What is sexlinked inheritance ? Explain inheritance of haemophilia in man.

Explanation

Sex-linked Inheritance Sex chromosomes contain genes that control the sex of an organism.
In addition to sex genes, these organisms also contain the genes to control body characters in sex chromosomes called sex-linked genes.
Inheritance of sex-linked genes is called sex-linked inheritance.
There are three types of sex-linked genes
(i) X-linked genes Sex-linked genes lie on X-chromosomes, e.g. colour blindness and haemophilia in man, eye colour in Drosophila.
(ii) Y-linked genes (holandric genes) Sex-linked genes lie on Y-chromosomes only, e.g. hypertrichosis.
(iii) X and Y-linked genes Sex-linked genes lie on homologous part of the X and Y-chromosomes, e.g. total colour blindness and nephritis.
Haemophilia (Bleeder.s disease) This is a sex-linked disease, where the patient will continue to bleed due to a minor cut in the body, as he or she cannot synthesise anti-haemophilic globulin (haemophilia-A) or plasma thromboplastin (haemophilia-B, also called Christmas disease) required for normal blood clotting.
It is caused by a recessive gene `X^(th)` located on the sex chromosome. Females have two X-chromosomes. For them to be haemophilic, it is necessary that each X-chromosome has a gene for haemophilia `(X^(h)X^(h))`.
The possibility of a female becoming a haemophilic is extremely rare because mother of such a female has to be at least a carrier and father should be haemophilic.


Such females are never born as the combination of these two recessive genes is lethal (fatal).
A female having only one gene for haemophilia `(XX^(h))`, appears normal because its dominant allele in the X-chromosome produces the factor required for normal blood clotting. Such females are called carriers. Such heterozygous females may transmit this disease to her sons.
In males, a single gene for the defect is also to express itself as the Y-chromosome carries no gene for this trait.
Hence, haemophiliia occurs only in males.