Studies of identical twins have_____.
A. proved conclusively the importance of biological factors in human development.
B. proved conclusively the importance of environmental factors in human development.
C. proved conclusively that human development is a result of the interaction between environmental and biological factors.
D. been based on extremely small samples and should be viewed as preliminary analyses at best.

Epigenetics is a regulatory system that controls the expression of genes without affecting the composition of the genes themselves. The regulation of genetic transcription has emerged as a key biological determinant of protein production and cell differentiation, and plays an important pathogenic role in a number of human diseases. It refers, in a broad sense, to the study of all those non-genetic factors involved in the determination of ontogeny.

Therefore, understanding it ,will be a key piece to unveil the interaction between external agents, lifestyles and cellular physiology. In this sense, almost identical twins are a great opportunity to study it.