These studies compare monozygotic and dizygotic twins for medical, genetic, or psychological characteristics to try to isolate genetic influence from epigenetic and environmental influence. Twins that have been separated early in life and raised in separate households are especially sought-after for these studies, which have been used widely in the exploration of human nature. Classical twin studies are now being supplemented with molecular genetic studies which identify individual genes. Unusual twinnings
Bi-paternal twins
This phenomenon is known as heteropaternal superfecundation. One 1992 study estimates that the frequency of heteropaternal superfecundation among dizygotic twins, whose parents were involved in paternity suits, was approximately 2.4%; see the references section, below, for more details.Dizygotic twins from biracial couples can sometimes be mixed twins, which exhibit differing ethnic and racial features. One such pairing was born in London in 1993 to a white mother and Caribbean father. Monozygotic twins of different sexes
Among monozygotic twins, in extremely rare cases, twins have been born with different sexes (one male, one female). 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).
Retrieved 2008-10-02. "TWINS Guide to the First Year" (PDF). TWINS Magazine. Fort Collins, Colorado. 2008. Archived from the original (PDF) on 2008-10-30. Retrieved 2008-10-06. Samson, Jennifer. "Facts About Multiples: An Encyclopedia of Multiple Birth Records". Archived from the original on 2009-10-15. Retrieved 2008-10-18. Taylor, Ally. "Twin Zygosity Test for Dichorionic Diamniotic (Di/Di) Twins! Zygosity Reveal". Retrieved 2018-03-30. Am J Med Genet C Semin Med Genet. 2009 May 15;151C(2):136-41. Not really identical: epigenetic differences in monozygotic twins and implications for twin studies in psychiatry. Haque FN, Gottesman II, Wong AH. Twin brothers promoted as Majors General together. Seneviratne brothers, who are twins and joined the Army on a same day, were promoted to the rank of Major General, again on the same day. == External links ==