Identical twin

Identical twins are twins formed when a zygote splits in two and develops into two individuals. They look alike, except that they are sometimes mirror images, but examination of details such as fingerprints can tell them apart. As they mature, even identical twins often become less alike, because of everything from scars to choices of hairstyle.

If the two individuals do not separate completely by birth, they are known as Siamese twins or conjoined twins.

Depending on the chromosome number in a species, one can have identical twins without a zygote splitting. Humans have n=23, and 2⁴⁶ is much bigger than the number of children anyone has had, so it doesn't happen; but for fruit flies, whose n=4, it happens all the time.

Famous twins include:

Mary-Kate and Ashley Olsen

Tia and Tamara Mowry

Mike and Jake 'Twin' Sullivan

Ann Landers and Dear Abby

Dick and Tom Van Arnsdale

See also: multiple births.