By comparing monozygotic and dizygotic twins, we can determine to what extent genetic factors and/or environmental factors are responsible for the development of diseases. The advantage of studies on twins: since twins are the same age, they usually grow up in the same environment and are therefore exposed to very similar environmental factors.

Why compare monozygotic and dizygotic twins?

With few exceptions, monozygotic twins have identical DNA. In dizygotic twins, by contrast, only half of the DNA is the same – just as with other siblings – because in both siblings the genetic material of father and mother has been completely mixed again.

An example:

If a monozygotic twin falls ill with asthma, the sibling will also develop asthma in 80% of the cases. In dizygotic twins the sibling only develops asthma in 5 –20% of the cases.

We can thus conclude that genetic factors play a very important role in the development of asthma. In monozygotic twins both siblings have inherited asthma genes from their parents. Since to a large extent they are exposed to the same environmental factors, both either get the disease or remain healthy.

In dizygotic twins, the probability that both siblings have inherited the unfavorable gene variant from father and mother is very small. Usually only one of the twins has inherited such an unfavorable constellation of genetic mutations from the parents, and so usually only one twin gets asthma.

Another example: the risk that with one affected twin, the second twin will also get chronic inflammation of the bowel is 54% for monozygotic twins. That shows that chronic inflammatory diseases are caused half by environmental factors and half by genetic factors. The genetic proportion in the development of the disease is still very high, but not as high as with allergies. In dizygotic twins, in which only half of the DNA is identical, both twins suffer from chronic inflammatory bowel disease in only 4% of the cases.