A woman who is a carrier for colour blindness marries a normal man. In their children, the disease may be seen in:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 50% of the sons affected and 50% of the daughters carriers
Colour blindness is a sex-linked recessive disorder; its gene is on the X chromosome. Let Xc be the X carrying the colour-blindness gene. The carrier mother is XcX and the normal father is XY.
| X (father) | Y (father) | |
| Xc (mother) | XcX – carrier daughter | XcY – colour-blind son |
| X (mother) | XX – normal daughter | XY – normal son |
Every daughter gets a normal X from the father, so no daughter is colour blind; half of them are carriers. A son has only one X, which comes from the mother, so half the sons are colour blind.
Option (1) All the daughters but not the sons — Incorrect. Each daughter gets the father’s normal X, so no daughter shows the disease.
Option (2) 50% of the sons affected and 50% of the daughters carriers — Correct. This matches the cross above.
Option (3) Both the sons and the daughters — Incorrect. Only sons can show the disease here; a daughter would need a colour-blind father.
Option (4) 50% of the daughters and 50% of the sons carriers — Incorrect. Sons cannot be carriers. With only one X, a son either has the gene and shows the disease, or does not have it.
NCERT Extract:
Mendelian Disorders – “… Males have only one X chromosome and females have two. The son of a woman who carries the gene has a 50 per cent chance of being colour blind. The mother is not herself colour blind because the gene is recessive. That means that its effect is suppressed by her matching dominant normal gene. A daughter will not normally be colour blind, unless her mother is a carrier and her father is colour blind.”