The following points highlight the four major types of congenital abnormalities found in man. The types are: 1. Down’s Syndrome 2. Colour Blindness 3. Albinism 4. Sickle-Cell Trait.
Type # 1. Down’s Syndrome:
It is hereditary abnormality due to genetic disorder, autosomal abnormality. It is also known as Mongolism or Mongoloid idiocy.
The individual possesses a small extra chromosome No 21, the total number of chromosomes being 47.
1. The extra chromosome is presumably due to nondisjunction or failure of homologous chromosome in the first meiosis, or of the two chromatids of a chromosome in the second meiosis to separate during the formation of ovum.
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2. It may also be caused by translocation between chromosome No 21 and 14 or 15 (translocation interchange).
About 0.15 per cent people suffer from Down’s syndrome. It is believed that the disease is related to the age of the mother and greatly increases with age. It is independent of the age of the father and preceding pregnancy of the mother.
Symptoms:
Stature short; stubby hands; hand and feet bear simian folds; a broad face; flat or stubby nose, obliquely set eyes with mongoloid eye-lid fold (though not similar); open mouth; fat and soft skin; flaccid muscles; mentally retarded and malformed heart.
Type # 2. Colour Blindness:
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The cone cells of the retina of human eye are of three- different types and respond to three primary colours—blue, green and red. The development of cone cells is controlled by colour gene. It is difficult or impossible for a person to distinguish between the colours of longer wave lengths due to lack of either the red or the green cones. This is known as red-green colour blindness.
The colour genes are sex linked and present in the female sex chromosome. ‘C’, the gene for normal vision is dominant over ‘c’ (recessive allele), gene for colour blindness. Colour blindness is almost absent in females, as they possess two female sex chromosomes (xx). Males have only one female sex chromosome (x).
Colour blindness is about 8 per cent in men but only about 0.4 per cent in females.
Genotypes:
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The genotype for a colour blind male k ‘c’, the recessive allele (no colour gene is present in male or y chromosome).
The genotype of a normal visioned male is ‘C’ (dominant).
The genotype of a color blind female is ‘cc’ (recessive).
The genotype of a normal visioned female is ‘CC’ (dominant).
The offspring’s of both the sexes receive x chromosome from a normal vision or a colour blind mother. The father transmits x chromosome only to the daughter.
Transmission of Colour Blindness:
A. Offspring’s of a normal man and a colour blind woman.
All the sons will be colour blind.
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All the daughters will be normal but earners.
B. Offspring’s of a colour blind son and a normal carrier daughter.
50 % of the offspring’s (male and female) will be normal, 50% of the offspring’s (male and female) will be colour blind.
C. Offspring’s of a colour blind male and a normal female (homozygous).
All the males will be normal.
All the daughters will be normal but carriers.
Type # 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 generalized.
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.
Type # 4. Sickle-Cell Trait:
In certain persons the erythrocytes (red blood cells) instead of being round as in normal human are sickle or crescent-shaped, and this is known as sickle-cell trait.
1. The disease sickle-cell trait is caused by mutation.
2. The amino acid, glutamic acid of the haemoglobin of a normal person is replaced by valine in sickle-cell haemoglobin.
3. As a result the normal haemoglobin and consequently the normal R.B.C. is converted into abnormal haemoglobin causing sickling R.B.C.
4. The blood of persons heterozygous for sickle-cell gene contains both normal haemoglobin (HbA) and mutant-(Hbs) alleles and the haemoglobin is a mixture of both kinds.
5. The persons homozygous for sickle- cell gene contain only mutant haemoglobin.
6. Sickle-cell trait occurs in Africans. The persons homozygous for sickle-cell gene suffer from anaemia, skin lesions, kidney damage and spleen enlargement, and die early.
7. The persons heterozygous for sickle- cell gene suffer from anaemia but they are immune to Plasmodium, causing malignant malaria.
The discipline of zoology dealing with animals directly or indirectly related to human economy is termed ‘economic zoology. This includes a vast number of animals embracing those destroying the products of our labour or natural resources (harmful) and also those providing us with materials of commercial value (beneficial). They are also called enemies and friends, respectively.