Question

What ratio is expected in offsprings if father is colour blind and mother's father was colour blind?

• A. 50% daughters are colour blind.
• B. All the sons are colour blind.
• C. All the daughters are colour blind.
• D. All the sons are normal.

Answer 1

The correct option is A 50% daughters are colour blind.

Colour blindness is a X-linked recessive disorder. Father transmits 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 colour blind man will be (X${}^c$Y). As son gets his X-chromosome from mother, the mother should be colour blind to have all affected sons; this makes option B wrong. The carrier female transmits its one copy of affected X-chromosome to 50% sons; option D is wrong. Daughters receive one X-chromosome from mother and other from father; to have all colour blind daughters, both parents should be affected which makes option C wrong. The carrier mother and colour blind man will inherit the disease to 50% sons and 50% daughters. 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