Question

A woman with normal vision marries a man with normal vision and gives birth to a colour blind son. Her husband dies and she marries a colour blind man. What is the probability of her children having the abnormality?

• A. 50% colour blind sons + 50% colour blind daughters
• B. All sons colour blind and daughter carrier
• C. All daughters colour blind and sons normal
• D. 50% sons colour blind and all daughters normal

Answer 1

The correct option is A 50% colour blind sons + 50% colour blind daughters

Colour blindness is a X-linked recessive disorder. Son gets its X chromosome from the mother, hence the normal mother of a colour blind son will be carrier (X${}^c$X). Among the children of a carrier female (X${}^c$X) and colour blind man (X${}^c$Y), 50% sons and 50% daughters will be affected. Correct answer is option A.
Parent generation : "X${}^c$X" x "X${}^c$Y"

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