Answer the following questions for autosomal conditions such as PKU.

If the first child of parents who are both heterozygous carriers of a recessive mutant allele is homozygous recessive, what is the chance the second child of the couple will be homozygous recessive? What is the chance the second child will be a heterozygous carrier of the recessive mutation?

Key Concepts

Autosomal Recessive Inheritance

Autosomal recessive inheritance occurs when a trait or condition is expressed only when an individual has two copies of a recessive allele, one inherited from each parent. In this case, both parents are heterozygous carriers, meaning they each have one dominant and one recessive allele. For a child to be homozygous recessive, they must inherit the recessive allele from both parents.

Punnett Square

A Punnett square is a diagram used to predict the genetic outcomes of a cross between two individuals. It helps visualize the possible combinations of alleles from the parents. In this scenario, a 2x2 Punnett square can be used to determine the probabilities of the offspring being homozygous recessive, heterozygous carriers, or homozygous dominant based on the parents' genotypes.

Probability of Genetic Outcomes

The probability of genetic outcomes refers to the likelihood of inheriting specific genotypes from parents. In the case of two heterozygous parents, the chance of having a homozygous recessive child is 25%, while the chance of having a heterozygous carrier is 50%. These probabilities remain constant for each child, independent of the outcomes of previous births.