and unmyelinated fibres of small diameter and normal count of large-diameter fibres are observed in sural nerve biopsy in CIPA patients [ 12 ]. However, the molecular test for the evaluation of mutation in NTKR1 is considered the confirmatory test for diagnosis of CIPA, but its availability is quite challenging.

In this study, we report a case of a one-year-old child suffering from CIPA, including the course of investigations and follow-up.

Case presentation

A one-year-old male child was brought to our primary care clinic by his mother. She stated that he had been teething for some time now and was biting and causing damage to his tongue. The child was examined, and healed lacerations were observed on the tongue. The child was also examined by the dental team, which commented that tongue biting could be a normal behaviour at this age.

The child presented to the clinic along with parents after approximately 14 months with swelling on his left arm. Parents visited the local emergency department and reported that they were not aware of any injury and had just observed the swollen arm of the child. There was no report of pain or crying observed in the child. On examination, the child was found to have a few-days-old fracture of ulna. Since no clear cause of the injury was found and the parents could not provide any explanation for the same, the protocol for the treatment of non-accidental injury was followed by the hospital. Due to the south Asian origin of parents and their poor English-speaking ability, there were some communication difficulties. They reasoned that since the child did not show any sign of distress, they were not aware of the injury and had not visited the doctor earlier. The hospital team became suspicious about the parents’ behaviour and unawareness and was not convinced by the narration of parents. The child was put under a safeguarding order, which meant a full detailed review was deemed necessary, and his inpatient medical investigations and treatment were initiated. During the course of the investigations, which included skeletal surveys, another healed fracture was found in the right lower leg. It was intimated to the parents that the child would not be discharged back to them until a complete investigation has been conducted. The parents were distressed and visited their family physician as they could not understand the safeguarding procedure. The child’s father informed the family physician (who communicated with them in their native language) that some family members in their home country have a similar problem of weak bones with no pain sensation. This information was communicated to the paediatric team, who accordingly conducted further investigations. These included genetic testing, which led to the diagnosis. The child was diagnosed with CIPA and was examined by various specialists. The child was eventually allowed to go home with the parents and continued to have follow-ups. The parents were found to have a consanguineous marriage, which could be the reason for recessive syndromes. They were recommended to consult with a genetic counsellor. Eighteen months after the diagnosis, the couple had a second child (female), who was also diagnosed with the same disorder. Follow-up of both children was conducted for the following five years and they were found to have mild learning difficulties and had delayed achievement of their milestones. Due to the history of multiple fractures, the children were at risk of developing Charcot joints. The male child had eczema, which was difficult to treat due to continuous scratching by the child since he did not have any sensation for pain. This caused significant damage to his skin. The younger sibling faced similar issues and had multiple fractures and eczema. There was some discussion with the family about testing the rest of the extended family who may have exhibited similar symptoms but this was complicated with the members living abroad.

Congenital insensitivity to pain has been classified as an autosomal recessive genetical disorder that is also termed as hereditary sensory and autonomic neuropathies. It can be categorised into five sub-types based on the stage of onset, additional symptoms and genetic mutations responsible for it [ 1 ]. It is caused by the mutation in NTRK1 gene, which results in failed differentiation and migration of neural crest cells. This leads to the total absence of small-diameter myelinated and unmyelinated nerve fibres, thereby causing loss of pain and temperature sensation. Moreover, innervation of sweat glands is also absent, causing anhidrosis [ 13 ]. Various symptoms associated with CIPA include congenital analgesia, self-mutilation/injury, multiple fractures, Charcot joints and anhidrosis [ 14 ]. There is also some association with mental retardation [ 15 , 16 ]. Reports have also suggested hyperhidrosis and impaired lacrimation associated with CIPA. Failure to thrive associated with insufficient weight gain in the child, along with repeated episodes of fever secondary to anhidrosis, were other symptoms reported in the case of CIPA [ 17 ].

In many reported cases of CIPA, the affected child is from consanguineous parents [ 18 ]. Similar to these cases, the marriage of the parents in this study was reported as consanguineous. Moreover, there was also some positive family history of similar conditions within the extended family. The child in our case was diagnosed at an early age with CIPA by the genetics team. Initially, the case presentation was confused with non-accidental injury and led to the separation of child from his parents. The parents were subjected to a child protection enquiry whilst the medical investigations were undergoing. This case was further complicated by poor communication and language difficulties of the parents. It is apparent from this case how such conditions could be confused with on-accidental injury. As the child grew up, it became increasingly difficult for the parents since there was a constant need for proper vigilance of the child to prevent injuries and self-mutilation. Mental health of the mother suffered from a continuous toll and pressure as any undesirable incident would cause a guilt in her for negligence in her duty towards the child. Other evidences also suggest that chronic conditions in children who require intense requirement of care have a negative effect on paternal mental health with the feelings of guilt or sorrow. It was also apparent during the course of follow-up that the child did not have any fear for painful stimuli as he would just jump off a chair or stairs, etc.

Treatment options

The treatment of CIPA is symptomatic and requires a multidisciplinary approach, incorporating both the physical and mental health issues of both patients and caregivers. Early diagnosis of disease during infancy could make parents more aware of risk factors, and thus, the accidents could be avoided with a constant awareness of the child’s activities. Special dental treatments have been recommended to avoid the severe outcomes due to self-mutilation by the child. Extraction of primary teeth of children with CIPA was recommended in the 1960s for avoiding self-mutilation. It was suggested to place both upper and lower full dentures. However, tooth extraction is an extreme step, and parents are usually reluctant to proceed with it [ 19 ]. Better options are now available that are more suitable to the quality of life of the CIPA child, such as wearing of dental guards along with strict vigilance by the parents. A team of multidisciplinary physicians along with dentists is needed for prevention of self-mutilation in CIPA patients.

There are other manifestations as well that are important and need to be considered. Anhidrosis in CIPA patients could cause recurrent fever and could be fatal if not diagnosed early. In addition, a high prevalence of infections has been observed. Skin conditions such as dermatitis and deep bone infections are other common issues associated with CIPA, and the most involved pathogen is Staphylococcus aureus . Antibiotic resistance observed in these patients limits the treatment options [ 20 ].

Conclusions

This interesting case, which was brought to the family medicine clinic, brings about many learning points. There was an importance in thorough history taking, including family history, which was vital in reaching a correct diagnosis. In areas where consanguineous relationships are common, healthcare professionals need to keep these factors in mind as it can mean an increased possibility of genetic conditions. If not diagnosed correctly, this can lead to not only a delay in appropriate management but also the consequences of an incorrect diagnosis, for example safeguarding concerns that came to light in this particular case. 

Genetic conditions, such as CIPA, manifest in childhood and therefore not only affect the child but also have a massive effect on the parents. They often require support for themselves due to the high level of complexities in managing the patient. Due to the nature of the condition, the parents need to be well educated in the management of the condition. Overall these types of conditions can be challenging for clinicians along with parents and require a multidisciplinary approach to enable the best outcomes.

The content published in Cureus is the result of clinical experience and/or research by independent individuals or organizations. Cureus is not responsible for the scientific accuracy or reliability of data or conclusions published herein. All content published within Cureus is intended only for educational, research and reference purposes. Additionally, articles published within Cureus should not be deemed a suitable substitute for the advice of a qualified health care professional. Do not disregard or avoid professional medical advice due to content published within Cureus.

The authors have declared that no competing interests exist.

Human Ethics

Consent was obtained by all participants in this study

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Case Study Based Questions for CBSE Class 12 Biology Board Exam 2024: Read this article for Last Minute Revision

Cbse class 12 biology important case study questions : practise important case study based questions for class 12 biology board exam. these case study based questions are important for the upcoming cbse class 12 biology board exam 2024 on march 19, 2024..

CBSE Class 12th Biology Board Exam 2023-24 Pattern

The paper will be of 70 marks and the time duration for completing the paper will be 3 hours.

The paper will have 33 questions divided into 5 sections.

Section–A 16 questions of 1 mark each,

Section–B 5 questions of 2 marks each;

Section–C 7 questions of 3 marks each;

Section–D 2 case-based questions of 4 marks each,

and Section–E 3 questions of 5 marks each.

CBSE Class 12 Biology Important Case Study Based Questions

Case Study 1:  Nondisjunction is the failure of homologous chromosomes to disjoin correctly during meiosis. It leads to the formation of a new cell with an abnormal amount of genetic material. A number of clinical conditions are the result of this type of chromosomal mutation. This results in the production of gametes containing a greater or lesser chromosomal amount than normal ones. Consequently, the individual may develop a trisomy or monosomal syndrome. Nondisjunction can occur in both Meiosis I and Meiosis II of the cellular division. It is also the main cause of many genetic disorders; however, its origin and process remain vague. Although it results in the majority of cases from errors in maternal meiosis II, both paternal and maternal meiosis I do influence it. Maternal age is considered a risk factor for trisomy, as well as recombination alterations and many others that can affect chromosomal segregation.

• It is the presence of an extra chromosome in a diploid cell.
• An aneuploid cell differs from other cells only in size.
• It can be less number of chromosomes in a diploid cell.
• Aneuploidy always affects female individuals.
• both i and iii
• both ii and iii
• i, iii and iv
• Errors in meiosis I is the only cause of aneuploidy
• Aneuploidy always affects sex chromosomes.
• Most of the aneuploidy results from errors in cell division involved in egg formation.
• Nondisjunction in meiosis I can lead to more abnormal cells than disjunction in meiosis II.
• both I and iii
• both iii and iv
• I, iii and iv
• Aneuploidy is not influenced by the mother’s age.
• Delivery before 30 years of age can decrease the incidence of aneuploidy in most cases
• The chance of aneuploidy increases up to 22 years of age.
• There is a dramatic increase in aneuploidy if the maternal age exceeds 30

• both ii and iv
• Chromosomal disorders
• Mendelian disorders
• Incomplete dominance
• All the above

Q5: Assertion: All types of genetic disorders are caused by chromosomal nondisjunction.

• Both assertion and reason are correct and the reason is the correct explanation of assertion
• Both assertion and reason are correct but the reason is not the correct explanation of the assertion
• Assertion is correct but the reason is incorrect
• Both assertion and reason are incorrect

Case Study 2:  A Representative Diagram of the Human Genome Project:

• Biotechnology
• Biomonitoring
• Bioinformatics
• Biosystematics

Q2:  Name a free living, non-pathogenic nematode, the DNA of which has been completely sequenced.

Answer: Caenorhabditis elegans

Q3: Summarize the methodology adopted in the Human Genome Project.

Answer: Expressed Sequence Tags (ESTs) : The approach focused on identifying all the genes that are expressed as RNA.

Sequence Annotation : The other took the blind approach of simply sequencing the whole set of genome that contained all the coding and non-coding sequence, and later assigning different regions in the sequence with functions.

Q4: What are SNPs’? How are they useful in human genomics?

• Identify disease-causing genes in humans
• Can be used to understand the molecular mechanisms of sequence evolution.

Q5: Mention at least four salient features of the Human Genome Project.

• Human genome contains 3164.7 million bp.
• Average gene consists of 3000 bases, but sizes vary greatly.
• Almost all (99.9 percent) nucleotide bases are exactly the same in all people.
• Less than 2 percent of the genome codes for proteins.

Case Study 3: Two blood samples of suspects ‘A’ and ‘B’ were sent to the Forensic Department along with sample ‘C’ from the crime scene. The Forensic Department was assigned the responsibility of running the samples and matching the samples of the suspects with that of the sample from the scene of the crime and thereby identifying the culprit.

• A radioactively labelled double stranded RNA molecule.
• A radioactively labelled double stranded DNA molecule.
• A radioactively labelled single stranded DNA molecule.
• A radioactively labelled single stranded RNA molecule.

Q3: What does ‘minisatellite’ and ‘microsatellite’ mean in relation to DNA Fingerprinting?

Answer:  Minisatellite: the repeating unit consists of 10-100 base pairs.

Microsatellite: the repeating unit consists of 2-6 base pairs.

Q3: How does polymorphism arise in a population?

Answer: Polymorphism (variation at the genetic level) arises due to mutations.

Q4: State the steps involved in DNA Fingerprinting in a sequential manner.

• DNA isolation
• DNA digestion with restriction enzymes.
• DNA fragment separation by electrophoresis.
• Hybridization
• DNA visualization under UV light.

Case Study 4: Bacteria like Streptococcus pneumoniae  and Haemophilus influenzae  are responsible for the disease pneumonia in humans which infects the alveoli (air-filled sacs) of the lungs. As a result of the infection, the alveoli get filled with fluid leading to severe problems in respiration. The symptoms of pneumonia include fever, chills, cough, and headache. In severe cases, the lips and fingernails may turn gray to bluish in colour. A healthy person acquires the infection by inhaling the droplets/aerosols released by an infected person or even by sharing glasses and utensils with an infected person. Dysentery, plague, diphtheria, etc., are some of the other bacterial diseases in man. Many viruses also cause diseases in human beings. Rhinoviruses represent one such group of viruses that cause one of the most infectious human ailments – the common cold. They infect the nose and respiratory passage but not the lungs.

The common cold is characterized by nasal congestion and discharge, sore throat, hoarseness, cough,

headache, tiredness, etc., which usually lasts for 3-7 days. Droplets resulting from the cough or sneezes of an infected person are either inhaled directly or transmitted through contaminated objects such as pens, books, cups, doorknobs, computer keyboards or mice, etc., and cause infection in a healthy person.

• By exhaling droplets of a non-infected person.
• By headache or leg pain.
• By eating fast food.
• By inhaling droplets of an infected person.

Q4:  How long does the common cold last?

Answer: 3-7 days

Q5:  Write any two symptoms of the common cold and pneumonia.

Answer: Cough and nasal congestion.

Case Study 5: When you insert a piece of alien DNA into a cloning vector and transfer it into a bacterial, plant, or animal cell, the alien DNA gets multiplied. In almost all recombinant technologies, the ultimate aim is to produce a desirable protein. Hence, there is a need for the recombinant DNA to be expressed. The foreign gene gets expressed under appropriate conditions. The expression of foreign genes in host cells involves understanding many technical details. After having cloned the gene of interest and having optimised the conditions to induce the expression of the target protein, one has to consider producing it on a large scale. Can you think of any reason why there is a need for large-scale production? If any protein encoding gene is expressed in a heterologous host, it is called a recombinant protein. The cells harbouring cloned genes of interest may be grown on a small scale in the laboratory. The cultures may be used for extracting the desired protein and then purifying it by using different separation techniques.

• A continuous culture system
• A stirred-tank bioreactor without in-lets and out-lets
• Laboratory flask of the largest capacity
• None of the above
• upstream processing
• downstream processing
• bioprocessing
• postproduction processing
• Human insulin
• Growth hormone
• cleaving and joining of DNA segments with endonuclease
• cleaving DNA segments with endonuclease and re-joining with ligase
• cleaving and re-joining DNA segments with ligase
• cleaving DNA segments with ligase and re-joining with endonuclease

Case Study 6: Gene Therapy

Read the following and answer the questions that follow:

• Replacing a disease-causing gene with a healthy copy of the gene
• Inactivating a disease-causing gene that is not functioning properly
• Introducing a new or modified gene into the body to help treat a disease
• Adenosine deaminase
• phenylketonuria
• Phenylalanine
• Bone marrow transplantation
• Southern blotting

Q4 Introduction of gene isolate from bone marrow producing ADA should be introduced at what age to

• acute diseases
• physiological diseases
• hereditary diseases
• infectious diseases
• CBSE Class 12 Biology Previous Year Question Papers with Solutions PDF Download
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• CBSE Class 12 NCERT Biology Solutions
• CBSE Class 12 Biology Chapter-wise notes
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5.1 Case Study: Genes and Inheritance

Created by: CK-12/Adapted by Christine Miller

Case Study: Cancer in the Family

People tend to carry similar traits to their biological parents, as illustrated by the family tree. Beyond just appearance, you can also inherit traits from your parents that you can’t  see.

Rebecca becomes very aware of this fact when she visits her new doctor for a physical exam. Her doctor asks several questions about her family medical history, including whether Rebecca has or had relatives with cancer. Rebecca tells her that her grandmother, aunt, and uncle — who have all passed away — had cancer. They all had breast cancer, including her uncle, and her aunt also had ovarian cancer. Her doctor asks how old they were when they were diagnosed with cancer. Rebecca is not sure exactly, but she knows that her grandmother was fairly young at the time, probably in her forties.

Rebecca’s doctor explains that while the vast majority of cancers are not due to inherited factors, a cluster of cancers within a family may indicate that there are mutations in certain genes that increase the risk of getting certain types of cancer, particularly breast and ovarian cancer. Some signs that cancers may be due to these genetic factors are present in Rebecca’s family, such as cancer with an early age of onset (e.g., breast cancer before age 50), breast cancer in men, and breast cancer and ovarian cancer within the same person or family.

Based on her family medical history, Rebecca’s doctor recommends that she see a genetic counselor, because these professionals can help determine whether the high incidence of cancers in her family could be due to inherited mutations in their genes. If so, they can test Rebecca to find out whether she has the particular variations of these genes that would increase her risk of getting cancer.

When Rebecca sees the genetic counselor, he asks how her grandmother, aunt, and uncle with cancer are related to her. She says that these relatives are all on her mother’s side — they are her mother’s mother and siblings. The genetic counselor records this information in the form of a specific type of family tree, called a pedigree, indicating which relatives had which type of cancer, and how they are related to each other and to Rebecca.

He also asks her ethnicity. Rebecca says that her family on both sides are Ashkenazi Jews (Jews whose ancestors came from central and eastern Europe). “But what does that have to do with anything?” she asks. The counselor tells Rebecca that mutations in two tumor-suppressor genes called BRCA1 and BRCA2 , located on chromosome 17 and 13, respectively, are particularly prevalent in people of Ashkenazi Jewish descent and greatly increase the risk of getting cancer. About one in 40 Ashkenazi Jewish people have one of these mutations, compared to about one in 800 in the general population. Her ethnicity, along with the types of cancer, age of onset, and the specific relationships between her family members who had cancer, indicate to the counselor that she is a good candidate for genetic testing for the presence of these mutations.

Rebecca says that her 72-year-old mother never had cancer, nor had many other relatives on that side of the family. How could the cancers be genetic? The genetic counselor explains that the mutations in the BRCA1 and BRCA2 genes, while dominant, are not inherited by everyone in a family. Also, even people with mutations in these genes do not necessarily get cancer — the mutations simply increase their risk of getting cancer. For instance, 55 to 65 per cent of women with a harmful mutation in the BRCA1 gene will get breast cancer before age 70, compared to 12 per cent of women in the general population who will get breast cancer sometime over the course of their lives.

Rebecca is not sure she wants to know whether she has a higher risk of cancer. The genetic counselor understands her apprehension, but explains that if she knows that she has harmful mutations in either of these genes, her doctor will screen her for cancer more often and at earlier ages. Therefore, any cancers she may develop are likely to be caught earlier when they are often much more treatable. Rebecca decides to go through with the testing, which involves taking a blood sample, and nervously waits for her results.

Chapter Overview: Genetics

At the end of this chapter, you will find out Rebecca’s test results. By then, you will have learned how traits are inherited from parents to offspring through genes, and how mutations in genes such as BRCA1 and BRCA2 can be passed down and cause disease. Specifically, you will learn about:

• The structure of DNA.
• How DNA replication occurs.
• How DNA was found to be the inherited genetic material.
• How genes and their different alleles are located on chromosomes.
• The 23 pairs of human chromosomes, which include autosomal and sex chromosomes.
• How genes code for proteins using codons made of the sequence of nitrogen bases within RNA and DNA.
• The central dogma of molecular biology, which describes how DNA is transcribed into RNA, and then translated into proteins.
• The structure, functions, and possible evolutionary history of RNA.
• How proteins are synthesized through the transcription of RNA from DNA and the translation of protein from RNA, including how RNA and proteins can be modified, and the roles of the different types of RNA.
• What mutations are, what causes them, different specific types of mutations, and the importance of mutations in evolution and to human health.
• How the expression of genes into proteins is regulated and why problems in this process can cause diseases, such as cancer.
• How Gregor Mendel discovered the laws of inheritance for certain types of traits.
• The science of heredity, known as genetics, and the relationship between genes and traits.
• How gametes, such as eggs and sperm, are produced through meiosis.
• How sexual reproduction works on the cellular level and how it increases genetic variation.
• Simple Mendelian and more complex non-Mendelian inheritance of some human traits.
• Human genetic disorders, such as Down syndrome, hemophilia A, and disorders involving sex chromosomes.
• How biotechnology — which is the use of technology to alter the genetic makeup of organisms — is used in medicine and agriculture, how it works, and some of the ethical issues it may raise.
• The human genome, how it was sequenced, and how it is contributing to discoveries in science and medicine.

As you read this chapter, keep Rebecca’s situation in mind and think about the following questions:

• BCRA1 and BCRA2 are also called Breast cancer type 1 and 2 susceptibility proteins.  What do the BRCA1 and BRCA2 genes normally do? How can they cause cancer?
• Are BRCA1 and BRCA2 linked genes? Are they on autosomal or sex chromosomes?
• After learning more about pedigrees, draw the pedigree for cancer in Rebecca’s family. Use the pedigree to help you think about why it is possible that her mother does not have one of the BRCA gene mutations, even if her grandmother, aunt, and uncle did have it.
• Why do you think certain gene mutations are prevalent in certain ethnic groups?

Attributions

Figure 5.1.1

Family Tree [all individual face images] from Clker.com used and adapted by Christine Miller under a CC0 1.0 public domain dedication license (https://creativecommons.org/publicdomain/zero/1.0/).

Figure 5.1.2

Rebecca by Kyle Broad on Unsplash is used under the Unsplash License (https://unsplash.com/license).

Wikipedia contributors. (2020, June 27). Ashkenazi Jews. In  Wikipedia.  https://en.wikipedia.org/w/index.php?title=Ashkenazi_Jews&oldid=964691647

Wikipedia contributors. (2020, June 22). BRCA1. In Wikipedia . https://en.wikipedia.org/w/index.php?title=BRCA1&oldid=963868423

Wikipedia contributors. (2020, May 25). BRCA2. In  Wikipedia.  https://en.wikipedia.org/w/index.php?title=BRCA2&oldid=958722957

Human Biology Copyright © 2020 by Christine Miller is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License , except where otherwise noted.

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• Case Study in Behavioral Genetics
• Case Study: "The Perfect 46"
• Case Study in Genetic Sports Advantage
• America’s Hidden History: The Eugenics Movement
• Case Study in Genetic Non Disclosure
• When a Case Study Isn't Hypothetical: Huntington's Disease
• Case Study in Three Parent Embryos
• Case Study in Prenatal Diagnosis and Sex Selection
• Case Study in Lethal Diseases and Autonomy
• Case Study in Personal Genome Services
• Case Study in Fertility Clinics and Designer Babies
• When a Case Study is Not Hypothetical
• Case Study in the Right NOT to Know
• Case Study in Genetic Testing for Sports Ability
• Case Study in Genetic Discrimination
• Case Study in DNA, Privacy and Human Cloning
• Case Study in Recombinant DNA Technology and Biosafety
• Case Study in Forensic Paternity Testing

Case Study in Genetics and Mental Illness

• Case Study in GM Food Animals
• Case Study in Tissue Ownership
• Case Study in Savior Siblings
• Case Study in DNA Fingerprinting
• Case Study in Incidental Findings
• About Genetics Generation
• Genetics Generation
• Women in Science

Case Study : Kellie is an 18-year old woman whose mother, Sharon, suffers from severe obsessive-compulsive disorder (OCD). Sharon's day is so overrun by repetitious rituals, such as locking and unlocking the front door, that she rarely leaves the house. Kellie has not yet developed any OCD tendencies. However, when Kellie learns that research has uncovered a link between genetic variations in the SLC1A1 gene on chromosome 9 and OCD, she questions whether she will develop the disorder later in life. Kellie goes to her doctor, Dr. Simpson, to ask about the possibility of genetic testing for the SLC1A1 gene variant. Although Dr. Simpson has access to the technology required for the testing, he has some concerns. He knows that most psychiatric disorders are triggered by both environmental and genetic factors. Any single gene is only partially responsible for causing OCD. Therefore, Dr. Simpson worries that if Kellie learns that she carries the predisposing variant of SLC1A1, the stress of this information will cause OCD to develop when it otherwise may not in the absence of an environmental trigger.

Please take our poll and leave a comment. Discussions about mental health are often more difficult than those regarding other medical issues.

References:

Cross-Disorder Group of the Psychiatric Genomics Consortium. Identification of risk loci with shared effects on five major psychiatric disorders: a genome-wide analysis . The Lancet 381(9875) , 1371-1379.

Image Credit to the National Human Genome Research Institute

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A patient worrying about cause of their disease is understandable. To completely understand the science behind the disease is paradoxical to welfare of a patient with worries. Medicine in an inexact science , they follow guidelines & protocol combined with experience in any sub-field e.g psychology . Successful (or) optimal treatment is about 'response ' of the doctor to the illness of the patient along with cultural or psychological conditions.

I understand why Dr. Simpson feels that a positive test result may trigger OCD-like behavior in Kellie that may not otherwise have developed. I think his hesitation in administering the test is warranted. However, if Kellie does in fact carry the mutation of the gene that is linked to OCD, there is a good chance she would exhibit that behavior at some point in her life regardless of whether she is tested by Dr. Simpson. I would hope that with the information from the newly available test Kellie and her doctors will be proactive in their monitoring and treatment of OCD so that she does not suffer as severely as her mother.

I think that the possible forensic application of a link between genetics and mental illness is fascinating. Assuming that some sort of causal connection or even mere correlation can be established, will courts allow expert testimony to establish if a criminal defendant is responsible for his/her misconduct? Such testimony could be admitted along with testimony from psychologists and/or psychiatrists.

People have a right to understand their genetic information. However, these kinds of multifactorial relationships to health conditions are really hard to understand. Doctors and medical scientists don't understand them. Why then would a doctor give patient information that doesn't help diagnose or treat an ALREADY existing condition? I am not sure the doctor should refuse any request for the test. However, a doctor should make sure there are appropriate state of health advisors available for the patient to carefully interpret any results. It's really a case by case basis, depending on the current health (mental and otherwise) of the patient. For this specific patient, what is the true benefit of knowing about one genetic variant?

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The Face of a Rare Genetic Disease

By Karobi Moitra

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This case study is designed to teach basic concepts of genetics by focusing on a rare disease, pseudoxanthoma elasticum (PXE).  Chromosome 16 is the narrator at the beginning of the case and introduces students to genes, chromosomes and mutations. The focus then shifts to the patient and his mother as she finds out about her son’s disease and her subsequent efforts to connect with patient advocacy groups for support. The case concludes with students watching a TED talk given by Sharon Terry, the real-life mother on whom this story is loosely based, so that students can connect on an emotional and human level with someone who has intimate experience as a parent of children with a rare genetic disease. The case is suitable for high school general biology classes, but it can also be used by biology major or non-major undergraduates in a lower-division biology class, or in any lower-division non-major class focused on human disease.

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Date Posted

• Explain the basic structure and function of chromosomes.
• Explain the relationship between genes, chromosomes, nucleus and the cell.
• Describe the genetic disease PXE.
• Explain the basic concepts of genetics.
• Understand mutations and their role in disease.
• Learn how to locate information about a particular disease or gene.
• Read and understand scientific articles and resources.
• Understand patient advocacy.
• Connect with the human face of genetic diseases.

Genetics; pseudoxanthoma elasticum; PXE; Sharon Terry; disease; advocacy; genetics; chromosomes; genes; mutation

Subject Headings

EDUCATIONAL LEVEL

High school, Undergraduate lower division

TOPICAL AREAS

Social issues

TYPE/METHODS

Teaching Notes & Answer Key

Teaching notes.

Case teaching notes are protected and access to them is limited to paid subscribed instructors. To become a paid subscriber, purchase a subscription here .

Teaching notes are intended to help teachers select and adopt a case. They typically include a summary of the case, teaching objectives, information about the intended audience, details about how the case may be taught, and a list of references and resources.

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Answer Keys are protected and access to them is limited to paid subscribed instructors. To become a paid subscriber, purchase a subscription here .

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Materials & Media

Supplemental materials.

• Science Didn’t Understand My Kid’s Rare Disease Until I Decided to Study It Sharon Terry talks about her children's rare disease PXE and how she decided to study it. This is a personal and true story of a mother who decided to do something to help her kids and other kids suffering from the rare disease PXE. Running time: 14:54 min. Produced by TEDMED, 2016.
• Mutation This video explains the concept of genetic mutations and how they relate to the central dogma of molecular biology. Running time: 7:02 min. Produced by Bozemanscience, 2012.

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Genetic disorders

• CBSE Class 12
• Chapter-5: Principles of Inheritance and Variation
• Genetic disorders Notes
• A TEXT OF BIOLOGY - CLASS XII
• Publication
• ACME SMART PUBLICATION

GENETIC DISORDERS

Pedigree Analysis :

A record of inheritance of certain genetic traits for two or more generations presented in the form of a diagram or family tree is called pedigree.

Parents are shown by horizontal line while their offsprings are connected to it by a vertical line.

The offsprings are also shown in the form of a horizontal line below the parents and numbered with arabic numerals.

Pedigree analysis is study of pedigree for the transmission of particular trait and finding the possibility of absence or presence of that trait in homozygous or heterozygous state in a particular individual.

It is useful for the genetic counsellors to advise intending couples about the possibility of having children with genetic defects like haemophilia, colour blindness, alkaptonuria, phenylketonuria, thalassaemia, sickle cell anaemia (recessive traits), brachydactyly, myotonic dystrophy and polydactyly (dominant traits).

Pedigree analysis indicates that Mendel's principles are also applicable to human genetics with some modifications found out later, like quantitative inheritance, sex linked characters and other linkages.

Symbols used in Pedigree analysis :

Proband is person from which case history starts. If it is male, it is called propositus , if it is female it is called proposita .

Example 8 : In the pedigree given below, indicate whether the shaded symbols indicate dominant or recessive allele. Also give genotype of the whole pedigree.

Solution: Since the shaded symbol appears in all the offsprings, father must be homozygous dominant while the mother homozygous recessive (AA × aa = all Aa) because in all other cases this possibility is absent (Aa × aa = 2Aa + 2aa; aa × AA = all Aa; aa × Aa = 2aA + 2aa). All the members of II generation will, therefore, be heterozygous (Aa). This is further confirmed by marriage of II-1 with homozygous recessive (Aa × aa = 2Aa, 2aa) and bearing children of both the parental types. Marriage of II-3 with the homozygous recessive can produce both recessive and heterozygotes as are present here.

MENDELIAN DISORDERS

Sickle-Cell Anaemia

It is an autosomal recessive disorder. In this disorder, the RBCs become sickle shaped under low O 2 concentration.

The affected persons die young.

Other heterozygous for this trait are having normal phenotype and long lived.

The disease is due to base substitution of sixth codon in the gene coding for chain of haemoglobin.

The middle base of a DNA triplet coding for the amino acid glutamic acid is mutated so that the triplet now codes for valine instead.

The mutant haemoglobin molecule undergoes polymerisation under low O 2 tension causing the change in the shape of RBC from biconcave disc to elongated sickle like structure.

Thalassaemia

It is recessive autosomal disease caused due to reduced synthesis of or polypeptide of haemoglobin. thalassaemia is a major problem, individuals suffering from major thalassaemia often die before ten years of age.

Phenylketonuria

Recessive autosomal disorder (Chromosome 12) related to phenylalanine metabolism to tyrosine. This disorder is due to absence of a liver enzyme called phenylalanine hydroxylase. Due to lack of this enzyme, phenylalanine follows another pathway and gets converted into phenylpyruvic acid. This phenyl pyruvic acid upon accumulation in joints causes arthritis and if it hits the brain, then it causes mental retardation known as phenyl pyruvic idiocy. These are also excreted through urine because of poor absorption by kidney.

Cystic Fibrosis

It is an autosomal recessive disorder common among Caucasian Northern Europeans. Persons suffering from this disease are having extremely salty sweat. It is due to decreased Na + and Cl – reabsorption in the ducts. Disease is due to a gene present on chromosome 7. Due to a defective glycoprotein, thick mucus develops in pancreas and lungs and formation of fibrous cyst occurs in pancreas.

Huntington's Chorea

It is an autosomal dominant disorder. The gene responsible for this disorder is present on chromosome 4. Disease is characterised by gradual degradation of brain tissue in the middle age and consequent shrinkage of brain.

Alzheimer's Disease

This autosomal recessive disease results in mental deterioration (loss of memory, confusion, anxiety) and ultimately the loss of functional capacities. The disease is due to deposits of -amyloid, a short protein in brain which results in degradation of neurons. It involves two defective alleles located on chromosome number 19 and 21. This disease is common in Down's syndrome.

Myotonic dystrophy is due to a dominant autosomal mutant gene located on the long arm of chromosome 19. Mild myotonia -atrophy and weakness of the musculature of the face and extremities, is most common.

Other Mendelian disorders :

(i) Alkaptonuria (Garrod, 1908) - Due to deficiency of oxidase enzyme.

(ii) Albinism (Chromosome 11) - Absence of tyrosinase

(iii) Tay-Sach's disease (Chromosome 15) - Absence of hexosaminidase B.

(iv) Gaucher's disease (Chromosomes 1) - Due to the inhibition of glucocerebrosidase enzyme action which leads to accumulation of cerebroside.

Other abnormalities due to autosomal dominant gene mutation

(i) Polydactyly - Presence of extra fingers and toes

(ii) Brachydactyly - Abnormal short fingers and toes

Abnormalities due to sex linked (X-linked) recessive gene mutation

(i) Haemophilia A - Due to lack of antihaemophilic-globulin.

Haemophilia B - Due to lack of plasma thromboplastin

(ii) Red-green colour blindness - Daltonism

Protanopia - Red colour blindness

Tritanopia - Blue colour blindness

Deuteranopia - Green colour blindness

(iii) Muscular dystrophy - Due to non-synthesis of protein dystrophin

Deterioration of muscles at an early stage

(iv) Lesch Nyhan syndrome -Deterioration of nervous system

Due to HGPRT deficiency (Hypoxanthin guanine

phosphoribosyl transferase)

CHROMOSOMAL DISORDERS

A. Autosomal abnormalities (Due to mutation in body chromosome)

(i) Down's Syndrome -It occurs due to trisomy of 21 st chromosome. The affected individual is short statured with small round head, furrowed tongue and partially open mouth. Palm is broad with characteristic palm crease and mental retardation. Physical and psychomotor development is retarded.

(ii) Edward's syndrome - Trisomy of 18 th chromosome

(iii) Patau's syndrome - Trisomy of 13 th chromosome

(iv) Cri du chat syndrome - Due to deletion in short arm of 5 th chromosome.

B. Allosomal or Sex Chromosomal Disorder

(i) Klinefelter's Syndrome - It occurs due to the trisomy of X-chromosome in male, resulting into a karyotype of 47, (44 + XXY). Individuals have long legs, sparse body hair, small prostrate gland, small testes, reduced mental intelligence and enlarged breasts (Gynaecomastia). Such individuals are sterile.

(ii) Turner's Syndrome - It is caused due to absence of one of the X-chromosome in female i.e. 45 with chromosome complement 44 + XO. Such females are sterile with undeveloped breast, short stature, reduced ovaries & absence of menstrual cycle.

(iii) Super female -AA + XXX, AA + XXXX

(iv) Jacob's syndrome or Super male -AA + XYY, also called as criminal syndrome.

Population Genetics

Hardy Weinberg equation is applied to know the distribution of traits and frequency of autosomal dominant recessive gene distribution in the entire population.

p = Dominant gene/allele

q = Recessive gene/allele

(p + q) 2 = p 2 + q 2 + 2pq = 1

Concept Builder

1. Archibald Garrod discovered "Inborn errors of metabolism in humans" -phenylketonuria, alkaptonuria and albinism.

2. Muscular dystrophy and haemophilia causing genes are sex-linked recessive lethal genes.

3. The symptomatic treatment of genetic diseases of man is called Euphenics.

4. Improvement of human race by improving environmental conditions is called Euthenics.

5. Improvement of the future qualities of mankind by selective breeding is called Eugenics.

6. F. Galton -Term "Eugenics".

7. Lejeune -Discovered Cri-du-chat syndrome

8. Darligton (1939) -The genes present in the cytoplasm are called plasmagenes.

Related Topic Name

Mendel's laws of inheritance, inheritance of one gene, inheritance of two genes, sex determination, enter your mobile number to sign up in edumple, enter your mobile number.

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12.7: Case Study Conclusion- Cancer and Chapter Summary

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• Page ID 64673

• Suzanne Wakim & Mandeep Grewal
• Butte College

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Case Study Conclusion: Cancer in the Family

Rebecca’s family tree, as illustrated in Figure \(\PageIndex{1}\), shows a high incidence of cancer among close relatives. But are genes the cause of cancer in this family? Only genetic testing, which is the sequencing of specific genes in an individual, can reveal whether a cancer-causing gene is being inherited in this family.

Fortunately for Rebecca, the results of her genetic testing show that she does not have the mutations in the BRCA1 and BRCA2 genes that most commonly increase a person’s risk of getting cancer. However, it does not mean that she doesn’t have other mutations in these genes that could increase her risk of getting cancer. There are many other mutations in BRCA genes whose effect on cancer risk is not known, and there may be many more yet to be discovered. It is important to continue to study the variations in genes such as BRCA in different people to better assess their possible contribution to the development of the disease. As you now know from this chapter, many mutations are harmless, while others can cause significant health effects, depending on the specific mutation and the gene involved.

Mutations in BRCA genes are particularly likely to cause cancer because these genes encode for tumor-suppressor proteins that normally repair damaged DNA and control cell division. If these genes are mutated in a way that causes the proteins to not function properly, other mutations can accumulate and cell division can run out of control, which can cause cancer.

BRCA1 and BRCA2 are on chromosomes 17 and 13, respectively, which are autosomes. As Rebecca’s genetic counselor mentioned, mutations in these genes have a dominant inheritance pattern. Now that you know the pattern of inheritance of autosomal dominant genes if Rebecca’s grandmother did have one copy of a mutated BRCA gene, what are the chances that Rebecca’s mother also has this mutation? Because it is dominant, only one copy of the gene is needed to increase the risk of cancer, and because it is on autosomes instead of sex chromosomes, the sex of the parent or offspring does not matter in the inheritance pattern. In this situation, Rebecca’s grandmother’s eggs would have had a 50% chance of having a BRCA gene mutation, due to Mendel’s law of segregation. Therefore, Rebecca’s mother would have had a 50% chance of inheriting this gene. Even though Rebecca does not have the most common BRCA mutations that increase the risk of cancer, it does not mean that her also mother does not, because there would also only be a 50% chance that she would pass it on to Rebecca. Therefore, Rebecca’s mother should consider getting tested for mutations in the BRCA genes as well. Ideally, the individuals with cancer in a family should be tested first when a genetic cause is suspected so that if there is a specific mutation being inherited, it can be identified and the other family members can be tested for that same mutation.

Mutations in both BRCA1 and BRCA2 are often found in Ashkenazi Jewish families. However, these genes are not linked in the chromosomal sense, because they are on different chromosomes and are therefore inherited independently, in accordance with Mendel’s law of independent assortment. Why would certain gene mutations be prevalent in particular ethnic groups? If people within an ethnic group tend to produce offspring with each other, their genes will remain prevalent within the group. These may be genes for harmless variations such as skin, hair, or eye color, or harmful variations such as the mutations in the BRCA genes. Other genetically based diseases and disorders are sometimes more commonly found in particular ethnic groups, such as cystic fibrosis in people of European descent and sickle-cell anemia in people of African descent. You will learn more about the prevalence of certain genes and traits in particular ethnic groups and populations in the chapter on Human Variation.

As you learned in this chapter, genetics is not the sole determinant of phenotype. The environment can also influence many traits, such as adult height and skin color. The environment also plays a major role in the development of cancer. 90 to 95% of all cancers do not have an identified genetic cause and are often caused by mutagens in the environment such as UV radiation from the sun or toxic chemicals in cigarette smoke. But for families like Rebecca’s, knowing their family health history and genetic makeup may help them better prevent or treat diseases that are caused by their genetic inheritance. If a person knows they have a gene that can increase their risk of cancer, they can make lifestyle changes, have early and more frequent cancer screenings, and may even choose to have preventative surgeries that may help reduce their risk of getting cancer and increase their odds of long-term survival if cancer does occur. The next time you go to the doctor and they ask whether any members of your family have had cancer, you will have a deeper understanding of why this information is so important to your health.

Chapter Summary

In this chapter, you learned about genetics — the science of heredity. Specifically, you learned that:

• Chromosomes are structures made of DNA and proteins that are encoded with genetic instructions for making proteins. The instructions are organized into units called genes, most of which contain instructions for a single protein.
• Humans normally have 23 pairs of chromosomes. Of these, 22 pairs are autosomes, which contain genes for characteristics unrelated to sex. The other pair consists of sex chromosomes (XX in females, XY in males). Only the Y chromosome contains genes that determine sex.
• Humans have an estimated 20,000 to 22,000 genes. The majority of human genes have two or more possible versions, called alleles.
• In Mendel's first set of experiments, he crossed plants that only differed in one characteristic. The results led to Mendel's first law of inheritance, called the law of segregation. This law states that there are two factors controlling a given characteristic, one of which dominates the other, and these factors separate and go to different gametes when a parent reproduces.
• In Mendel's second set of experiments, he experimented with two characteristics at a time. The results led to Mendel's second law of inheritance, called the law of independent assortment. This law states that the factors controlling different characteristics are inherited independently of each other.
• Mendel's laws of inheritance, now expressed in terms of genes, form the basis of genetics, the science of heredity. Mendel is often called the father of genetics.
• The position of a gene on a chromosome is its locus. A given gene may have different versions called alleles. Paired chromosomes of the same type are called homologous chromosomes and they have the same genes at the same loci.
• The alleles an individual inherits for a given gene make up the individual's genotype. An organism with two of the same allele is called a homozygote, and an individual with two different alleles is called a heterozygote.
• The expression of an organism's genotype is referred to as its phenotype. A dominant allele is always expressed in the phenotype, even when just one dominant allele has been inherited. A recessive allele is expressed in the phenotype only when two recessive alleles have been inherited.
• In sexual reproduction, two parents produce gametes that unite in the process of fertilization to form a single-celled zygote. Gametes are haploid cells with only one of each pair of homologous chromosomes, and the zygote is a diploid cell with two of each pair of chromosomes.
• Examples of human autosomal Mendelian traits include dimples and earlobe attachment. Examples of human X-linked traits include red-green color blindness and hemophilia.
• Two tools for studying inheritance are pedigrees and Punnett squares. A pedigree is a chart that shows how a trait is passed from generation to generation. A Punnett square is a chart that shows the expected ratios of possible genotypes in the offspring of two parents.
• Multiple allele traits are controlled by a single gene with more than two alleles. An example of a human multiple allele trait is ABO blood type.
• Codominance occurs when two alleles for a gene are expressed equally in the phenotype of heterozygotes. A human example of codominance occurs in the AB blood type, in which the I A and I B alleles are codominant.
• Incomplete dominance is the case in which the dominant allele for a gene is not completely dominant to a recessive allele, so an intermediate phenotype occurs in heterozygotes who inherit both alleles. A human example of incomplete dominance is Tay Sachs disease, in which heterozygotes produce half as much functional enzyme as normal homozygotes.
• Polygenic traits are controlled by more than one gene, each of which has a minor additive effect on the phenotype. This results in a continuum of phenotypes. Examples of human polygenic traits include skin color and adult height. Many of these types of traits, as well as others, are affected by the environment as well as by genes.
• Pleiotropy refers to the situation in which a gene affects more than one phenotypic trait. A human example of pleiotropy occurs with sickle cell anemia, which has multiple effects on the body.
• Epistasis is when one gene affects the expression of other genes. An example of epistasis is albinism, in which the albinism mutation negates the expression of skin color genes.
• Examples of genetic disorders caused by single-gene mutations include Marfan syndrome (autosomal dominant), sickle cell anemia (autosomal recessive), vitamin D-resistant rickets (X-linked dominant), and hemophilia A (X-linked recessive). Very few genetic disorders are caused by dominant mutations because these alleles are less likely to be passed on to successive generations.
• Nondisjunction is the failure of replicated chromosomes to separate properly during meiosis. This may result in genetic disorders caused by abnormal numbers of chromosomes. An example is Down syndrome, in which the individual inherits an extra copy of chromosome 21. Most chromosomal disorders involve the X chromosome. An example is Klinefelter's syndrome (XXY, XXXY).
• Prenatal genetic testing, for example, by amniocentesis, can detect chromosomal alterations in utero . The symptoms of some genetic disorders can be treated or prevented. For example, symptoms of phenylketonuria (PKU) can be prevented by following a low-phenylalanine diet throughout life.
• Cures for genetic disorders are still in the early stages of development. One potential cure is gene therapy, in which normal genes are introduced into cells by a vector such as a virus to compensate for mutated genes.

Chapter Summary Review

• Different alleles of the same gene are located at the same locus on homologous chromosomes.
• Different alleles of the same gene are located at different loci on homologous chromosomes.
• Different genes of the same alleles are located at the same locus on homologous chromosomes.
• Different alleles of the same gene are located at different loci on the same chromosome.
• What do A and a represent?
• If the person expresses only the phenotype associated with A , is this an example of complete dominance, codominance, or incomplete dominance? Explain your answer. Also, describe what the observed phenotypes would be if it were either of the two incorrect answers.
• Explain how a mutation that occurs in a parent can result in a genetic disorder in their child. Be sure to include which type of cell or cells in the parent must be affected in order for this to happen.
• What is an allele that is not expressed in a heterozygote called?
• True or False . Sex is determined by a gene on an autosome.
• True or False . In sexual reproduction, parents and offspring are never identical.
• True or False . In humans, a gamete will have 23 chromosomes.
• True or False . The expression of an organism’s phenotype produces its genotype.
• True or False . It is entirely likely for a gene to have more than two alleles.
• two factors of the same characteristic separate into different gametes.
• there are dominant and recessive factors.
• factors controlling different characteristics are inherited independently of each other.
• there are two factors that control inheritance.
• are on homologous chromosomes.
• are on the same chromosome.
• are on adjacent chromosomes.
• are on non-homologous chromosomes.
• Half of their daughters will have red-green color blindness.
• All of their daughters will have red-green color blindness.
• All of their sons will have red-green color blindness.
• All of their children will have red-green color blindness.
• A trait that has three alleles
• A trait that is controlled by two genes
• A trait that is controlled by a single gene with one dominant and one recessive allele
• A trait that has two alleles, both of which are expressed equally in the phenotype

Attributions

• Pedigree by Rachel Henderson by CK-12 licensed CC BY-NC 3.0
• Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0

Genetic & Chromosomal Disorders | Biology Class 12 - NEET PDF Download

Mendel’s works on the principle of inheritance in genetics remained a mystery for quite some time. Even though his works were not accepted during his era, later it was rediscovered and gained credibility. Currently, Mendel’s work is fundamental for studying inheritance pattern in living organisms. In addition, it helped to discover and predict how genetic disorders function. Let’s learn about pedigree analysis and how it helps in predicting genetic disorders.

What is a Pedigree Analysis?

Pedigree analysis is a  chart that represents a family tree,  which displays the members of the family who are affected by a genetic trait. Here, the rows represent the generations of a family, squares represent males and circles represent females. In many cases, including various plant and animal species,  scientists use pedigree analysis to analyse the inheritance of phenotypes, or traits, using mating experiments called crosses.

Mendel’s experiments revealed that the ‘factors’, what we know as genes, are responsible for the inheritance of traits. They are also accountable for the disorders prevailing in living organisms. Genes are the hereditary unit of organisms, responsible for structural and functional changes in them. Besides this, it is the cause of variation in organisms which can either result in a good or bad trait.

To study the inheritance pattern in living organisms.

To analyze the structure and function of genes.

To determine the genetic disorders prevailing in a family.

To predict the variation in organisms.

• DNA sequences are made up of various, which, in turn, code for a particular protein. Any changes in this sequence, e.g. mistakes during DNA replication may lead to a change in the genetic codes or chromosomal aberrations. 
• This can be transferred from parents to offspring. Inheritance of altered genes causes genetic disorders in offspring. The Mendelian disorders may arise due to change or alteration in one gene. Their genetic inheritance is governed by Mendelian genetics. Mendelian disorders mostly occur in families with a certain pattern reflecting the alteration in a single gene. 
• Prediction of these disorders is based on family history and can be done with the help of a family tree. This process of analysis of a number of generations of a family is called the pedigree analysis. 

• Pedigree analysis is a strong tool in human genetics which helps to predict the pattern of inheritance, even when data is limited. A family tree can be represented by a pedigree chart with all the members of a family. They may be having a genetic disorder or maybe carrier of the disease.  In the pedigree analysis, standard symbols are used to distinguish between different family.

Mendelian Disorder

“Mendelian disorders are the genetic disorders caused at a single genetic locus.”

What are Mendelian Disorders?

Mendelian disorder is a type of genetic disorder primarily resulting due to alterations in one gene or as a result of abnormalities in the genome. Such a condition can be seen since birth and be deduced on the basis of family history using the family tree. The analysis hence carried out is known as pedigree analysis.

These genetic disorders are quite rare and may affect one person in every thousand or a million. Genetic disorders may or may not be inherited. Inheritable genetic disorders usually occur in the germline cells, whereas in non-inheritable genetic disorders the defects are generally caused by new mutations or due to some changes in the DNA. For instance, cancer may either be caused by an inherited genetic condition, or by a new mutation caused by the environmental causes or otherwise.

Types of Mendelian Genetic Disorders

According to Mendel’s’ laws of inheritance, the different types of Mendelian disorders include:

• Autosomal dominant.
• Autosomal recessive.
• Sex-linked dominant.
• Sex-linked recessive.
• Mitochondrial.

The various types of Mendelian disorders can be identified easily from the pedigree analysis.

Chromosomal aberrations

DNA replication

Pedigree analysis

Genetic inheritance

Examples of Mendelian Disorders

Few examples of the Mendelian disorder in humans are

• Sickle cell anaemia

Muscular dystrophy

• Cystic fibrosis

Thalassemia

Phenylketonuria.

• Colour blindness
• Skeletal dysplasia

Haemophilia

• This is a type of sex-linked recessive disorders. According to the genetic inheritance pattern, the unaffected carrier mother passes on the haemophilic genes to sons.
• It is a very rare type of disease among females because for a female to get the disease, the mother should either be hemophilic or a carrier but the father should be haemophilic.
• This is a disorder in which blood doesn’t clot normally as the protein which helps in clotting of blood is affected. Therefore, a person suffering from this disease usually has symptoms of unexplained and excessive bleeding from cuts or injuries.
• This type of genetic disorder is caused when the affected gene is located on the X chromosomes. Therefore, males are more frequently affected.

Sickle-Cell Anaemia

• This is a type of autosomal recessive genetic disorder.
• According to Mendelian genetics, its inheritance pattern follows inheritance from two carrying parents.
• It is caused when the glutamic acid in the sixth position of the beta-globin chain of haemoglobin molecule is replaced by valine. The mutant haemoglobin molecule undergoes a physical change which changes the biconcave shape into the sickle shape.
• This reduces the oxygen-binding capacity of the haemoglobin molecule.
• This genetic disorder is autosomal recessive in nature.
• It is an inborn error caused due to the decreased metabolism level of the amino acid phenylalanine.
• In this disorder, the affected person does not have the enzyme that converts phenylalanine to tyrosine. As a result, phenylalanine accumulation takes place in the body and is converted into many derivatives which result in mental retardation.

• This is a type of disorder in which the body makes an abnormal amount of hemoglobin. As a result, a large number of red blood cells are destroyed which leads to anemia.
• It is an autosomal recessive disease.
• Facial bone deformities, abdominal swelling, and dark urine are some of the symptoms of thalassemia.
• It is an inherited disease which is mainly caused due to abnormal hemoglobin synthesis. It is transferred by one of the parents who is a carrier of this disease due to either deletion of particular key gene fragments or a genetic mutation.

 - Alpha-thalassemia – A disorder in which one of the genes of alpha-globin has a mutation or abnormality.

 - Beta-thalassemia – The genes of beta-globin are abnormal.

It develops when there is some abnormality in any one of the genes that are involved in the production of hemoglobin and this defect is inherited from the parents. If any of the parents have thalassemia, the baby is more likely to develop this disease so-called thalassemia minor. If both the parents suffer from this disease, you are more likely to get the disease.

There are no symptoms at an early stage but are likely to be a disease carrier. It is the most common disease in people of Asia, Africa, the Middle East, Turkey, and Greece.

Sickle cell anemia

Color blindness

Cystic Fibrosis

• This is an autosomal recessive disorder.
• This disease affects the lungs and the digestive system and the body produces thick and sticky mucus that blocks the lungs and pancreas.
• People suffering from this disorder have a very short life-span.

Types of Hemophilia

Haemophilia exists in two forms:

• Hemophilia A: It is caused specifically by a mutation in the Factor VIII gene on the X chromosome.
• Hemophilia B: This is caused by a mutation in the Factor IX gene on the X chromosome.

Hemophilia Prevention

Since haemophilia is a hereditary condition, it cannot be prevented; but it can be diagnosed and help the mother understand the risks of having a baby with haemophilia. The female members of the family are the only carriers of this syndrome. If there is a history of haemophilia in a family, it is better to consult a physician and have a blood test to examine the clotting factors and to perform a molecular genetic test to examine the carriers in their genes. As per the studies conducted on this inherited genetic disorder, the genes from the mother can be transmitted to both her children. Among them, there are 50% chances that her son will have haemophilia A or B and 50% chances that her daughter will be a carrier of this gene.

Symptoms of Hemophilia

The signs and symptoms of haemophilia vary based on the levels of clotting factors present. These clotting factors are substances in the blood affect the process of blood coagulation. If the clotting factors are slightly reduced, then the bleeding is observed only after the surgeries. If the clotting factors are completely reduced, then spontaneous bleeding is observed.

Symptoms of spontaneous bleeding include

• Many large or deep bruises.
• Joint pain and swelling (caused by bleeding)
• Unexplained bruises or bleeding.
• Blood in urine or in stools.
• More bleeding for a normal cut or injury.
• Nosebleeds for no apparent reason.
• Excessive bleeding in tooth gums.
• Unusual bleeding after vaccinations.

Colour Blindness

Introduction

Colour blindness can be simply defined as trouble in seeing or identifying colours like blue, green and red. There are some rare cases where a person cannot see and identify any colours at all. A person with this syndrome also finds difficulties in differentiating the colours with shades. This syndrome is also called a colour vision problem or colour vision deficiency. Colour blindness was discovered by an English chemist named John Dalton in the year 1798. During the discovery, he was also suffering from colour blindness. He wrote his first article about colour blindness, which was based on his own experience. Colour blindness is also called as Daltonism, which is named after its discoverer – John Dalton.

None of the above

What are Chromosomal Abnormalities (Disorders)?

Chromosomal abnormalities are the type of genetic disorders caused due to the change in one or many chromosomes or the abnormal arrangement of the chromosomes. There are different types of chromosomal abnormalities as follows:

1. Aneuploidy – It is a condition in which there is a loss or gain of chromosomes due to abnormal segregation of genes during cell division.

2. Polyploidy –  It is a condition in which the count of the entire set of chromosomes increases due to the failure of cytokinesis in cell division. It is mostly observed in plants.

In humans, when there is an extra copy of a chromosome in one of the pairs, it is called trisomy and when one of the chromosome from the pair is lacking, it is called monosomy.

Normal human beings have forty six chromosomes arranged in twenty three pairs. However, even a slight variation from this pattern causes abnormalities. Chromosomal disorders are caused either due to changes in chromosomal number or changes in chromosomal structure. Changes in chromosomal number occur due to non-disjunction of chromosomes which is the failure of chromatids to dis-join during cell division leading to either aneuploidy or euploidy. Aneuploidy is a condition where one or more chromosomes are either gained or lost. Aneuploidy is of two types – trisomy and monosomy. A normal diploid individual has 2n number of chromosomes. However, when an extra chromosome is added, the condition is known as trisomy. On the other hand, when a chromosome is lost or absent, the condition is known as monosomy.

Euploidy is a polyploidic condition, where more than two haploid sets of chromosomes are formed due to the failure of cytokinesis. Depending on the number of chromosomal sets added, euploidy can be triploidy, tetraploidy, pentaploid and so on.

Other common examples of chromosomal abnormalities are down syndrome, Klinefelter syndrome, and Turner syndrome.

1. Down’s Syndrome 

It is caused due to the presence of an extra copy of the twenty first chromosome.The symptoms are a swollen face, bulging and slanting eyes, a small mouth and a protruding and furrowed tongue.

2. Klinefelter’s Syndrome

Down's Syndrome

Klinefelter's Syndrome

3. Turner’s Syndrome

Turner’s Syndrome where one of the X-chromosomes is absent in females. Chromosomes in these females will be forty five with XO. Females with this syndrome show symptoms like a webbed neck, constriction of the aorta, poor breast development, under-developed ovaries and short stature.

Changes in chromosomal structure can take several forms. Some of these changes include a portion of a chromosome getting deleted, duplicated or transferred to another chromosome. While some of these structural changes in a chromosome are inherited others take place accidentally when reproductive cells are being formed or during early foetal development. Jacobsen Syndrome and Cri-du Chat Syndrome are some disorders caused due to changes in chromosomal structure. Therefore, chromosomes hold the genetic keys to all the functions of our body and any change in the number or structure of a chromosome can lead to chromosomal disorders.

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Class 12 Biology (India)

Course: class 12 biology (india)   >   unit 5.

• An introduction to genetic mutations
• Pedigree for determining probability of exhibiting sex linked recessive trait
• Pedigrees review
• (Choice A)   ee A ee
• (Choice B)   Ee B Ee
• (Choice C)   X E Y ‍   C X E Y ‍  
• (Choice D)   X e Y ‍   D X e Y ‍  
• (Choice E)   EE E EE

• Biology Important Questions
• Class 12 - Biology
• Chapter 5: Principles Inheritance Variation

Important Questions for Class 12 Chapter 5 Principles of Inheritance and Variations

Inheritance is the transfer of genes from parents to the offsprings. The principles of inheritance and variation were explained by Gregor Mendel in his experiments on a pea plant. He stated three laws of inheritance on the basis of his observations with the pea plant:

• Law of Dominance
• Law of Segregation
• Law of Independent Assortment

Very Short Answer Type Questions

Q.1. What is the cross known as when the progeny of F1 and a homozygous recessive plant is crossed? State its advantage.

A.1. The cross is a test cross. It is advantageous to determine the genotype of the parent plant.

Q.2. What are the criteria for selecting organisms to perform crosses to study the inheritance of a few traits?

A.2. The following criteria are adopted for selecting organisms:

• The traits should be easily visible.
• The organisms should have different traits.
• They should have a short life span
• They must be true breeds
• The pollination procedure should be simple.
• The traits can be manipulated easily
• Random mating of gametes should take place

A.3. The pedigree shows an autosomal recessive disorder. The parents are the carrier of the disease so the disease will be visible in only a few offsprings. The other offsprings will be either a carrier or non-carrier.

Q.4. Why did Mendel self-pollinate the tall F1 plants to get the F2 generation and crossed a pure breeding tall plant with a pure breeding dwarf plant to obtain the F1 generation?

A.4. The genotype of 50% of the offspring will resemble one parent and the rest 50% will resemble the other parent. The F1 generation obtained from the cross is heterozygous. So selfing the F1 generation is sufficient to obtain the F2 generation. It would also help to understand the inheritance of selected traits over generations.

Q.5. How are the alleles of a gene different from each other? What is its importance?

A.5. Alleles are the alternative forms of the same gene. For eg., a gene for height comprises of two alleles, one for tall (T) and the other for the dwarf (t). They differ in their nucleotide sequence due, which results in different phenotypes.

Importance :

They are essential in studying the inheritance and behaviour patterns.

They show variations in the population due to contrasting phenotypes of a character.

Q.6. How far are the genes and environment responsible for the expression of a particular trait?

A.6. The genes remain active throughout our lives, switching on and off their expression in response to the environment. The external factors such as light, temperature, nutrition, etc. are responsible for the gene expression exhibiting changes in the phenotype. Genes provide potentiality while the environment provides an opportunity for the expression of the traits.

Q.7. What is the genetic basis of the wrinkled phenotype of pea seed?

A.7. A single gene determines the shape of the seed. The (R) is for the round shape, which is dominant over (r) for the wrinkled seed. If homozygous alleles control the seed shape, it will depict the phenotype of same alleles, for eg., RR (round), rr (wrinkled). If the alleles are heterozygous, the phenotype of the dominant allele will be expressed, for eg., Rr (round).

Q.8. Why does an individual have only two alleles even if a character shows multiple alleleism?

A.8. The multiple forms of an allele that occurs on the same gene locus are known as multiple alleles. But an individual carries only two alleles. This happens because a zygote is formed by the fusion of haploid sperm and egg. They have only one allele for each trait. When the zygote becomes diploid, it has two alleles for each trait.

Q.9. How is a mutation induced by the mutagen? Explain with examples.

A.9. The mutagen changes the base sequence by insertion, deletion or substitution and induces mutation .

Q.10. Differentiate between dominance, co-dominance and incomplete dominance.

A.10.  Dominance is the phenomenon in which one variant of a gene masks the effect of a different variant of the same gene.

Co-dominance is the relationship between two alleles of a gene. In this none of the alleles are recessive and the phenotype of all the alleles are expressed.

Incomplete dominance is a form of intermediate inheritance in which one allele for a specific trait is not expressed completely over its paired allele.

Q11.Define the chromosomal theory of inheritance?

A11. The chromosomal theory of inheritance is defined as the fundamental theory of genetics, which recognizes chromosomes as the carriers of genetic material.

Q12. Define Linkage?

A12. In genetics, the linkage is defined as the tendency of genes to remain combined together during the inheritance. This phenomenon was first observed and reported by William Bateson and R.C. Punnet in the early 1900s.

Short Answer Type Questions

Q.1. How is it possible for a child to have a blood group O if the parents have blood groups A and B?

A.1. Case I- If the father is I A and mother is I B , the child will have blood groups AB, A, B, O. Case II- If a father is I A and mother is I B , the child will have the same blood groups as in the case I, i.e., AB, A, B, and O. Thus if the parents have heterozygous alleles, the child will have blood group O.

Q.2. Explain Down’s syndrome.

A.2. Down’s syndrome is an autosomal genetic disorder caused by trisomy at chromosome 21, i.e., there is an extra copy of chromosome 21. This condition affects an individual both physically and mentally. Children born with Down’s syndrome have a flat nose and small ears. They face problem in thinking, understanding and reasoning throughout their lives. They might have trouble hearing and seeing. They are often dwarf.

Q.3. Why is it that women exceeding 40 years of age have more chances of having a child with Down’s syndrome?

A.3. The women exceeding 40 years of age have more chances of having a child with Down’s syndrome because increased age affects the meiosis of chromosomes adversely. The meiosis remains incomplete until fertilization. It remains arrested at prophase-I and the chromosome is unpaired. If the fertilization occurs after a very long gap, the chromosomes will have to remain unpaired for a longer time. The longer the time of unpairing, the greater are the chances of its non-disjunction, and hence conditions like trisomy arise.

Q.4. How was it known that the genes are located on chromosomes?

A.4. The chromosomal theory of inheritance proposed by Bovine and Sutton stated that the genes are present on specific locations on a chromosome. Later, Thomas Morgan observed mutation in the eye colour of the fruit flies and based on the inheritance patter concluded that the gene responsible for the eye colour is located on the X-chromosome.

Q.5. A plant with yellow flowers was crossed with a plant with red flowers. The F1 progeny obtained had orange flowers. What is the inheritance pattern? A.5. The inheritance is incomplete dominance . In this, a new intermediate phenotype between the two original phenotypes is obtained. One allele for a specific trait is not completely expressed over the other allele for the same trait.

Q.6. Mention the characteristics of a true-breeding line. A.6. Characteristics if true breeding is as follows:

• It undergoes self-pollination.
• It depicts stability in the inheritance for several generations.
• Provide gametes with similar traits, hence used as parents for artificial hybridization .
• Homozygous recessive plants are used to identify the genotype through a test cross.

Q.7. Who had proposed the chromosomal theory of inheritance?

A.7.  Theodor Boveri and Walter Sutton are the two scientists who were credited with developing the Chromosomal Theory of Inheritance during the early 1900s.

Q.8. What is recombination? Mention its applications with reference to genetic engineering.

A.8. Recombination is the process of producing a new combination of genes by crossing over during meiosis.

Applications:

It is a means of introducing new traits.

Variability is increased, which is necessary for natural selection.

It is used for preparing linkage chromosome maps.

The desired recombinants produced as a result of crossing over are selected by the plant breeders to produce new crop varieties.

Q.9. Why does sickle-cell anaemia persist in the human population when it is believed that the harmful alleles get eliminated from the population after a certain time?

A.9. Sickle cell anaemia is an autosomal recessive disease in which the red blood cells become sickle-shaped, inhibiting the oxygen-carrying capacity of the blood. Despite this, it protects the carrier from malaria. Individuals with heterozygotes HbAS survive more than the homozygotes HbSS because they are not exposed to the same severity of risks.

Q.10. Define artificial selection. Has it affected the process of natural selection?

A.10. Artificial selection is the intentional breeding of plants and animals where the breeders select the desired traits and make them breed to produce offsprings with the required characteristics. It is an ancient method of genetic engineering. It surely affects the process of natural selection. The individuals cannot evolve on their own. The process is a threat to biodiversity. The traits are not selected considering the fitness of the organism.

Q11.What are Sex chromosomes?

A11.  Sex chromosomes are defined as a pair of chromosomes, which determine whether an individual is male or female. In all mammals, including humans, have sex chromosomes X and Y  in their cells . Females have two X chromosomes(XX), and males have an X and a Y chromosome (XY).

Q12.What are chromosomes and who discovered chromosomes?

A12.  Chromosomes are thread-like structures present within the nucleus of a cell. Each species has a unique number of chromosomes and it varies from one organism to another. Humans have 23 pairs of chromosomes and Humans have 23 pairs of chromosomes.  

Carl Wilhelm von Nageli, a Swiss botanist, discovered chromosomes. He was the first person to observe chromosomes in plant cells in the year 1842.

Long Answer Type Questions

Q.1. What is aneuploidy? Differentiate between aneuploidy and polyploidy.

A.1. Aneuploidy is the chromosomal abnormality in which one or more chromosomes are gained or lost during meiosis due to the non-disjunction of chromosomes.

Differences between aneuploidy and polyploidy:

Polyploidy is a type of chromosomal aberration containing an entire extra set of chromosome. It may be triploid or tetraploid. This phenomenon is common in plants. It is, however, lethal in animals.

Q.2. Describe the individuals with the following chromosomal abnormalities:

• Trisomy at chromosome 21

1) Trisomy – Trisomy results in an autosomal linked genetic disorder known as Down’s syndrome. The individuals exhibit the following characteristics:

• Protruding tongue
• Slanting eyes
• Short height
• Mental retardation
• Under-developed genitals and gonads

2) XXY – The presence of an additional copy of an X-chromosome results in Kleinfelter’s syndrome . The patient exhibits the following characteristics:

• The male individual possesses feminine characteristics.
• Development of breasts in males
• Male is sterile
• Poor beard growth
• Feminine voice

3) XO – Loss of X-chromosome results in Turner’s syndrome. Characteristics:

• The female is sterile.
• The ovaries are immature.
• Webbed neck
• Thorax is shield-shaped
• Under-developed breasts.
• Puffy fingers
• Uterus is small

The standard dihybrid ratio observed is 9:3:3:1. If the two genes interact with the values will deviate. This is because when the genes are linked they do not exhibit independent assortment and remain together in the gametes and the offsprings. The dihybrid ratio thus obtained is 3:1.

Q.5. Why is Drosophila used extensively for genetic studies?

A.5. Drosophila is extensively for genetic studies because it has the following characteristics:

They have a life span of two weeks.

They can be grown in the laboratory on simple synthetic medium.

A large number of progenies are produced by a single mating.

The male and the female Drosophila can be differentiated easily.

It has many variations easily visible under a simple microscope.

Q6.List out the characteristics of the chromosome theory of Inheritance.

The important characteristics of the chromosome theory of Inheritance are:

• Fertilization restores diploid condition.
• Chromosomes segregate and assort independently.
• Homologous chromosomes separate at the time of meiosis.
• Both chromosomes, as well as genes, exist in pairs within the diploid cells.
• Gamete contains only one chromosome of a particular type and only one of the two alleles of a character.

Q7.Define autosome, hemizygous, homozygous, and heterozygous?

Autosome– All chromosomes apart from the sex chromosomes are called the Autosomes. The number of autosomes differs from one organism to another. Humans have 44 number or 22 pairs of autosomes .

Hemizygous– It is a condition in which an organism has only one copy of a gene or DNA sequence present in diploid cells.

Homozygous — It is a condition in which an organism has two similar alleles of a given gene (XX).

Heterozygous –It is a condition in which an organism has two different alleles of a given gene (XY) .

Q8.What are Sex-linkage?

A8. Sex linkage can be defined as the phenotypic expression of an allele, which is dependent on the individual’s gender. It describes the presentation of the chromosome and the sex-specific patterns of inheritance . Sex linkage is directly tied to the sex chromosomes – homogametic sex and heterogametic sex. In mammals, the homogametic sex  (XX) is female and the heterogametic sex (XY) is male. Thus the sex-linked genes are carried on the X chromosome.

Q9. Why is colour blindness more prominent in males than females?

A9. Colour blindness is a sex-linked disorder and the genes responsible are present on the X-chromosome. To become affected by the disease, the female should possess the alleles for colour blindness on both the X-chromosomes. If the allele is present on only one chromosome, the female becomes a carrier of the disease. Since males have only one X-chromosome, it carrying the allele renders them affected. That is why males are more prone to colour blindness .

Q10. Why did scientists select fruit flies for his genetics experiments?

A10.  Drosophila melanogaster is a small common fly species, which belongs to the family Drosophilidae. This species is generally known as the vinegar fly or a fruit fly.  

In the year 1830, Drosophila melanogaster was established as a key model organism for biomedical science and it is due to the considerable biological similarity to mammals and an abundance of available genetic tools.

Like humans, these fruit flies species have a similar distribution of chromosomes. An individual with a pair of X chromosomes is female fruit fly and an individual with one X and one Y chromosome is male.

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Also Acess  Class 12 Biology Sample papers  and  class 12 Biology Previous Year Question Papers

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