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.

If a mother's egg splits before being fertilized, it can result in semi-identical twins, who would share 75% of their genetic material. (Identical twins share 100% of genetic material, while fraternal twins and other ordinary siblings share 50%.)

Depending on the chromosome number in a species, one can have genetically identical offspring 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.

Identical supertwins are also possible; the largest number of identical children born is five, which has happened only one time in history with all the children surviving. (Identical quintuples have possibly occurred once or twice more in medical history in centuries past, but these children did not survive.) These were the Dionne Quintuplets, born in May 1934 in Ontario, Canada.

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.