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Nonallelic genes that cooperate in the expression of a single characteristic. The genes may be linked or unlinked. In some cases a standard Mendelian ratio is maintained, but there may be unexpected phenotypes. Thus in the inheritance of flower colour in sweet peas, purple-flowered offspring may be obtained when two white-flowered plants are crossed. This is because flower colour is affected by two genes and if either is a double recessive then no colour develops. In other cases, no unusual phenotypes appear but there may be some apparent modification of the Mendelian ratio in the F2, such as 9:3:4 (= 9:3:[3 + l]), or 9:7 (= 9:[3 + 3 + l]).
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