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?

Verified step by step guidance

1. Identify the inheritance pattern: Since the condition is autosomal recessive, the disease manifests only when an individual has two copies of the mutant allele (homozygous recessive).
2. Define the genotypes of the parents: Both parents are heterozygous carriers, meaning their genotype is $Aa$, where $A$ is the normal allele and $a$ is the mutant allele.
3. Set up a Punnett square to determine the possible genotypes of their offspring by combining the alleles from each parent: the possible combinations are $AA$, $Aa$, $aA$, and $aa$.
4. Calculate the probability of the child being homozygous recessive ($aa$): count the number of $aa$ outcomes over the total possible outcomes in the Punnett square.
5. Express the probability as a fraction or percentage to represent the chance that the first child will be homozygous recessive for the mutation.

Key Concepts

Autosomal Recessive Inheritance

Autosomal recessive inheritance occurs when a trait or disorder is expressed only if an individual inherits two copies of a mutant allele, one from each parent. Carriers have one normal and one mutant allele but typically do not show symptoms. Diseases like PKU follow this pattern.

Genotype Probability in Offspring

When both parents are heterozygous carriers (Aa), the possible genotypes for their child are AA, Aa, or aa. Using a Punnett square, the probability of the child being homozygous recessive (aa) is 25%, as each allele is inherited independently.

Phenotypic Expression of Homozygous Recessive

Individuals who are homozygous recessive (aa) for a mutation typically express the autosomal recessive condition, such as PKU. This means the disease phenotype manifests only when both alleles are mutant, highlighting the importance of genotype in predicting disease risk.