Question

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.

Answer 1

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.

| $X$| $Y$
---|---|---
${X^c}$| $X$ ${X^c}$| ${X^c}$ $Y$
$X$| $X$ $X$| $X$ $Y$

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.