Law of segregation

The observation that the two alleles in an individual can separate, with half of the progeny inheriting one allele and half of the progeny inheriting the other allele, is known as Mendel’s law of segregation. It helps explain why progeny do not always resemble their parents. For example, two round pea plants with the genotype Rr can be crossed to produce a wrinkled pea plant. The two alleles, R and r, do not mix or change each other even though they are present in the same individual. Each allele can contribute to the next generation.

Like any good hypothesis (a scientific “law” is simply a hypothesis that is supported by an overwhelming amount of data), Mendel’s law of segregation allows us to make predictions that can be tested by gathering more data (i.e. from additional experiments). We can also use this as an opportunity to practice using Punnett squares. For each of the following crosses, draw a Punnett square and give the predicted genotypic and phenotypic ratios among the progeny. In each case, the capital letter represents the allele for the dominant trait.

Punnett square practice:
(Click on the below crosses to reveal the answer)

1) Pp x pp

2) PP x pp

3) Yy x Yy

Mendel’s experiments were consistent with these predictions. Since then, thousands of other experiments involving many different traits in many different plants and animals have been shown to be consistent with these predictions, which are based on Mendel’s law of segregation. Thus, from his experiments with garden peas, Mendel deduced this basic rule of genetic inheritance that applies to all sexually reproducing plants and animals.

As we move on, you will want to be very comfortable with the ratios from the different monohybrid crosses (shown below). Preferably, you should know the ratios without needing to draw a Punnett square.


Genotypic ratio

Phenotypic ratio


all AA

all A-

aa x aa

all aa

all aa

AA x aa

all Aa

all A-

Aa x AA

1 AA : 1 Aa

all A-

Aa x aa

1 Aa : 1 aa

1 A- : 1 aa

Aa x Aa

1 AA : 2 Aa : 1 Aa

3 A- : 1 aa

Notice that the cross A- x aa gives different results depending on whether the A- parent is homozygous or heterozygous. Thus, crossing an individual with a dominant phenotype to a recessive homozyogote is called a test cross, because it can be used to determine the genotype of the parent with the dominant phenotype.