Editing Twin (section)

===Monozygotic twins of different sexes===
Among monozygotic twins, in extremely rare cases, twins have been born with different sexes (one male, one female).<ref>{{Cite journal|first=R |last=Schmidt |author2=EH Sobel |author3=HM Nitowsky |author4=H Dar |author5=FH Allen Jr |title=Monozygotic twins discordant for sex |journal=Journal of Medical Genetics |volume=13 |issue=1 |pages=64–68 |date=February 1976 |pmid=944787 |pmc=1013354 |doi=10.1136/jmg.13.1.64}}</ref> When monozygotic twins are born with different sexes it is because of chromosomal defects. The probability of this is so small that multiples having different sexes is universally accepted as a sound basis for ''in utero'' clinical determination that the multiples are not monozygotic.

Another abnormality that can result in monozygotic twins of different sexes is if the egg is fertilized by a male sperm but during cell division only the X chromosome is duplicated. This results in one normal male (XY) and one female with [[Turner syndrome]] (45,X).<ref>{{cite web |title=The Zygosity Puzzle |url=https://www.twinsmagazine.com/images/zygosityMA03.pdf |access-date=2015-04-18 |url-status=dead |archive-url=https://web.archive.org/web/20160304081204/https://www.twinsmagazine.com/images/zygosityMA03.pdf |archive-date=2016-03-04}}</ref> In these cases, although the twins did form from the same fertilized egg, it is incorrect to refer to them as genetically identical, since they have different [[karyotype]]s.