parents from the northern country decide to have a rest on the south. Certainly, they get sunburn, their skin become brown. At the same time they have a conception of a child. What color of skin will this child have? Of course, white. So sunburn is not hereditary sign. The levels of modificational variability depend on «norm of reaction». There are wide and limited norms of reaction (fig. 2). The examples neither of wide nor of reaction in human being are: height, weight, color of skin; and limited norm of reaction are: eye color; number of tooth, hands, legs; blood groups.