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A couple has two sons and two daughters. Only one son is colourblind and the rest of the siblings are normal. Assuming colourblindness is sex-linked, which one of the following would be the phenotype of the parents?

(a) Mother would be colourblind, father would be normal

(b) Father would be colourblind, mother would be normal

(c) Both the parents would be normal

(d) Both the parents would be colourblind

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Answer is : (c) Both the parents would be normal

Colourblindness is a X-linked recessive disease, i.e. an heterozygous mother does not show the disease and is a carrier. But a father cannot be a carrier of the disease as it has single X-chromosome.

In the given question, the son is colourblind which means it had inherited Xc from the mother. But another son is normal. This shows that the mother is heterozygous for the disease.

The expected cross for the question will be

This shows that both the parents would be normal if they have one colourblind son and one normal son.

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