## Identical twins are a model for studying epigenetic processes

Epigenetic changes occur not only during intrauterine development, but also throughout the life of the organism. This is evidenced by research results in which scientists analysed epigenetic modifications in the genome of identical twins. In the case of identical twins, they both develop from one egg fertilized by a single sperm and therefore have the same genotype. At the level of genetic information, such twins are completely identical, and in the first years of life they are almost indistinguishable at the level of phenotype as well. Before birth, they shared a common space in utero, and since their childhood experiences were more or less the same, they will also have the same epigenetic traces. However, this is true only as long as they live under the same conditions and are affected by the same factors. The older the twins get, the more their epigenomes differ because they are exposed to different conditions. A different environment, different relationship experiences that evoke different emotions, and a different attitude towards their health mean that although they still have the same DNA, it may change which genes are expressed, and which are not. This is also proven by a study of older identical twins, where a comparison of the total content and distribution of modified bases and histones showed that the less time the twins lived together, the more their epigenomes differed.

For this reason, identical twins are not likely to develop each genetic disease with the same probability. For example, in the case of schizophrenia, if it has developed in one of the twins, there is less than a 50% chance that it will develop in the other. Similarly, the risk of developing the autoimmune systemic disease lupus erythematosus is also 50%, while for Alzheimer's disease, the probability is 40%. The question arises to what extent heredity is responsible for health and to what extent the environment in which a person lives. The results of the studies concluded that genes are responsible for only about 49% of the studied traits, characteristics and the environment is responsible for the remaining 51%.