Read the given passage and answer the following questions:
Colour blindness is a sex-linked disorder due to defect in either red or green cone of eye resulting in failure to discriminate between red and green colour. This defect is due to mutation in certain genes present in the X chromosome. It occurs in about 8 per cent of males and only about 0.4 per cent of females. This is because the genes that lead to red-green colour blindness are on the X chromosome.
Rahul and Shipra are husband and wife and both are suffering from colour blindness. What is the chance that their daughter will have colour blindness?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 100%
Colour blindness is an X-linked recessive trait. Let Xc be the X chromosome carrying the gene. A colour-blind man (Rahul) is XcY. A colour-blind woman (Shipra) must carry the gene on both her X chromosomes, so she is XcXc.
| Xc (Rahul) | Y (Rahul) | |
| Xc (Shipra) | XcXc daughter, colour blind | XcY son, colour blind |
| Xc (Shipra) | XcXc daughter, colour blind | XcY son, colour blind |
Every daughter gets Xc from her father and Xc from her mother. With no normal X at all, every daughter is colour blind.
Option (1) 50% — Incorrect. A 50 per cent chance is the figure for the son of a carrier woman. Here the mother is affected, not a carrier, so there is no normal X to pass on.
Option (2) 100% — Correct. Both X chromosomes of every daughter carry the gene, so the chance is 100 per cent.
Option (3) 0% — Incorrect. A daughter escapes colour blindness only if she receives at least one normal X. Neither parent has a normal X to give.
Option (4) 25% — Incorrect. A 25 per cent chance would need one parent to have a normal X chromosome. Neither parent has one here.
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.”