Twinning is the process which results in the birth of twins or higher order multiples instead of the more usual singleton. Twin studies can be used in determining genetic contribution to normal development and disease processes.

Keywords: monozygotic; dizygotic; discordance; twin studies; higher multiples

Figure 1.

Familial DZ twinning. Five sets of DZ twins occurring in one family demonstrating probable autosomal dominant inheritance with incomplete penetrance.

Figure 2.

Placentation in twins. Each developing fetus is surrounded by an inner amniotic membrane enclosing the amniotic sac and an outer chorionic membrane continuous with the placenta. The arrangement of the membranes may be useful in determining zygosity and the timing of the splitting event.

Figure 3.

Familial MZ twinning. Pedigrees 1 and 2 show familial MZ twinning with apparent autosomal dominant inheritance. Pedigree 3 shows a family with both DZ and MZ twinning. MZ twins are denoted as (male twins) or (female twins).

Figure 4.

Zygosity testing by analysis of hypervariable DNA. Lanes 1a and 1b contain DNA from each of a pair of DZ twins and show differences in the pattern of DNA fragments. Lanes 2a and 2b contain DNA from each of a pair of MZ twins, showing an identical pattern of DNA fragments.


Further Reading

Bamforth F, Machin G and Innes M (1996) X‐chromosome inactivation is mostly random in placental tissues of female monozygotic twins and triplets. American Journal of Medical Genetics 61: 209–215.

Derom C, Vlietinck R, Derom R, van den Burghe H and Thiery M (1998) Population‐based study of sex proportion in monoamniotic twins. New England Journal of Medicine 319: 119–120.

Goodship J, Carter J and Burn J (1996) X‐inactivation patterns in monozygotic and dizygotic female twins. American Journal of Medical Genetics 61: 205–208.

Hall J (1996) Invited editorial comment: Twins and twinning. American Journal of Medical Genetics 61: 202–204.

James W (1980) Sex ratio and placentation in twins. Annals of Human Biology 7: 273–276.

Jorgensen A, Philip J, Christensen B et al. (1992) Opposite patterns of X‐inactivation in MZ twins discordant for red‐green colour deficiency. American Journal of Human Genetics 51: 291–298.

Lupski J, Garcia G, Zoghbi H, Hoffman E and Fenwick D (1991) Discordance of muscular dystrophy in monozygotic female twins: evidence supporting asymmetric splitting of the inner cell mass in a manifesting carrier of Duchenne dystrophy. American Journal of Medical Genetics 40: 354–364.

Machin G (1996) Some causes of genotype and phenotypic discordance in monozygotic twin pairs. American Journal of Medical Genetics 61: 216–228.

Machin G and Keith L (1999) An Atlas of Multiple Pregnancy – Biology and Pathology. New York: Parthenon.

Newman B, Selby J, King M et al. (1987) Concordance for type 2 (non‐insulin dependent) diabetes mellitus in male twins. Diabetologia 30a: 763–768.

Philips D (1993) Twin studies in medical research: can they tell us whether diseases are genetically determined? Lancet 341: 1008–1009.

Proceedings of the 10th Jubilee International Workshopon Multiple Pregnancy (1996) Archives of Perinatal Medicine 2. Poznan, Poland: Scientific Publishers OWN.

Sperber G, Machin G and Bamforth F (1994) Mirror‐image dental fusion and discordance in monozygotic twins. American Journal of Medical Genetics 51: 41–45.

Spitz L (1996) Conjoined twins. British Journal of Surgery 83: 1028–1030.

Westergard T, Wohlfahrt J, Aaby P and Melbye M (1997) Population based study of rates of multiple pregnancies in Denmark, 1980–94. British Medical Journal 314: 775–779.

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How to Cite close
Bamforth, Fiona(Apr 2001) Twinning. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1038/npg.els.0001885]