BIOLOGY: GENETIC DISORDERS

BIOLOGY: GENETIC DISORDERS

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About genetic disorders

Huntington's disease is inherited in the autosomal dominant fashion

• Genetic disorders are disorders that are passed on from generation to generation
• They are caused by abnormalities in genes or chromosomes
• Some genetic disorders may also be influenced by non-genetic environmental factors. Eg: cancer
• Most genetic disorders are relatively rare and only affect one person in thousands or millions
• To recollect, males have XY chromosome pairs while females have XX pairs

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Single Gene Disorders

• Single gene disorders result from the mutation of a single gene
• They can be passed onto subsequent generations in multiple ways
• Single gene disorders include sickle cell disease, cystic fibrosis Huntington disease

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Multiple gene disorders

• Multiple gene disorders result from mutation on multiple genes in combination with environmental factors
• They do not have a clear pattern of inheritance, which makes it difficult to assess risk of inheriting a particular disease
• Examples include heart disease, diabetes, hypertension, obesity, autism

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TYPES OF SINGLE GENE GENETIC DISORDERS

1. Autosomal dominant

   

   Sickle cell disease is inherited in the autosomal recessive pattern
   1. Only one mutated copy of the gene is necessary for inheritance of the mutation
   2. Each affected person usually has one affected parent
   3. There is a 50% chance that the child will inherit the mutated gene
   4. Autosomal dominant disorders usually have low penetrance i.e. although only one mutated copy is needed, only a small portion of those who inherit that mutation will develop the disorder
   5. Eg: Huntington’s disease, Marfan syndrome
2. Autosomal recessive
   1. Two copies of the gene must be mutated for a person to be affected
   2. An affected person usually has unaffected parents who each have one mutated gene
   3. There is a 25% chance that the child will inherit the mutated gene
   4. Eg: Cystic fibrosis, sickle cell disease, Tay-Sachs disease, dry earwax, Niemann-Pick disease
3. X-linked dominant
   1. X-linked dominant disorders are caused by mutations on the X chromosome
   2. Males and females are both affected by such disorders. However, males are affected more severely
   3. For a man with a X-linked dominant disorder, his sons will all be unaffected (since they receive their father’s Y chromosome) while his daughters will all be affected (since they receive his X chromosome)
   4. A woman with a X-linked dominant disorder has a 50% chance of passing it on to progeny
   5. Eg: Hypophosphatemic rickets, Rett syndrome, Aicardi syndrome
4. X-linked recessive

   

   X-linked recessive with a carrier mother
   1. Caused by mutations on the X-chromosome
   2. Males are affected more frequently than females
   3. The sons of a man affected by a X-linked recessive disorder will not be affected, while his daughters will carry one copy of the mutated gene
   4. The sons of a woman affected by a X-linked recessive disorder will have have a 50% chance of being affected by the disorder, while the daughters of the woman have a 50% chance of becoming carriers of the disorder
   5. Eg: colour blindness, muscular dystrophy, hemophilia A
5. Y-linked disorders
   1. Caused by mutations on the Y chromosome
   2. Y chromosomes are present only in males
   3. The sons of a man with Y-linked disorders will inherit his Y chromosome and will always be affected while the daughters will inherit his X chromosome and will never be affected
   4. Eg: male infertility
6. Mitochondrial disorders
   1. These disorders are caused by mutations in the mitochondrial DNA
   2. Only mothers can pass on mitochondrial disorders to children, since only egg cells (from the mother) contribute mitochondria to the developing embryo
      
   3. Eg: Leber’s Heriditary Optic Neuropathy

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