Question

Phenylketonuria (PKU) is a heritable condition in humans involving inability to break down the amino acid phenylalanine because of lack of a certain enzyme. If not diagnosed and treated very shortly after birth, PKU’s develop severe mental disabilities and usually do not reproduce. Almost all PKU children, therefore, are born to parents who are not PKU’s.

A). Is the gene responsible for PKU dominant or recessive? Explain how you know.

B) The normal (non-PKU) brother of a PKU seeks the advice of a genetic counselor before a contemplated marriage.

a.Neither of his parents have PKU. What are the genotypes of his parents? (use any letter you choose)

b. What is the chance that he is a carrier for PKU (heterozygous)? Explain how you know. Use a Punnett Square if possible.

Answer 1

A) Answer: it is due to a recessive allele because PKU offsprings are produced from non PKU's it means skipping of generation is observed. And non-PKU carries a recessive allele for PKU.

B) The normal (non-PKU) brother of a PKU seeks the advice of a genetic counsellor before a contemplated marriage.

Because his parents (both father and mother are heterozygous for the PKU, means they are carrier for PKU)

Let take the non-PKU allele = 'P' and PKU allele = 'p'

So the genotype of parents are Pp (they are normal but carrier for PKU)

now draw a Punnett square for possibilities or chances or probability of being heterozygous

	P	p
P	PP (normal not carrying PKU)	Pp (heterozygous - carrier)
p	Pp (heterozygous - carrier)	pp (affected by PKU)

so the probability of being heterozygous is 50%