Why Twins Don't Always Look Identical

Same parents, sometimes same DNA — so why don't twins and siblings look like copies of each other? Genetics has a fun answer.

Twins are a perfect natural experiment for resemblance. Why do 'identical' twins still have tell-apart differences, and why can fraternal twins look no more alike than any siblings? Here's the breakdown.

Identical twins: same DNA, different lives

Identical (monozygotic) twins start from one fertilized egg, so they share essentially the same DNA. Yet they're rarely *perfectly* identical. Small differences accumulate from the womb onward: positioning, blood supply, and later environment and habits — the nurture side of the equation. Different expressions, sun exposure, and weight changes gradually make even matched DNA look subtly distinct.

Fraternal twins: just siblings, same birthday

Fraternal (dizygotic) twins come from two separate eggs, so genetically they're ordinary siblings who happen to share a birthday. They can look quite different — different hair, eyes, build — for the same reason any two siblings can.

Why ordinary siblings vary so much

Each child inherits a random half of each parent's genes, reshuffled fresh every time. Two siblings can draw very different hands from the same deck — one favoring mom, one favoring a grandparent — which is the same mechanism behind why a baby favors one parent.