Question

Haemophilia is more common in males because it is a

• A. recessive trait which is able to express in homozygous condition
• B. dominant trait carried by Y chromosome
• C. recessive trait which is able to express in hemizygous condition
• D. dominant trait which is able to express in heterozygous condition

Answer 1

The correct option is C recessive trait which is able to express in hemizygous condition

Haemophilia is an X linked recessive trait which is caused due to the presence of a mutated gene ‘h’ on the X chromosome of humans.

A human female would be haemophilic if both the X chromosome had h gene ( X${}^h$X${}^h$) condition. But this condition is very rare in females as it is lethal. Usually the females with such a genotype die early in the embryonic life.

But in males, the case is different. They contain one X chromosome and one Y chromosome. The presence of a mutant allele over the X chromosome is enough to show the phenotypic expression. This is because there is no counter allele on the corresponding Y chromosome. This is the hemizygous condition.
In males, a homozygous condition is not possible because of the presence of two different chromosomes. This condition is not lethal in males. So, haemophilia is mostly seen in males.
It is not a dominant trait and is not present on the Y chromosome.
In females, this disorder is not expressed phenotypically in the heterozygous condition. Though the female carries a single mutant allele, she is not diseased. She leads a normal life.