Question

12. Kevin is very short due to a condition known as de novo X-linked achondroplasia, which was caused by a denovo germline mutation in an X-linked gene that occurred in his mother. Do we expect that has mother has achondroplasia? Why? (2 points) Do we expect that Kevin's son would have achondroplasia? Why? (2 points)

Answer 1

Let us indicate Kevin's genotype as XaY, where Xa is the ressesive allele for the X-linked mutation for condition achondroplasia.

The allele is expressed in Kevin because the Y allele does not mask the effect of Xa allele here.

Let us draw the inheritance pattern of Kevin's genotype from his parents.

Genotype of Kevin's parents: XaX x  XY

Gametes: Xa X X Y

Genotype of Kevin: XaY

(OR)

Genotype of Kevin's parents: XaXa x  XY

Gametes: Xa Xa X Y

Genotype of Kevin: XaY

From the above pattern we can infer that Kevin inherited the recessive allele from his mother. For Kevin's mother to have the condition achondroplasia, she must have both the alleles recessive that is she must inherit one recessive allele each from her parents; which is rare. So we do not expect that Kevin's mother has achondroplasia. However she is a carrier of the X-linked mutation for achondroplasia.

As the mutation is X-linked, Kevin cannot pass the condition to his son. We do not expect Kevin's son to have achondroplasia because Kevin cannot pass the condition to his son. For Kevin's son to have the condition, Kevin's wife has to be a carrier of the X-linked mutation.