Question

A normal woman, whose father was colour blind person, is married to a normal man. Her sons would likely to be

• A. 75% colour blind
• B. 50% colour blind
• C. All normal
• D. All colour blind

Answer 1

The correct option is B 50% colour blind

Colour blindness is a X-linked recessive disorder. One copy of the affected gene in males in each cell is sufficient to cause the disorder (X${}^c$Y). Females with two copies of the affected gene show the disorder (X${}^c$X${}^c$). According to the question, father of female was affected; as we know that father passes his X-chromosome to daughters, the female has received the affected X-chromosome and is the carrier. This is because females heterozygous (X${}^c$X) for this trait are normal but serve as a carrier of the disease. The carrier mother will pass her single copy of affected X-chromosome to 50% sons (X${}^c$Y). The 100% daughter will have the normal vision because the father is normal. The correct answer is option B.
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 colour blind boy	XY normal boy