Question

X-linked recessive genes can be easily expressed in which of the following?

• A. Males only
• B. Females only
• C. Equally expressed in both males and females
• D. Sex linked recessive genes are not expressed at all

Answer 1

The correct option is A Males only

We know from our previous discussions (Mendel’s experiments on pea plants) that a recessive trait is the one which remains unexpressed in the $F_1$ generation but reappears in the $F_2$ generation because it requires two identical alleles to be expressed. A single copy of a recessive allele cannot express itself in the presence of its dominant counterpart.

Similarly, from the experiments carried out by Morgan on Drosophila, we came across white- eyed flies in the $F_2$ generation. The genotype of the white eyed male was $X^{w}Y$. Here we see that there is a single copy of the recessive allele on the X chromosome. There is no allele on the Y chromosome. So here we can say that a single copy of recessive allele can easily express itself in the absence of another recessive allele or dominant allele, if it is located on the X chromosome in males. In females however, this is not possible. Since they have 2 copies of X chromosome, they would require 2 copies of the recessive allele to express the recessive trait.
So, X-linked recessive genes can easily be expressed in males only. In females, it is very rare. It is also not possible to completely suppress the expression of X-linked recessive genes.