CBSE Class 12-science Answered

Males are hemizygous for the X chromosome, i.e., they have only one copy of every gene on the X chromosome. Females, on the other hand, have two and thus, can be homozygous or heterozygous for any locus on this chromosome. As a result, every X-linked gene is expressed in males as there is nothing to "mask" a recessive genotype, as there would be in females.

Let us consider the trait of haemophilia in humans. This is a group of bleeding disorders in which the blood takes longer than usual to clot. The gene responsible for this disease resides on the X chromosome and is recessive. More men (1 in 10,000) than women are affected (1 in 100,000,000), as the disease is recessive in women but is always expressed in men, since men with that allele do not have another allele that might cover it up.
The disease often skips a generation and no haemophiliac man ever bears a haemophiliac son (if the mother carries two normal X chromosomes). This is because only daughters inherit an X chromosome from his father while a son always receives his father's Y chromosome. Assuming the mother carries 2 normal chromosomes, their children won't suffer from the disease, but all daughters will be carriers (heterozygous for the haemophilia gene). The only way for a woman to be affected by haemophilia is if both, her father is a haemophiliac and her mother is a carrier (and thus she has a 50% chance of inheriting the disease), or if both parents are affected (in which case all children will inherit the disease).