Recessive conditions clinically manifest only when an individual has two copies of the mutant allele. When just one copy of the mutant allele is present, an individual is a carrier of the mutation but does not develop the condition. When two carriers mate, each child has a 25% chance of being homozygous wild-type (unaffected); a 25% chance of being a homozygous mutant (affected); or a 50% chance of being heterozygous (unaffected carrier). In the given picture, the genotype of II (3) is Aa (heterozygous and unaffected).
