 Recessive inheritance

In a fully penetrant autosomal recessive model with normal allele $A$ and disease allele $a$, $aa$ is affected and $AA,Aa$ are unaffected. A carrier cross $Aa\times Aa$ has offspring probabilities $1/4,1/2,1/4$ for $AA,Aa,aa$, respectively, from the two independent parental meioses.