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A cross of Aa with aa

If one parent is Aa and the other is aa, only the first has a dominant allele to pass on. Half of the gametes from Aa carry A, half carry a. All the gametes from aa carry a. The children therefore become Aa or aa, in the ratio one to one. Those who are Aa show the dominant trait. Those who are aa show the recessive trait. This square is often used when you want to know whether an individual with a dominant appearance is a carrier, by crossing it with a recessive individual.

Try it yourself

Punnett square

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

one carrier and one recessive

Symbols

$Aa$	shows the dominant trait
$aa$	shows the recessive trait

Example

The offspring are Aa or aa, equally often.

Share with the dominant trait: $1/2 \cdot 100 = 50$ %.

A child with the recessive trait must have received a from both. Then the parent with the dominant appearance is a carrier.

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Part of Biology 1: Heredity and genetics.