Answer the following questions for autosomal conditions such as PKU.

Parents who are each heterozygous carriers for a recessive mutant allele have a child who does not have the condition. What is the chance this child is a heterozygous carrier of the condition?

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. In the case of PKU, both parents must carry the recessive allele for their child to potentially inherit the condition. If a child inherits one normal allele and one recessive allele, they will be a carrier but will not express the condition.

Punnett Square

A Punnett square is a diagram used to predict the genetic makeup of offspring from two parents. By arranging the alleles of each parent, it allows for the visualization of possible genotypes in their children. For heterozygous parents (Aa x Aa), the Punnett square shows the probabilities of offspring being homozygous dominant, heterozygous, or homozygous recessive.

Carrier Probability

Carrier probability refers to the likelihood that an individual possesses one copy of a recessive allele without expressing the associated condition. In the scenario where both parents are heterozygous carriers, there is a 50% chance that their child will be a carrier (heterozygous) for the condition, as indicated by the Punnett square outcomes.