Question

In a family of four including a normal mother, a normal father, a color blind son and a normal son; who do you think has the defective X gene other than the affected son?

(a) The mother

(b) The father

(c) The normal son

(d) The effected son only

I had been asked this question during an online interview.

Asked question is from Sex Linked Inheritance topic in division Chromosomes and Sex Determination of Cytogenetics

Answer 1

Right answer is (a) The mother

The explanation is: Males have only one X chromosome, so if the father or normal son carried the defective X gene they would show the phenotypic effect. Since the female has two copies of X chromosome, only in homozygous form the recessive gene like colorblindness is expressed. So mother here is a carrier.