Question

2. Hemophilia is a sex-linked recessive trait. Show, through Punnett squares, how, before clotting factor was available, it was virtually impossible for girls to get hemophilia. And show how now, with clotting factor available, girls can now get hemophilia.

Answer 1

Hemophilia is a inherited disorder. It is characterised by the inability to stop bleeding ( i. e , blood doesn't clots properly ). Hemophilia A and Hemophilia B are caused by   the deficiency of clotting factor VIII and clotting factor XI respectively.

Case 1 -If mother is a carrier of hemophilia and father is unaffected, the girl child must be a carrier of the disease or not at all.

Case 2 - To get a girl with hemophilia , 1 of her gene of sex chromosome must be dominant ( X^HX / X^HX^h) . The probability of getting a girl child with hemophilia is given below.

a) Crossing between affected mother (X^HX^h) and affected father (X^HY).

b) Crossing between affected mother ( X^HX^h)and unaffected father (XY).

c) Crossing between unaffected mother ( XX) and affected father ( X^HY).

Conclusion - The girl child is impossible to get hemophilia ,when the mother ( XX^h ) is just a carrier or by an affected father (X^hY ) .

A girl is hemophilic ,either her mother (X^HX / X^HX^h ) or father is affected ( X^HY ) .

* XX - unaffected mother

XY - unaffected father

X^HX - affected daughter with dominant gene

XX^h - carrier daughter with recessive gene

X^HX^h - affected daughter with both dominant gene and recessive gene

X^HY - affected son with dominant gene

X^hY - affected son with recessive gene