Mating between two individuals that are equivalently heterozygous at the same locus.
A monohybrid is created through mating of two individuals that are true breeding but not, for a specific locus, homozygous for the same alleles. Indeed, usually there are phenotypic differences between the two homozygotes, that is, such that each is true breeding but for different traits associated with a single character. Thus, AA × aa → Aa, where Aa is the monohybrid.
The monohybrid cross in this example is Aa × Aa. Note that, in specifying this cross, no mention has been made as to which allele is dominant and which recessive. Indeed, dominance-recessive relationships are not essential to describing or even understanding a monohybrid cross. Instead, what is important is solely that alleles A and a are not identical.
Note that the genotypic ratio among progeny for a monohybrid cross are expected to be 1:2:1, that is, one AA, two Aa, and one aa, on average. Note, though, that this is not necessarily a description of the expected phenotypic ratio, which instead does depend on the dominant-recessive relationships between the alleles involved. Indeed, specifically one never expects to obtain a 3:1 genotypic ratio from a monohybrid cross.
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