3:1 Ratio

∞ generated and posted on 2016.12.04 ∞

Consequence of a monohybrid cross given complete dominance of one allele over the other.

A 3:1 Ratio is the relative fraction of phenotypes among progeny (offspring) results following mating between two heterozygotes, where each parent possesses one dominant allele (e.g., A) and one recessive allele (e.g., a) at the genetic locus in question—the resulting progeny on average consist of one AA genotype (A phenotype) for every one aa genotype (a phenotype) for every two Aa genotype (A phenotype), thus three A phenotypes for every one a phenotype.

With 3:1 ratios there are three progeny with the dominant phenotype for every one (on average) with the recessive phenotype. Note that explicitly these are phenotypic ratios rather than ratios of genotypes.

For a more complicated version of the same theme, see 9:3:3:1 ratio and Mendelian ratio. Note the use of a Punnett square in the following figure:

Figure legend: B and W are alleles, indeed, as contained within sperm and eggs. BB, BW, and WW are all genotypes, created by the fertilization of egg by sperm. The associated phenotypes, 'black' and 'white' are as indicated with both BB and BW black and WW white. Note the ratio of three black progeny from this mating to one white. The mating itself was BW × BW, which themselves were both black rather than white in phenotype, that is, black/B is dominant phenotypically to white/W in this hypothetical mating by an unspecific species.

3:1 ratios are what is most commonly taught when learning Mendelian genetics and therefore what we might feel is the simplest of all possible cases. The truth, though, is that 3:1 ratios seem simple only because of familiarity (assuming, of course, that you are familiar with 3:1 ratios ☺).

The simplest of crosses instead is between two homozygotes of the same type. Furthermore, codominance and incomplete dominance actually are much simpler to follow since their phenotypic ratios and genotypic ratios are identical.

In fact, there is an emphasis on 3:1 ratios when learning genetics not because they represent the simplest of cases but instead because they represent a relatively hard while at the same time hugely important case, illustrating the impact of dominant-recessive relationships between alleles on mating outcomes.


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