Your grandmother's eyes
People say you have your grandmother's eyes — that's genetics at work, instructions copied from parents to child. Some instructions win out over others, which is why two brown-eyed parents can still have a blue-eyed baby.
Alleles: two copies of each gene
You inherit one allele of each gene from each parent. A dominant allele (written with a capital, A) masks a recessive one (lowercase, a). So AA and Aa both show the dominant trait; only aa shows the recessive trait.
The Punnett square
A Punnett square crosses the two parents' alleles to show every equally-likely combination for their offspring.
Simulation · Punnett square
AA
aA
aA
aa
Dominant trait: 3/4 · recessive: 1/4
Choose each parent's genotype and read the offspring ratio. Try Aa × Aa — the classic 3:1 dominant-to-recessive split.