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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.
Identical twins have been known to behave differently according to psychologists. They have their own individual personalities to enable themselves to be identified as one person. Many identical twins share all their time together so people assume that they behave alike just as they look alike however, this is not necessarily the case. Twins establish their own individual likes and dislikes and their behavior in many ways is different apart from their sibling. Not to say they are totally different but there are some obvious signs of differences if you carefully observe two identical twins at the same time.
Famous twins include:
See also: multiple births.