Biology · Principles of Inheritance and Variation · NEET
Males are XY. They have only ONE X chromosome. If that single X carries the recessive colour-blind allele (Xc), there is no second X to hide it, so the boy is colour-blind. Females are XX. A girl needs the bad allele on BOTH X's (XcXc) to be colour-blind. Getting two bad X's is much rarer than getting one, so far more males are affected. This is the number one point NEET tests about X-linked recessive traits.
Yes, but it is rare. A girl is colour-blind only if she is XcXc, meaning both her X chromosomes carry the recessive allele. For this, her father must be colour-blind (XcY, he gives Xc) AND her mother must be at least a carrier (XcX) or colour-blind. If she has even one normal X (XCXc), she is a carrier with normal vision, not colour-blind.
A carrier is a woman with genotype XCXc. She has one normal allele (XC) and one colour-blind allele (Xc). Because the normal allele is dominant, she can see colours normally. But she still carries the hidden Xc and can pass it to her children. A carrier mother gives each son a 50% chance of being colour-blind. Only females can be silent carriers; a male with Xc is always affected because he has no second X.
For a colour-blind SON, the allele always comes from the MOTHER, because a son gets his only X from his mother (and Y from father). A colour-blind son proves the mother carried at least one Xc. A father passes his X only to his daughters, never to his sons. So a colour-blind father can make his daughters carriers, but he can never directly make his son colour-blind.
A colour-blind grandfather (XcY) gives his Xc to his daughter, who becomes a carrier (XCXc) but sees colours normally. That daughter can then pass the Xc to her son, who becomes colour-blind (XcY). So the trait skips the daughter's generation and appears in the grandson. This zig-zag from grandfather to grandson through a carrier daughter is called criss-cross inheritance.
Father is XcY, mother is XCXC (homozygous normal). Sons get their X from the mother, which is always XC, so every son is XCY = normal vision, probability of colour-blind son = 0. Daughters get Xc from father and XC from mother, so every daughter is XCXc = carrier, but she sees normally. So no child is colour-blind. This exact cross was NEET 2016 (answer 0).
If a colour-blind man marries a woman who is homozygous for normal colour vision, the probability of their son being colour-blind is
A woman has an X-linked condition on one of her X chromosomes. This chromosome can be inherited by
If a colour-blind female marries a man whose mother was also colour-blind, what are the chances of her progeny having colour blindness?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is recessive, and the gene is on the X chromosome, so it is called X-linked recessive. The normal colour-vision allele (XC) is dominant over the colour-blind allele (Xc).
Males: XCY = normal, XcY = colour-blind. Females: XCXC = normal, XCXc = carrier (sees normally), XcXc = colour-blind. There is no carrier male because a male has only one X.
Per NCERT, it is a defect in the red or green cones of the eye, so the person fails to tell red and green apart. It is caused by mutation in genes on the X chromosome.
Yes. If the mother is a carrier (XCXc) and the father is normal (XCY), their son has a 50% chance of being colour-blind (XcY), even though both parents see colours normally.
Yes, both are X-linked recessive disorders, so they follow the same inheritance rules: more males affected, carrier mothers, and criss-cross inheritance. Study them together for NEET.
