Question

If a haemophilic son is born to a normal mother and a haemophilic father, which of the following can be the genotype of the mother?

• A. $X^{h}Y$
• B. $X^{+}{X}^h$
• C. $X^{+}Y$
• D. $X^{+}{X}^{+}$

Answer 1

Answer: (B)

$X^{+}{X}^h$

Haemophilia is an X-linked recessive disorder. Since males are hemizygous for chromosome, One copy of the affected gene in males in each cell is sufficient to cause the disorder (X${}^h$Y ). Thus, the genotype of an affected son will be X${}^h$Y. Since father transmits its X chromosome to the daughters, not to son, so the haemophilic father would not affect the disease inheritance of his son. Son receives its X chromosome from the mother which means that the mother should have at least one copy of affected allele to transmit the disease to her son. Mother with both normal copies of X-chromosome cannot have a diseased son. XY is genotype of male; human females have two copies of X chromosomes as their sex chromosomes. Females with two copies of the affected gene show the disorder (X${}^h$X${}^h$ or X${}^h$X${}^h$). Females heterozygous for this trait will be normal but serve as a carrier of the disease for her sons. Thus, the correct answer is option B.