Twin Studies in Medical Genetics

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Abstract
It is the aim of twin studies to obtain results which are not only valid for twins, but apply to the whole population. Therefore the following questions have to be answered first: do twins differ from non-twins in the trait under study? Do different nongenetic factors act upon MZ and DZ twins which alter the probability of manifestation of a trait, even before birth? There are important differences in embryogenesis and placental blood flow in mono- and dichorionic twins; this can influence the normal fetal development. Therefore the value of twin studies alone in analysing the genetic component in the etiology of congenital malformations is rather ambiguous. Twin studies beyond the newborn period can be classified into four approaches: (1) Case reports; (2) Accumulated case reports; (3) Limited representative sample; (4) Unlimited representative sample. The most frequent systematic method in medical genetics is the establishment of all twins in a defined population of probands (3). Another successful application in the last few years has been in pharmacogenetics. Although no simple mode of inheritance could be discovered, it was possible to estimate the genetic component within the interindividual variability of the metabolism of certain drugs (nortriptyline, antipyrine, phenylbutazone, ethanol). Now, additional non-twin research is needed to work out single factors within the observed polygenic systems.