Question

What are the chances of colour blind daughters and sons being born in a marriage of normal man marrying a normal woman whose father was colour blind?

• A. All sons are normal and all daughters are colour blind.
• B. Both the sons and daughters are phenotypically normal.
• C. All the sons are colour blind and all daughters are normal.
• D. Both the sons and daughters are colour blind.
• E. 50% sons are colour blind and all daughters are phenotypically normal.

Answer 1

Answer: (E)

50% sons are colour blind and all daughters are phenotypically normal.

Colour blindness is a X-linked recessive disorder. Father transmit its X chromosome to the daughters so the colour blind father will inherit its affected X chromosome to the daughter. Hence, the genotype of carrier female will be (X${}^c$X) and that of normal man will be (XY). The carrier female for colour blindness will inherit the disease to 50% son only. The daughters received their normal copy of X-chromosome from the normal father and will be phenotypically normal. Correct answer is option E.
Parent generation: "X${}^c$X" x "XY"

Gametes : X${}^c$X --> XY	X${}^c$	X
X	X${}^c$X normal carrier girl	XX normal girl
Y	X${}^c$Y colour blind boy	XY normal boy