Question

In humans the gene for red and green colour blindness is located on the sex chromosome. If a male is colour blind and is a child of normal visioned parents, then who among the following in his family is likely to be colour blind?

• A. Maternal grandfather
• B. Maternal grandmother
• C. Paternal grandfather
• D. Paternal grandmother

Answer 1

Answer: (A)

Colour blindness is a X-linked recessive disorder. Son gets its X-chromosome from mother; since the male is affected, he has got the X${}^c$ allele from his mother. According to question, his parents are normal which means that father does not carry the disease; presence of disease in father would have expressed due to its hemizygous genotype for X-chromosome. And mother is carrier who passed her affected allele to male. Father is normal, this rules out the disease to be inherited from paternal grandparents; options C and D are wrong. Father transmits its X chromosome to the daughters so the carrier mother have inherited her affected X chromosome from father which means that maternal grandfather’s genotype is X${}^c$Y; option A is correct. If maternal grandmother was carrier; the mother of male would be diseased as she will get one affected allele from father and one from mother; but mother is carrier here, option B is incorrect. Correct answer is option A.