Identical twins are genetically identical: they have the same genes. They are formed by a fertilised egg dividing into two separate individuals, and are always of the same sex. They may be called monozygotic or MZ twins (mono = one; zygote = fertilised egg). They contrast 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.
Identical twins are natural clones. Because they carry the same genes (and this can be proved), they may 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.
Twins in animals[change]
- King R.C. Stansfield W.D. & Mulligan P.K. 2006. A dictionary of genetics, 7th ed. Oxford. p461
- Advanced methods are not described here; just a simplified account of how the problem can be approached.
- A good account is given in 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
- 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.
- 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