3:1 Ratio - Biology As Poetry (2024)

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

there are three

with the

for every one (on average) with the

. Note that explicitly these are phenotypic

rather than

of

.

For a more complicated version of the same theme, see

and

. Note the use of a

in the following figure:

3:1 ratios are what is most commonly taught when learning

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

instead is between two

of the same type. Furthermore,

and

actually are much simpler to follow since their

and

are identical.

In fact, there is an emphasis on 3:1 ratios when learning

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

between

on

.