The chromosomes present in the diploid cells of the majority of the sexually reproducing animals are of two types: autosomes bearing genes for somatic characters and sex chromosomes bearing genes for sex. Sex chromosomes also carry some genes for non-sexual characters such as colour blindness and haemophilia. Such genes which are always associated with sex chromosomes are called sex-linked genes. In man and Drosophila the sex chromosomes (X and Y) are unequal in size and shape, X being larger and rod shaped whereas Y is small and slightly curved. In birds and butterflies the sex chromosomes (Z and W) are also unequal in shape and size, Z being larger than W. In Mendelian pattern of inheritance, the genes for contrasting characters were located on autosomes but not on the sex chromosomes. Secondly, the result of reciprocal cross is same as normal cross which is not the case with sex linked inheritance. There are three types of sex-linked genes depending upon their association with particular chromosome.
Some important points of sex linked inheritance are as follows:
- It is a criss-cross inheritance as the father passes its sex-linked character to his daughter who in turn passes it to the grandson.
- Daughter does not express the recessive trait but act as carrier in the heterozygous condition.
- Female homozygous for recessive trait expresses the trait.
- Any recessive gene borne by the X chromosome of male is immediately expressed as Y chromosome has no allele to counteract.
Sex linked inheritance : examples in men
Colour Blindness
Colour blindness is an example of sex linked character. Those who suffer from red green colour blindness cannot distinguish between red and green colour. The gene for this defect is located on X chromosome.
When a normal woman is married to a colour blind man, their children (daughters and sons) have normal colour vision. But when their daughters were married to normal man, 50% of their sons are colour blind and the remaining 50% are normal, while the daughters were all normal.
If a colour blind woman marries a normal man, their daughters are normal but all their sons are colour-blind. When these F1 daughters are married to colour blind men, colour blind sons and daughters are born in equal number.
Haemophilia
Haemophila is another popular example of sex linked inheritance in human beings. It is caused by a mutant gene (h) present in X chromosome and recessive to normal gene and is, therefore, suppressed in heterozygous condition. Individuals suffering from this disease lack a factor responsible for clotting of blood. So in the absence of blood clotting substance, a minor cut or injury may cause prolonged bleeding leading to death. This disease in man is generally restricted to male members.