Identical twins start out as genetically identical: they have the same alleles. They are formed by a fertilised egg dividing into two separate individuals. 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 eggs fertilised by two separate sperms, 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. 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.
Identical twins are natural clones. 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 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. Take a group of identical twins and a group of fraternal twins, and a group of siblings from the population. Measure them for various traits. Do a statistical analysis (such as analysis of variance). This tells you to what extent the trait is inherited. You will find that all those traits which are partly inherited will be significantly more similar in identical twins.
Studies like this may be carried further, by comparing identical twins brought up together with identical twins brought up in different circumstances. That gives a handle on how much circumstances can alter the outcomes of genetically identical people.
The person who first did twin studies was Francis Galton, Darwin's half-cousin, who was a founder of statistics. His method was to trace twins through their life-history, making many kinds of measurement. Unfortunately, though he knew about mono and dizygotic twins, he did not appreciate the real genetic difference. Twin studies of the modern kind did not appear until the 1920s.
Wilhelm Weinberg made the first estimate of the rate of twinning. Realizing that identical twins would have to be the same sex, while non-identical twins could be either same or opposite sex, Weinberg derived a formula for estimating the frequency of MZ and DZ twins from the ratio of same and opposite sex twins to the total of maternities. Weinberg also estimated that the heritability of twinning itself was close to zero. That means the capacity to have twins is not hereditary.
Not quite so identicalEdit
Monozygotic twins are genetically nearly identical and they are always the same sex unless there has been a mutation during development. Monozygotic twins always have different phenotypes. Twins may express different sexual phenotypes, normally from an XXY Klinefelter's syndrome zygote splitting unevenly.
Although monozygotic twins are genetically almost identical, a 2012 study of 92 pairs of monozygotic twins found that monozygotic twins acquire hundreds of genetic differences early in foetal development. This is caused by mutations (or copy errors) taking place in the DNA of each twin after the splitting of the embryo. The researchers summarised:
- "These findings provide direct evidence that early somatic point mutations do occur and can lead to differences in genomes between otherwise identical twins, suggesting a considerable burden of somatic mutations among the trillions of mitoses that occur over the human lifespan.".
It is estimated that, on average, a set of monozygotic twins will have about 360 genetic differences that occurred early in foetal development.
Another cause of difference between monozygotic twins is epigenetic modification. These are caused by differing environmental influences throughout their lives, which affects which genes are switched on or off. A study of 80 pairs of monozygotic twins ranging in age from three to 74 showed that the youngest twins have relatively few epigenetic differences. The number of epigenetic differences increases with age. Fifty-year-old twins had over three times the epigenetic difference of three-year-old twins. Twins who had spent their lives apart (such as those adopted by two different sets of parents at birth) had the greatest difference. However, certain characteristics become more alike as twins age, such as IQ and personality. This phenomenon illustrates the influence of genetics in many aspects of human characteristics and behaviour.
Twins in animalsEdit
Twinning is normal in many mammals, such as rats, cats, dogs. Identical twins are normal in a few species. The nine-banded armadillo, Dasypus novemcinctus, gives birth to identical quadruplets.
- King R.C. Stansfield W.D. & Mulligan P.K. 2006. A dictionary of genetics, 7th ed. Oxford. p461
- European Society of Human Reproduction and Embryology (2007). "Time-lapse recordings reveal why IVF embryos are more likely to develop into twins. Researchers believe the laboratory culture could be the cause". Press release. http://www.eshre.com/emc.asp?pageId=939. Retrieved 2008-09-30.
- Advanced methods are not described here; just a simplified account of how the problem can be approached.
- Bodmer W. & McKie R. 1994. The book of man: the quest to discover our genetic heritage. Abacus, London. p188–197 ISBN 0-349-10620-7
- Ridley, Matt 2003. The agile gene: how nature turns on nurture. Harper, Chapter 3, A convenient jingle. ISBN 0-00-639448-5
- Bouchard T.J. et al 1990. Sources of human psychological differences: the Minnesota study of twins raised apart. Science. 250 223/8.
- Thompson P.M. et al 2001. Genetic influence on brain structure. Nature Neuroscience. 5 83/4.
- Bulmer M. 2000. Francis Galton, pioneer of heredity and biometry. Johns Hopkins, Baltimore MD. p67
- Galton F. 1875. The history of twins, as a criterion of the relative powers of nature and nurture. J. Anthropological Inst. 5, 329–348.
- Crow, James F. (1999). "Hardy, Weinberg and language impediments". Genetics 152: 821–825. PMID 1460671. http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1460671.
- Edwards J.H; Dent T. & Kahn J 1966. Monozygotic twins of different sex. Journal of Medical Genetics 3 (2): 117–123. doi:10.1136/jmg.3.2.117. PMC 1012913. PMID 6007033.
- Machin, G.A. 1996. Some causes of genotypic and phenotypic discordance in monozygotic twin pairs. American Journal of Medical Genetics 61 (3): 216–228. doi:10.1002/(SICI)1096-8628(19960122)61:3<216::AID-AJMG5>3.0.CO;2-S. PMID 8741866.
- Schmid 2000.. "Prenatal diagnosis of heterokaryotypic mosaic twins discordant for fetal sex.". Prenat Diagn 20 (12): 999–1003. PMID 11113914.
- During the development of a foetus errors in cell division result in some genetic changes: Rui Li et al 2013. Somatic point mutations occurring early in development: a monozygotic twin study. Journal of Medical Genetics. 
- Fraga, Mario F.; Ballestar, Esteban et al (2005). "Epigenetic differences arise during the lifetime of monozygotic twins". Proc. Natl. Acad. Sci. U.S.A. 102 (30): 10604–9. doi:10.1073/pnas.0500398102. PMC 1174919. PMID 16009939.
- Segal, Nancy L. (1999). Entwined lives: twins and what they tell us about human behavior. New York: Dutton. ISBN 0-525-94465-6. OCLC 40396458.
- Plomin, Robert (2001). Behavioral genetics. New York: Worth Pubs. ISBN 0-7167-5159-3. OCLC 43894450.
- Mandler,G. (2001) Apart from genetics: What makes monozygotic twins similar? Journal of Mind and Behavior. 22, 147-159.
- Cyranoski D. 2009. Developmental biology: two by two. Nature. 458, p826.PubMed
- Newman H.H. Patterson J.T. 1909. A case of normal identical quadruplets in the Nine-Banded Armadillo, and its bearing on the problems of identical twins and of sex determination. Biological Bulletin, 17: 181-187