Identical twins develop from a single fertilised egg that splits, so they share essentially the same genetic material and are always the same sex. Fraternal twins develop from two separate eggs fertilised at the same time — genetically they are ordinary siblings who happen to share a pregnancy, and they may be different sexes.
The core difference
- identical (monozygotic) — one egg, one sperm, splitting early. Nearly the same genome; always the same sex.
- fraternal (dizygotic) — two eggs, two sperm. Genetically ordinary siblings; may be different sexes.
What the words literally say
Fraternal comes from Latin frater, brother — which is why it strikes people as odd when applied to two sisters. It is used for all non-identical twins regardless of sex; the technical term dizygotic ("two zygotes") avoids the problem entirely.
A common misconception
Identical twins are not always indistinguishable. Environment, position in the womb and later development produce real differences, and even fingerprints differ. "Identical" refers to the origin, not to the outcome.
Related terms
Monozygotic and dizygotic are the precise terms and are preferred in medical writing. Mirror twins describes a subset of identical twins whose features appear reversed.
Frequently asked questions
Can fraternal twins be different sexes?
Yes. They come from two separate eggs, so they can be any combination of sexes — unlike identical twins, who are always the same sex.
Why are they called fraternal if they are sisters?
Because the word comes from Latin frater, brother, and was applied broadly. The technical term dizygotic avoids the mismatch.
Do identical twins have identical fingerprints?
No. They share essentially the same genome, but fingerprints form under conditions in the womb and differ between them.