Describe the essence of multiple gene actions

With the multiple action of one gene, two variants of the splitting of two characters can take place. In one case, both signs can manifest themselves in a ratio of 3 (AA, Aa, Aa); 1 (aa). In the second case, one sign is manifested in the ratio 3 (AA, Aa, Aa): 1 (aa), and the other sign in the ratio 1 (AA); 3 (Aa, Aa, aa).
This is due to the fact that the genes that determine morphological characters affect physiological functions (change the vitality, fecundity of individuals) or are fatal (cause the death of the embryo or organism at its birth).

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