Answer the following questions for autosomal conditions such as PKU.

If both parents are heterozygous carriers of a mutant allele, what is the chance that their first child will be homozygous recessive for the 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 mutant allele, one inherited from each parent. In the case of conditions like Phenylketonuria (PKU), both parents must be carriers (heterozygous) for their child to potentially inherit the homozygous recessive genotype, which leads to the expression of the disorder.

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 two heterozygous parents (Aa x Aa), the Punnett square reveals the probabilities of offspring being homozygous dominant, heterozygous, or homozygous recessive.

Genotype Probability

Genotype probability refers to the likelihood of an offspring inheriting a specific combination of alleles from its parents. In the case of two heterozygous parents, the probability of their child being homozygous recessive (aa) is 25%, as derived from the Punnett square analysis. Understanding these probabilities is crucial for predicting the inheritance of genetic conditions.