Tom's brother suffers from phenylketonuria (PKU), a recessive disorder. Tom and the brothers' parents do not have PKU. What are the chances that Tom is a carrier of the recessive PKU allele?

a. 1/4

b. 1/3

c. 1/2

d. 2/3

e. 4/3

1/2.

Explanation:

Autosomal recessive disorder may be defined the disorder in which the trait or disease will only be express in the homozygous recessive conditions. The heterozygous condition represent the carrier of the trait.

PKU trait will only be express in the recessive condition only. Both the parents must be carrier of the trait as the tom's brother suffers from PKU. Their cross can be shown as follows:

Parents Aa * Aa.

Progeny AA (normal trait), Aa and Aa (carrier), aa (expression of trait).

There are 50% or 1/2 chances for the Tom's to be the carrier of the trait.

Thus, the correct answer is option (c).