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A cross of Aa with Aa

A dominant allele, written A, masks a recessive allele a. A person who is Aa shows the dominant trait, but can pass on both alleles. When two such people have a child, you make a Punnett square with A and a from each. The four cells are equally likely. Three of them have at least one A and show the dominant trait. Only the cell aa shows the recessive trait. That is why a recessive trait can skip a generation and appear again in the children of two carriers.

Try it yourself

Punnett square

\mathrm{Aa} \times \mathrm{Aa}

both are carriers

Symbols

$A$	dominant allele
$a$	recessive allele

Example

The cells are AA, Aa, aA and aa. The ratio is $1 + 2 + 1 = 4$ equally likely outcomes.

Probability of aa: $1/4 \cdot 100 = 25$ %.

Aa and aA are the same genotype. Do not count them as two different traits.

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Part of Biology 1: Heredity and genetics.