Twins

A twin is one of two offspring produced in the same pregnancy.[1] Twins can either be monozygotic (in common parlance, "identical"), meaning that they develop from one zygote that splits and forms two embryos, or dizygotic ("fraternal") because they develop from two separate eggs that are fertilized by two separate sperm.
In contrast, a fetus which develops alone in the womb is called a singleton, and the general term for one offspring of a multiple birth is multiple. It is theoretically possible for two singletons to be identical if all 23 chromosomes in both gametes from the mother and father were to be exact matches from one birth to the next. While this is statistically improbable (less than one in one billion-billion-billion chance) under natural conditions, a controlled pairing may someday be possible.
Fraternal/Sororal (dizygotic) twins
Eight-month-old sororal twin girls nappingFraternal or dizygotic (DZ) twins (also referred to as "non-identical twins", "dissimilar twins", "biovular twins", and, in cases of females, sororal twins) usually occur when two fertilized eggs are implanted in the uterus wall at the same time. When two eggs are independently fertilized by two different sperm cells, fraternal twins result. The two eggs, or ova, form two zygotes, hence the terms dizygotic and biovular.
Dizygotic twins, like any other siblings, have an extremely small chance of having the same chromosome profile. Like any other siblings, dizygotic twins may look similar, particularly given that they are the same age. However, dizygotic twins may also look very different from each other. They may be of different sexes or the same sex. The same holds true for brothers and sisters from the same parents, meaning that fraternal and/or sororal twins are simply siblings who happen to be the same age.
Monozygotic ("identical") twins
Comparison of zygote development in monozygotic and dizygotic twins. In the uterus, a majority of monozygotic twins (60–70%) share the...