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 some critical limits were crossed (e.g. physical, chemical, biological factors).

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.

E.g. there was a research few years ago, when the doctor tried to teach one of the twin to climb the stairs, the second twin didn´t practice it. However, both of them were able to climb the stairs after few weeks, the trained one did it a week earlier. It shows that the environmental influence could help them to learn something faster, but at the end the results were the same for both of the children.

Use of Twin Studies
Thank to studies of twin pool it was discovered the genetic connection between our genes and some diseases - heart diseases, vascular diseases or kidney diseases. We always expected the role of genes in these diseases, but nowadays we know much more about their origin, e.g. there is a genetic base of CRP (C-reactive protein) and albumin (renal protein) production. The more CRP is produced, the higher risk of cardiovascular disease.

Related articles

 * Family Studies, Pedigree Analysis
 * CRP
 * Albumin