From Citizendium, the Citizens' Compendium
A Monohybrid cross is a cross between individuals who are identically heterozygous at one locus, for example, Bb x Bb (see the Punnett square below).
Monohybrid inheritance is the inheritance of a single characteristic. The different forms of the characteristic are usually controlled by different alleles of the same gene. For example, a monohybrid cross between two pure-breeding plants (homozygous for their respective traits), one with yellow seeds (the dominant trait) and one with green seeds (the recessive trait), would be expected to produce an F1 (first) generation with only yellow seeds because the allele for yellow seeds is dominant to that of green. A monohybrid cross compares only one trait.
Monohybrid Cross in Punnett Square
For a monohybrid cross, the parental genotypes are homozygous, and their progeny, now the F1 generation, are all heterozygous. The above table, a Punnett square, illustrates the results of a cross between two F1 heterozygous individuals. The result is a 1:2:1 genotypic ratio, and a 3:1 phenotypic ratio.
Usage of Monohybrid Cross
Generally, the monohybrid cross is used to determine the F2 generation from a pair of homozygous grandparents (one grandparent dominant, the other recessive) which results in a F1 generation that are all heterozygous. The pairing of these offspring results in a monohybrid cross and results in the F2 generation, with a 75% chance for the dominant phenotype and a 25% chance for the recessive phenotype.
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