Show steps by steps complete solutions and punnet square 0. Phenylketonuria is c

Question

Show steps by steps complete solutions and punnet square 0. Phenylketonuria is caused by a mutation in the gene encoding the enzyme phenylalanine hydroxylase (PAH). The mutation results in the accumulation of phenylalanine which, in turn, causes damage to the developing nervous system and is manifested in severe mental retardation. If a person is homozygous for the mutated form of the PAH gene, phenylketonuria develops, but if the person is heterozygous for PAH (one allele normal, the other mutated) he or she will not develop the disease. Sophie and her husband are both heterozygous for the PAH gene. Sophie is pregnant with twins. What is the probability that both twins will develop phenylketonuria if they are fraternal (non- identical) twins? What is the probability that they both develop phenylketonuria if they are identical twins?

Explanation / Answer

According to the information, both the mother and father are heterozygous i.e. Pp for the alleles. Thus, the Punnett square can be formulated as below:

Genotype: PP: Pp: pp :: 1:2:1

Phenotype: Normal : Affected :: 3:1

Here, it is important to note that the fraternal or identical twins follow the same order of genotype and phenotype as the single individuals. This is because the presence of twins does not affect the pattern of inheritance.

Thus, the probability that the twins will carry the disease in both cases would be 25%.

P p P PP Pp p Pp pp

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