The following points highlight the three major congenital abnormalities seen in man. The abnormalities are: 1. Down’s Syndrome 2. Colour Blindness 3. Albinism.
Congenital Abnormality # 1. Down’s Syndrome:
It is a hereditary abnormality due to genetic disorder. It is also known as Mongolism or Mongoloid idiocy.
1. The person is characterised by a broad face, flat or stuoocy nose, obliquely set eyes, open mouth, fat and soft skin, and flaccid muscles.
2. The cells of such person contain three No.21 chromosomes, which is smallest in size.
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3. Trisomy of the chromosome occurs possibly due to nondisjunction or failure of homologous chromosomes in the first meiosis, or of the two chromatids of a chromosome in the second meiosis to separate.
Congenital Abnormality # 2. Colour Blindness:
Inability to differentiate between red and green colour is colour blindness. It is a sex linked abnormality and more common in man than in woman. A recessive gene ‘C’ located on the X chromosome regulates colour blindness.
1. The gene-type of a homozygous female for recessive allele is CC.
2. Offspring’s of both the sexes receive X chromosome from a colour blind mother.
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3. Unlike, the father transmits X chromosome only to the daughter.
4. Offspring’s of a colour blind woman and a normal man.
(a) All the males will be colour blind.
(b) All the daughters will be normal but carriers.
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5. Offspring’s of a carrier daughter and a colour blind man.
(a) 50% of the offspring’s (male and female) will be normal.
(b) 50% of the offspring’s will be colour blind.
Congenital Abnormality # 3. Albinism:
It is a congenital absence of pigment from the skin, hair, choroid coat and iris. It may be partial or localised and total or generalised.
1. Melanin, a black pigment is synthesised in melanocytes following a series of enzymatic reactions. The amino acid tyrosine plays an important role in it.
2. One-gene one-enzyme hypothesis advocates that a gene controls synthesis of one enzyme. The enzyme is responsible for a specific metabolic reaction.
3. Albinism is the result of a genetic block involving a step in the pathway of melanin formation from tyrosine, possibly due to gene mutation.