Twins and Twin Studies

We recognize two types of twins - monozygotic (The egg is fertilized by one sperm and its split occurs after the conception.) and dizygotic (Two eggs are fertilized by two separated sperms.). Monozygotic twins have all the same genes and they are the same sex too. However, they are more interesting for the twin studies, just one third of twins are monozygotic. On the other hand, dizygotic twins are much more common and share usually just half of their genes, so we can say that their relationship corresponds to the relationship of siblings. The dizygotic twins studies have the same significance like normal family studies.

Twins-reared-together
Both of the twins grow up in the same family and they are exposed to the same external influences. In the case of monozygotic twins we can then recognize their genetic susceptibility to disease. The probability that just one of monozygotic twins from the same family becomes ill is very low, as compared with the dizygotic.

Twins-reared-apart
This type of studies are based on the comparisons of twins, who were adopted into different families and the conditions in which they grew up were not the same, so we can evaluate the role of environment on the disease. If just one of the twins suffers from the genetic disease, we assume the influence of threshold effect – it is the situation when the symptoms occur after long asymptomatic period, because critical limits were crossed.

Co-twin Control Studies
If both of the twins have some healthy problem, we offer the treatment to only one of them and then we compared the results of the cure. It is not ethical to try with the drug therapy, so we can apply it e.g. on rehabilitation.