Two ways twins form
A woman usually releases one ovum per cycle, so a single baby is by far the most common outcome of a pregnancy. Occasionally, however, two babies develop together and are born as twins. Twins arise in two quite different ways, and which way they form decides whether they are genetically the same or merely brothers and sisters who happen to share a birthday.
Identical (monozygotic) twins
Identical twins form from one ovum fertilised by one sperm. The single zygote divides by mitosis and then splits into two separate embryos early in development. Because both embryos come from the same zygote, they have exactly the same genes, are always the same sex, and look very alike. They often share one placenta. If the zygote splits only partly, the twins may remain joined; these are known as conjoined twins.
Fraternal (dizygotic) twins
Fraternal twins form when two ova are released and each is fertilised by a different sperm. This gives two separate zygotes with different combinations of genes. Fraternal twins are no more alike than ordinary brothers and sisters; they may be the same or different sexes and usually have separate placentas.
| Feature | Identical | Fraternal |
|---|---|---|
| Number of ova | One | Two |
| Genes | Same | Different |
| Sex | Always same | Same or different |
Example
A boy and a girl born together must be fraternal twins, because identical twins share all their genes and are therefore always the same sex.
Remember
Mono = one zygote splits = identical. Di = two zygotes = fraternal. Identical twins share all genes; fraternal twins share about half, like normal siblings.