## The epigenetic landscape illustrates the importance of epigenetics

The first indications that the fate of cells may depend on the environment in which they live was highlighted in the 1950s by developmental biologist Conrad Hal Waddington, who studied the development of multicellular organisms. He based his model, called the epigenetic landscape, on his observations (Figure 6.2A). In the epigenetic landscape model, the cell represents a ball that changes its direction during the development of the organism depending on which path it takes. In fact, the direction of the cell depends on which metabolic pathways will be turned on, which themselves depend on the environmental conditions, and based on their combination, the resulting phenotype is formed. As a result, in a multicellular organism, all cells do not perform the same function, but are specialized for specific tasks (Figure 6.2B).

According to estimates, the adult human body contains 37 trillion (3.7x1013) cells. After the cells have gone through the complete process of differentiation, there are approximately 200 types of cells in the body, which differ from each other in their function and structure. Each of the body's cells has the same DNA, but not all genes are transcribed in individual types of specialized cells. After all, why would it be necessary to synthesize liver enzymes in brain neurons? In addition, in female mammals, including humans, one entire chromosome is inactivated - the sex chromosome X. This inactivation is necessary to compensate for the gene dosage, because females have two X chromosomes, unlike males, who have only one X chromosome (their second sex chromosome is the Y chromosome). The need for a mechanism to turn genes on and off is therefore obvious.