twin studies

Twinning in human pregnancies occurs either by the separation of a single fertilised egg, or by the fertilisation of two different eggs. In the case where twins result from a single egg, they will be genetically identical or monozygous. Those arising from different eggs are dizygous. Dizygous twins therefore include twin pairs consisting of one male and one female and all dizygous twins are no more genetically similar than any other two siblings. Cases where both twins in a pair suffer from the same disease or carry the same trait are said to be ‘concordant’ for that trait or disease. Where a single twin is affected the pair is said to be ‘discordant’.
Studies of twins are important in genetic research into a variety of both inherited and non-inherited diseases. As this aims to give an overview to the genetic contribution to a given illness, for example Diabetes or Multiple Sclerosis. There are three main approaches commonly used in different twin studies. The first, th...