Question

A colourblind man, both of whose parents had normal vision and whose paternal and maternal grandparents had normal vision, probably inherited the gene for colourblindness from his

• A. Maternal or paternal grandmother
• B. Maternal or paternal grandfather
• C. Father
• D. Mother

Answer 1

Answer: (D)

Mother

Colourblindness is a X-linked recessive disorder. Since, males are hemizygous for chromosome, one copy of the affected gene in each cell is sufficient to cause the disorder (X${}^c$Y). Females have two X chromosomes and hence need two copies of the affected gene to cause the disorder (X${}^c$X${}^c$). Females heterozygous (X${}^c$X) for this trait be normal but serve as a carrier of the disease. According to the question, the colourblind man has parents with normal vision, so the father can not transmit the disease. This is because if the father would carry affected allele, he would express the disease. Since maternal grandparents also had normal vision, the maternal grandfather can not transmit the disease. Hence, the maternal grandmother and the mother must be carriers for the disease and have transmitted it to the man. Correct option is "D".
Grandparent generation : (X${}^c$X) x XY

Gametes : (X${}^c$X) --> XY	X${}^c$	X
X	X${}^c$X carrier girl (mother)	XX normal girl
Y	X${}^c$Y colourblind boy	XY normal boy

Parent generation : (X${}^c$X) x XY

Gametes : (X${}^c$X) --> XY	X${}^c$	X
X	X${}^c$X carrier girl	XX normal girl
Y	X${}^c$Y colourblind (The man)	XY normal boy