∞generated and posted on 2023.05.16∞
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
ratiosthere are three
progenywith the
for every one (on average) with the
recessive phenotype. Note that explicitly these are phenotypic
ratiosrather than
ratiosof
genotypes.
For a more complicated version of the same theme, see
9:3:3:1 ratioand
Mendelian ratio. Note the use of a
Punnett squarein 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 geneticsand 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
crossesinstead is between two
hom*ozygotesof the same type. Furthermore,
codominanceand
incomplete dominanceactually are much simpler to follow since their
phenotypic ratiosand
genotypic ratiosare identical.
In fact, there is an emphasis on 3:1 ratios when learning
geneticsnot 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 relationshipsbetween
alleleson
mating outcomes.
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