Question

1. Examine each pedigree shown (a-c). The shaded square/circle shows an inherited phenotype. For each (a, b, c), write a sentence or two that indicates whether it is (or is not) consistent with X-linked recessive inheritance. If you decide the pedigree IS consistent, for full credit you must prove this by setting up a Punnett square or a forked line diagram. (6 pts) 이 이 SSG God

Answer 1

(a) Here the phenotype is consistent with X-linked recessive inheritance.

For the purpose of understanding let us consider the recessive gene be represented as Xr.

Here as the female offspring is homozygous for the recessive trait in generation 2, her phenotype would be XrXr, while that of her affected father would be XrY. As the offspring is homozygous for the recessive trait, the mother would be a carrier for the recessive trait. Therefore her phenotype would be XrX.

The following diagram explains the inheritance:

Phenotype of parents   XrY x XrX

↓

Phenotype of offsprings XrXr XrX XY

(b) Here the phenotype is consistent with X-linked recessive inheritance.

Here as the male offspring is affected in generation 2, so his phenotype would be XrY. This indicates that he received one recessive gene each from one of his parents. As his father is not affected, the father's phenotype would be XY. The mother would be a carrier and her phenotype would be XrX.

The following diagram explains the inheritance:

Phenotype of parents XY x XrX

↓

Phenotype of offsprings XrY XrX XY

(c) Here the phenotype is not consistent with X-linked recessive inheritance.

As the female in generation 1 is homozygous for the recessive trait (XrXr), both the male offsprings in generation 2 should be affected. Here only one of the male offspring is affected. Therefore the phenotype is not consistent with X-linked recessive inheritance.