Identical twins
Identical twins start out as genetically identical: they have the same alleles. They both share the same sack in the womb. They are always of the same sex, and are monozygotic or MZ twins (mono = one; zygote = fertilised egg). This contrasts with fraternal twins, who are formed by two separate eggs fertilised by two separate sperms, and who are not always the same sex (DZ = dizygotic). Both types of twin are carried in the same uterus at the same time, so their birth environment is the same.