Question

1. If a child has PKU, what is its genotype? What are the genotypes of its parents (use A for the dominant allele and a for the recessive allele)?

2. Is it possible to count the number of PKU carriers in a population? Why or why not?

3. If 50 of the 500,000 people in Atlanta have PKU, how many people are carriers?

4. In Atlanta, if a woman with PKU marries a man without symptoms of PKU, what is the chance that they will have a child with PKU?

5. If two people without symptoms in Atlanta have a child, what is the chance that the child will have PKU?

Answer 1

1) Phenyl ketonuria is a homozygous recessive disease, for a child to be affected with the disease, he or she has to be homozygous recessive for the allele.

A- dominant

a- recessive

so the genotype of the affected child is aa.

the possible genotypes of the parents are Aa, Aa and Aa, aa

( aa individuals if recognized earlier can be raised with out developing the symptoms pf phenylketonuria by avoiding foods with tryptophan)

Aa * Aa

	A	a
A	AA ( normal)	Aa( normal)
a	Aa( normal)	aa ( affected)

Aa * aa

	A	a
a	Aa( normal)	aa ( affected)

2) No, phenylketonuria is an autosomal recessive trait, individuals who are heterozygous ( carriers ) for the trait are not affected, they are phenotypically normal so we can not distinguish between heterozygotes and homozygous dominant individuals, so we cannot  count the number of PKU carriers in a population.