Question

3) In humans, hemophilia (a disease that affects the blood clotting process) is x-linked. A young man, John,
has hemophilia but his parents do not.
a) What are the genotypes of John and his parents?
b) If John’s parents have additional children, what are the probabilities that John’s future sisters and
brothers will not have hemophilia?
c) Of John’s 4 grandparents, only his father’s father has hemophilia. What are the possible genotypes
of the 4 grandparents? From which of the grandparents could John have inherited hemophilia?

Answer 1

a) let us assume normal allele is X and hemophilia allele is x

John's parents are not affected and john is affected. So one of the parent is carrier. Since it is x linked recessive trait then mom should be a carrier. Then their genotypes are XY and Xx

Then john's genotype is xY- affected.

Chances of brothers and sisters not being affected.

The possible gametes - XX, XY, Xx and xY

Probability of being unaffected child is 3/4. Since father is not affected there is no chance for sister to be affected. But they can be a carrier.

So probability of being unaffected children is 3/4

C) john's paternal grandfather being affected but not transmitted to his father since father x chromosome is transmitted only to daughter. Then paternal grandmother genotype is XX, because john's father is not affected.

So he should be inherited the gene from one of maternal grandparents. His mother is carrier, so she might be inherited from her father. So her father should be affected.

If her mother also affected then she would be affected. So john inherited this disease from his maternal grandfather.

john's paternal and maternal grandparents genotype is same.grandfathers genetype are xY and grand mothers genotype are XX