Identical twins are genetically identical: they have the same genes. They are formed by a fertilised egg dividing into two separate individuals, and are always of the same sex. They may be called monozygotic or MZ twins (mono = one; zygote = fertilised egg). They contrast 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.