Why do recessive traits from older generations suddenly resurface in one individual?

— Deva Narayanan

A: Genes work in pairs. Every person inherits two copies of most genes, one from each parent. Some gene variants, called dominant alleles, express themselves even when only one copy is present. Others, called recessive alleles, express themselves when a person inherits both copies.

Someone with one recessive allele alongside a dominant one is called a carrier: they will show no sign of the trait. So a family may carry the allele for a particular eye colour, yet a single person won’t display it if in every generation, each carrier paired with someone who had the dominant allele.

But when two carriers have a child together, each parent may pass on their recessive allele, and if both do, the child inherits two copies. Thus, the trait will finally express itself.

Chance has a starring role. When eggs and sperm form, genes get shuffled and distributed at random, creating a vast range of possible combinations, which is why even full siblings can inherit different traits from the same parents. This also determines whether a given child of two carriers receives the recessive allele from both, one or neither parent.

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