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NCERT Exemplar · Q41

Q.Why is the frequency of red-green colour blindness many times higher in males than that in the females?

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Red-green colour blindness is X-linked recessive, so males need only one defective gene copy to be affected, while females need two — making the condition far more common in males.

Red-green colour blindness is one of the clearest examples of sex-linked inheritance in humans, and its pattern of occurrence beautifully illustrates why the X and Y chromosomes matter so much in determining who gets certain genetic conditions.

The genes responsible for red and green colour vision sit on the X chromosome. When a mutation occurs in these genes, the person loses the ability to distinguish between red and green hues. The crucial point is that this trait is recessive — meaning the normal, functional version of the gene masks the defective one if both are present.

Now consider the chromosomal difference between males and females. Males carry one X and one Y chromosome (XY), while females carry two X chromosomes (XX). This seemingly simple difference has profound consequences for colour blindness.

A male needs only a single copy of the defective gene to be colour blind. Because he has just one X chromosome, if that X carries the mutant gene, there is no second X to provide a normal copy that could compensate. The Y chromosome does not carry genes for colour vision at all, so it offers no backup. The male is therefore colour blind.

A female, on the other hand, has two X chromosomes. If one carries the defective gene, the other X usually carries a normal copy. Since the trait is recessive, that one normal copy is enough to give her normal colour vision. She becomes a carrier — genetically carrying the condition but not expressing it. Only when a female inherits the defective gene on both her X chromosomes does she become colour blind, and this is statistically much rarer.

Note

A carrier female can pass the defective gene to her children. If she has a son, he has a 50% chance of inheriting the affected X and being colour blind. If she has a daughter, that daughter has a 50% chance of being a carrier herself. …

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