Sarah Izhar Content Writer
Content Writer
Genetic Disorders occur when there is a change in the DNA sequence. This change in the sequence of DNA is called mutation.
- Mutation can occur in a single gene or multiple genes.
- These changes are either inherited from parents or acquired during their lifetime.
- Examples of disorders that are inherited are sickle cell anemia and hemophilia.
- Certain types of cancers or neurofibromatosis are diseases that are acquired due to factors like smoking, drinking, etc.
What are Genetic Disorders?
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The disorders that affect the DNA sequence or genes are called genetic disorders. Either a single gene is affected or multiple genes experience mutation.
- Genes are defined as the basic hereditary material or unit that holds genetic information as DNA which is later translated into protein.
- These proteins carry various biological functions in the human body.
- Any change in the gene causes a change in the instructions due to which the protein does not function well.
- Genetic disorders are either present by birth or acquired during one’s life due to some external factors.
Types of Genetic Disorders
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Some disorders are present by birth i.e. they are innate while some others are developed over time i.e. they are acquired due to external factors. In this section, we will discuss the different types of genetic disorders.
Mendelian Disorders
Mendelian disorders are one of the rarest types of disorders that are caused by a change in a single gene.
- These disorders are identified by analyzing the family history using pedigree analysis.
- Mendelian disorders are present at the time of birth.
Mendelian Disorders
Some examples of Mendelianl disorders are:
- Cystic fibrosis (autosomal recessive)
- Haemophilia (sex-linked recessive)
- Albinism (autosomal recessive)
- Sickle cell anemia (autosomal recessive)
Chromosomal Disorders
Any change in the number or structure of chromosomes is called a chromosomal disorder.
- There can either be a deletion or an addition in the number of chromosomes
- For example, in trisomy 21 there is a duplication of the 21st chromosome.
- In Turner syndrome, one of the X chromosomes is missing completely or partially.
Some of the examples of chromosomal disorders are:
- Down’s Syndrome: Addition of X Chromosome 21.
- Turner’s Syndrome: Absence of X Chromosome.
- Klinefelter's Syndrome: Addition of X Chromosome.
- FragileX Syndrome: Increase in the number of FMR1 Gene.
Multifactorial Genetic Inheritance
Multifactorial genetic inheritance is also called polygenic inheritance.
- It is defined as the inheritance of a characteristic or trait from multiple genes.
- These are mainly caused by environmental influences and gene alterations.
Examples of multifactorial genetic inheritance are:
Mitochondrial Inheritance
Mitochondrial inheritance results in mutations in the non-nuclear mitochondrial DNA and is passed down from mother to child. The following are some of the illnesses induced by mitochondrial inheritance:
- Leber’s Hereditary Optic Atrophy
- Myoclonic Epilepsy with ragged red fibres
- Mitochondrial encephalopathy
- Lactic Acidosis
Mitochondrial Inheritance
Monogenic Disorder
Monogenic Disorder or inheritance is the opposite of polygenic inheritance. In this type, a single trait or characteristic is determined by a single gene.
- Examples of monogenic inheritance include cystic fibrosis, Duchenne muscular dystrophy, sickle cell disease, and Tay-Sachs disease
Causes of Genetic Disorders
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The genetic material present in our body signals the genes to produce proteins. These protein molecules start interactions to keep the body healthy.
- However, due to changes in the DNA sequences, the protein-making instructions of genes are disrupted.
- Sometimes, the proteins the body contains do not properly function.
- The external causes of genetic disorders include UV radiation, exposure to smoke, and chemicals, and smoking cigarettes.
Symptoms of Genetic Disorders
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There are various symptoms of genetic disorders, a few of which are mentioned below:
- Changes in behavior
- Breathing issues
- Poor brain functioning
- Delayed speech
- Difficulty in moving due to muscle stiffness.
- Poor growth
- Seizures or stroke
- Poor vision and hearing ability
- Difficulty in swallowing
- Limb and facial abnormalities
- Digestive problems
Examples of Genetic Disorders
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Various other examples of genetic disorders include:
- Thalassemia
- Obesity
- Kleinfelters syndrome
- Hypertension
- Huntington’s disease
- Hemochromatosis
- Cancer
- Cystic Fibrosis
- Leber’s Hereditary Optic Atrophy
Things to Remember
- Mutations that occur in one gene are called monogenic while multifactorial is the changes caused in multiple genes.
- Any change in the chromosome number is called a chromosomal disorder, for example, Down Syndrome.
- Prenatal diagnostic testing is a method for diagnosing genetic disorders.
- Mendelian disorders are caused by mutations in a single gene.
- Colon cancer is an example of a complex disorders.
Sample Questions
Ques. What is pedigree analysis? Suggest how such this analysis can be useful. (3 marks)
Ans. Pedigree analysis is the study of pedigree for the transmission of a particular trait and finding the possibility of absence or presence of that trait in a homozygous or heterozygous state in a particular individual. Pedigree analysis helps-
(i) in analysis of transmission of character in family over generation.
(ii) in genetic counselling of diseases like haemophilia.
(iii) to identify whether a particular genetic disease is due to a recessive gene or a dominant gene.
(iv) to identify the possible origin of the defective gene in the family or in a population.
Ques. Mention any two autosomal genetic disorders with their symptoms. (3 marks)
Ans. Sickle cell anaemia: Haemoglobin has less than O2 transport, sickle shaped RBCs etc.
Phenylketonuria: Mental retardation (due to accumulation of phenylalanine in brain), hypopigmentation of skin & hair, eczema etc.
Ques. How do genes and chromosomes share: similarity from the point of view of genetic studies? (3 marks)
Ans. Similarities between Chromosomes and Genes:
- Both occur in pairs.
- Both segregate at the time of gamete formation such that only one of each pair is transmitted to a gamete.
- Independent pairs segregate independently of each other in both.
Ques. “Genes contain the information that is required to express a particular trait.” Explain. (3 marks)
Ans. The genes present in an organism show a particular trait by way of forming a certain product. This is facilitated by the process of transcription and translation (according to central dogma of genetics).
Ques. How does a mutagen induce mutation? Explain with examples. (2 marks)
Ans. A mutagen is a physical or chemical agent that changes the genetic material, usually DNA. Different mutagens act on the DNA differently. Powerful mutagens may result in chromosomal instability, causing chromosomal breakages and rearrangement of the chromosomes such as translocation, deletion, and inversion.
Ionizing radiations such as X-rays, gamma rays and alpha particles may cause DNA breakage and other damages. Ultraviolet radiations with wavelength above 260 nm are absorbed strongly by bases, producing pyrimidine dimers. Radioactive decay, such as 14C in DNA which decays into nitrogen.
Ques. What is Down’s syndrome? Give its symptoms and cause. Why is it that the chances of having a child with Down’s syndrome increases if the age of the mother exceeds forty years? (3 marks)
Ans. Down’s syndrome is a human genetic disorder caused due to trisomy of chromosome no. 21. Such individuals are aneuploid and have 47 chromosomes (2n + 1).
The symptoms include mental retardation, growth abnormalities, constantly open mouth, dwarfness etc.
The reason for the disorder is the non-disjunction (failure to separate) of homologous chromosomes of pair 21 during meiotic division in the ovum.
The chances of having a child with Down’s syndrome increase with the age of the mother (+40) because ova are present in females. Since their birth and therefore older cells are more prone to chromosomal non-disjunction because of various physico-chemical exposures during the mother’s life-time.
Ques. How was it concluded that genes are located on chromosomes? (2 marks)
Ans. Morgan confirmed Mendelian laws of inheritance and the hypothesis that genes are located on chromosomes. Morgan had discovered that eye colour in Drosophila expressed a sex-linked trait. All first-generation offspring of a mutant white-eyed male and a normal red-eyed female would have red eyes because every chromosome pair would contain at least one copy of the X chromosome with the dominant trait.
Ques. If a father and son are both defective in red-green colour vision, is it likely that the son inherited the trait from his father? Comment. (2 marks)
Ans. Gene for colour blindness is X-chromosome linked, and sons receive their genes solely from their mother, not from their father. Male-to-male inheritance is not possible for X-linked traits in humans. In the given case the mother of the child must be a carrier (heterozygous) for the colour blindness gene.
Ques. It is said that the harmful alleles get eliminated from the population over a period of time, yet sickle cell anaemia is persisting in the human population. Why? (3 marks)
Ans. The harmful alleles get eliminated from the population over a period of time, yet sickle cell anaemia is persisting in the human population because SCA is a harmful condition which is also a potential saviour from malaria.
Those with the benign sickle trait possess a resistance to malaria infection. The pathogen that causes the disease spends part of its cycle in the red blood cells and triggers an abnormal drop in oxygen levels in the cell. In carriers, this drop is sufficient to trigger the full sickle-cell reaction, which leads to infected cells being rapidly removed from circulation and strongly limiting the infection’s progress. These individuals have a great resistance to infection and have a greater chance of surviving outbreaks. This resistance to infection is the main reason the SCA allele and SCA disease still exist. It is found in the greatest frequency in populations where malaria was and often still is a serious problem.
Ques. What is Klinefelter’s Syndrome? (2 marks)
Ans. This genetic disorder is also caused due to the presence of an additional copy of X-chromosome resulting in a karyotype of 47, XXY.
Such an individual has overall masculine development, however, the feminine development (development of breast, i.e., Gynaecomastia) is also expressed. Such individuals are sterile male.
Ques. What is Turner’s Syndrome? (2 marks)
Ans. Such a disorder is caused due to the absence of one of the X chromosomes, i.e., 45 with XO. Such females are sterile as ovaries are rudimentary besides other features including lack of other secondary sexual characters.
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