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A 9:3:3:1 ratio of phe-notypes among the offspring of a single cross. Such a ratio can only be obtained providing that all the following conditions are met: two genes at different loci are involved, e.g. gene A/a and gene B/ b; the immediate parents of these offspring are heterozygous for both genes, i.e. AaBb; these heterozygotes are selfed or crossed with one another, i.e. AaBb x AaBb; the genes are unlinked, i.e. independent assortment; there is no interaction between the genes, e.g. no epista-sis; A is dominant to a, and B is dominant to b, i.e. no codominance, etc.; fertilization is random, and all the gametes, zygotes, and offspring have an equal chance of survival. If there is reason to expect a 9:3:3:1 ratio in a particular cross and this result is not obtained, one or more of the above conditions is not being fulfilled. For example, the ratio may be modified to 9:3:4 (= 9:3:[3+1]) in some cases of gene interaction (epistasis). A dihybrid ratio is an example of a simple Mendelian ratio, being an expansion of the monohybrid 3:1 ratio, viz: (3:1)2.
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