Principles of Inheritance and Variation Question 141
141. Which of the following most appropriately describes haemophilia?
[2016]
(a) Recessive gene disorder
(b) $X$ - linked recessive gene disorder
(c) Chromosomal disorder
(d) Dominant gene disorder
Show Answer
Answer : b
Hints & Solution
(b) Haemophilia A and haemophilia B are inherited in an X-linked recessive pattern. The genes associated with these conditions are located on the $X$ chromosome, which is one of the two sex chromosomes. In males (who have only one $X$ chromosome), one altered copy of the gene in each cell is sufficient to cause the condition. In females (who have two $X$ chromosomes), a mutation would have to occur in both copies of the gene to cause the disorder. Because it is unlikely that females will have two altered copies of this gene, it is very rare for females to have haemophilia. A characteristic of X-linked inheritance is that fathers cannot pass X-linked traits to their sons.
