Question

A couple with normal vision bear a colour blind child. Work out a cross to show how it is possible and mention the sex of the affected child.

Answer 1

Hint: The X-linked recessive inheritance (colour blindness) refers to genetic abnormalities resulting from mutations in the X-chromosome genes. In recessive inheritance, the two corresponding genes have to be mutated to cause disease. Perhaps if one gene in the pair is defective, there seems to be no or slight disease.

Complete answer:
According to the question, it is given that the couple is having normal vision whereas the child is colour blind. The possibility can be in such a way that the male parent is normal and the female parent is the carrier for the colour blindness since colour blindness is an X- linked recessive inheritance. When this case is depicted using a cross, the below results are possible. Out of four offsprings, there is a carrier daughter, affected son, normal daughter, and normal son. Hence, the sex of the child affected by colour blindness will be a male.

Additional information:
In individuals, the development of X-linked recessive traits follows a specific series of three phases. The first is that affected fathers never pass on X-linked recessive features to their sons because the fathers offer Y chromosomes to their sons. Second, X-linked recessive traits will be more frequently displayed in males than in females. This is based on the fact that males only have one X chromosome and hence one X mutated to be affected. The very last trait seen would be that the X-linked recessive traits seem to skip generations, indicating that the affected grandfather does not have an affected son.

Note: X-linked inheritance states that the gene that induces the trait or condition is located on chromosome X. Females have two X chromosomes; males have one X and one Y chromosomes. Genetic traits on the X chromosome could be recessive or dominant. Their presence in men and women is just not the same. Genes on a Y chromosome do not suit the genes on the X chromosome. In females, X-linked recessive genes are expressed only when there are two variants of the gene.