What is Identical twins?
'''Identical twins''' start out as [[Genetics|genetically]] identical: they have the same [[allele]]s. They both share the same sack in the [[Uterus|womb]]. They are always of the same [[sex]], and are '''monozygotic''' or MZ twins (mono = one; [[zygote]] = fertilised egg). This contrasts with ''fraternal twins'', who are formed by two separate [[Ovum|eggs]] fertilised by two separate [[sperm]]s, and who are not always the same sex (DZ = dizygotic). Both types of [[twin]] are carried in the same [[uterus]] at the same time, so their [[birth]] [[environment]] is the same. Research shows that the frequency of monozygotic twinning is one in 240 births. Fraternal twins are twice as common.<ref>King R.C. Stansfield W.D. & Mulligan P.K. 2006. ''A dictionary of genetics'', 7th ed. Oxford. p461</ref> [[In vitro fertilization]] (IVF) techniques are more likely to create dizygotic twins. For IVF deliveries, there are nearly 21 pairs of twins for every 1,000.<ref>{{cite press release |title=Time-lapse recordings reveal why IVF embryos are more likely to develop into twins. Researchers believe the laboratory culture could be the cause |publisher=European Society of Human Reproduction and Embryology |date=2007 |url=http://www.eshre.com/emc.asp?pageId=939 |accessdate=2008-09-30 |archivedate=2007-09-21 |archiveurl=https://web.archive.org/web/20070921094734/http://www.eshre.com/emc.asp?pageId=939 }}</ref> == Twin research == Identical twins are natural [[clone]]s. Because they start out with the same genes, they can be used to investigate how much [[heredity]] contributes to individual people. This is the [[nature vs nurture]] question. Studies with twins have been quite interesting. If we make a list of characteristic [[Trait (biology)|traits]], we find that they vary in how much they owe to heredity. For example: *[[Eye]] colour: entirely inherited. *[[Weight]], [[height]]: partly inherited, partly environmental. *Which [[language]] you speak: entirely environmental. The way the studies are done is like this.<ref>Advanced