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A colour blind child was born to a normal couple. Work out a cross to show how it is
possible. Mention the sex of the child.

Last updated date: 14th Jun 2024
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Hint:-Colour blindness is a sex-linked trait. It is associated with the $X$ chromosome.
$X$ linked traits are passed on from the mother to the son.
$X$ linked traits are more common in males than in females.

Complete Answer:-
Colour blindness is a disease in which a person cannot distinguish between different colours. The gene for colour blindness is carried by the $X$ chromosome.
The presence of the trait is indicated as ${X^c}$.
We consider a couple in which the male partner is completely normal (not having colour blindness). His genotype will be written as $X$$Y$.
Now, the female partner is a carrier of colour blindness (${X^c}$ $X$). This means that she is not colour blind but carries the gene on one of her chromosomes. The blindness does not present in her because the extra X chromosome compensates for the anomaly. If this defective gene is present in a male there is no extra X chromosome for compensation. That is why the presence of the abnormal gene even only on one of the sex chromosomes presents the disease in males.


The progenies with the genotyping phenotype are as follows:
$X$${X^c}$ - carrier female
${X^c}$ $Y$ - colour blind Male
$X$$X$- normal female
$X$$Y$ - normal Male
Thus, we see that even when both parents are normal one of the children is colour blind. The sex of the colour-blind child is male.

Note:- Haemophilia is another example of an X linked trait which is more common in males than in females. A daughter can be colour blind only if her mother is a carrier or a colour blind and her father is also colour blind. Genetic counselling can be done to prevent a child with such anomalies.