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Last updated date: 28th Nov 2023
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MVSAT Dec 2023

A haemophilic man marries a normal-homozygous woman. What is the probability that their son will be hemophilic?
a. 75%
b. 50%
c. 25%
d. 0%

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Hint: Hemophilia is an inherited genetic disorder where it mainly involves with impaired blood clotting, which means that person with hemophilia bleed more time, and their clotting of capability reduced, in people with mild cases cannot be observed until the people with hemophilia met any injuries or accidents.

Complete answer:
Hemophilia is a Sex-linked disease and it is X linked recessive disease.
There are mainly two types of hemophilia, they are
Hemophilia A - This is mainly due to the low amount of clotting factor VIII.
Hemophilia B - This is also due to the low level of clotting factor IX.

This hemophilia is mainly inherited through the X chromosome with a non-functional gene.
As we all know that females have XX chromosomes, and males possess XY chromosomes.
As above said it is an X linked recessive disease, if the female has a defective allele on one chromosome, it is not expressed as the other X chromosome which is dominant may mask the gene expression. So heterozygous females are always Carriers.

For the females to express the disease both chromosomes should be defective and recessive, which is homozygous with defective genes.
In the above question, they ask for the probability of son of hemophilic man and homozygous normal women
Here Man is hemophilic which means his X chromosome is defective, and the mother is normal, so in the son X chromosome definitely comes from mother, but not from father so the answer is 0%.

Hence, the correct answer is option (D).

Note: Hemophilia is more common in males than females because even if the mother is a healthy female but carrier, there is a 50% chance of getting the disease because the X chromosome in males is carried from mothers. So males are more to hemophilia than females.