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How Triplets or Higher Order Multiples are formed
Identical triplets occur is almost the same way as identical twins, but instead of the egg dividing once, one of those halves divides again to create three genetically identical babies. In the rare cases of identical quads, both halves would divide to form 4 identical babies. The Dionne quints of Canada were identical quintuplets, the only set in the world.
Fraternal triplets would occur if three eggs were released by the mother and each fertilized by a different sperm. The same would occur for fraternal quads if 4 eggs were released and fertilized, as well as 5 being released for quints. The chances of having spontaneous (not from IVF or fertility drugs) quads and quints isn't as common, spontaneous quints are quite rare, fraternal or otherwise.
Mixed higher order multiples can often be a combination of identical and fraternal. The most common type of spontaneously conceived triplets are "a pair and a spare", two identical and one fraternal.Two eggs would be released by the mother and fertilized, and only one would split to form identical twins. This can occur with quadruplets, you could have 2 identical and 2 fraternal (three eggs released, one splits, this seems to be the most common after all fraternal), 2 sets of identical twins (2 eggs released, both split), or 3 identical and 1 fraternal (2 eggs released, one splits once, and then again). The same can happen with quintuplets, you could have 2 identical, 3 fraternal (which seems to be the most common after all fraternal), or any combination of identical and fraternal, although it is very rare to see more than 2 identical in a set of quints. Combinations for sextuplets and higher would be similar, although it is highly unlikely to see identicals in sextuplets and higher since almost all of them come from fertility drugs. (Although there was one case where the sextuplets were 3 identical and 3 fraternal).
Boy/Girl Identicals?
There has been very rare cases (about 3 I have found), of opposite sex identical twins because of genetic abnormality. In these rare cases, identical boys would be conceived and one twin, during the twinning process, loses a Y chromosome (boys have chromosome type XY while girls have XX), creating a girl by default, but a girl with only one X chromosome (chromosome type XO). The girl will suffer from Turner's Syndrome, which is distinguishable by their short stature, folds of skin at the neck, abnormal development of secondary sexual characteristics, an intellectual deficit known as space-form blindness. The male co-twin would be unaffected. Turner's syndrome can occur in any birth, even that of fraternal female twins, or identical female twins. It also occurs in single births, so an individual with Turner's syndrome is not automatically a twin. Turner's Syndrome occurs in about 1 in 10,000 of all births.
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