Tom's brother suffers from phenylketonuria (PKU), a recessive disorder. Tom and the brother's 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. 4/3
• E. 2/3

Answer: (E)

Verified step by step guidance

1. Understand that phenylketonuria (PKU) is a recessive genetic disorder, meaning that an individual must inherit two copies of the recessive allele to express the disorder.
2. Since Tom's brother has PKU, he must have inherited two recessive alleles (one from each parent). This means both parents are carriers of the PKU allele.
3. Tom's parents do not have PKU, indicating they are heterozygous carriers (one dominant allele and one recessive allele). Represent this as 'Pp' where 'P' is the dominant allele and 'p' is the recessive allele.
4. To determine Tom's genotype, consider the possible combinations of alleles he could inherit from his parents. The possible combinations are: PP, Pp, and pp. Since Tom does not have PKU, he cannot be 'pp'.
5. Calculate the probability that Tom is a carrier (Pp). From the Punnett square, the possible genotypes for Tom are PP, Pp, and Pp. Therefore, the probability that Tom is a carrier (Pp) is 2 out of 3, or \( \frac{2}{3} \).