Question

X–linked recessive gene is
A. Always expressed in males.
B. Always expressed in the female.
C. Lethal
D. Sublethal

Answer 1

Hint: As we all know that male carries one X chromosome and female carries two X chromosomes. From two X chromosomes, one gets inactivated and stays in a cell as a Barr body. So, the chances of which X chromosome will get expressed become fifty-fifty.

Complete answer:
The human female has XX as sex chromosomes and the human male has XY as a sex chromosome. Here, X and Y are the genes responsible for sex determination. Female has two X chromosomes and only one of them gets expressed. So, if there is any mutation in one gene, another one will mask the effect of the mutated gene. This type of female is known as a carrier. While in the male, they only have one X chromosome so it will express itself, and if there is any mutation in the X chromosome that will get expressed to and will show the symptoms.

Let’s understand the above diagram to get the answer.

In the given diagram, the mother is a carrier which means she is having a mutation on the X chromosome but it is not expressed because the mutated X chromosome got inactivated as we know that in females one X chromosome is active and another one is inactive. Now, she passes one good X chromosome to her first boy, he doesn’t get the disease but the second one receives a mutated chromosome and that chromosome will get expressed and he will have a disease. And similarly, one daughter will be a carrier and the other will be normal.

If the mother is affected and has a mutation on both the X chromosomes, every boy/ male born from her, will have the disease and every daughter will be a carrier.

So, the correct answer is A, which is “always expressed in male”.

Note:
Always remember that boys have only one X chromosome and they only receive it from mother. So if the mother is having any mutation on the X chromosome, there are high chances that her male progeny will express that X chromosome and will show the symptoms.