Carrying out a genetic cross

Monohybrid crosses

Genetic crosses of single gene combinations (monohybrid inheritance) can be shown and examined using Punnett squares. These show the possible offspring combinations that could be produced, and the probability of these combinations can be calculated.

Worked example 1

The height of pea plants is controlled by a single gene which has two alleles: tall and short.

The tall allele is dominant and is shown as T.

The small allele is recessive and is shown as t.

Complete this Punnett square to show the possible allele combinations of the offspring produced when two pea plants are bred.

In this Punnett square the top row shows the alleles of parent 1 and the left-hand column shows the alleles of parent 2.

TT
t
t

Parent 1 is TT - tall

Parent 2 is tt - short

TT
tTtTt
tTtTt

If you add the combinations into each box you see that all the possible offspring have the same allele combination - Tt.

Note: You should always write the dominant allele first.

This means that all the offspring produced will be tall.

Worked example 2

In this genetic cross, Parent 1 is still tall but is heterozygous with alleles Tt. Parent 2 is short and is homozygous with alleles tt.

In this Punnett square the top row shows the female alleles and the left-hand column shows the male alleles.

Tt
tTttt
tTttt

Half of the possible offspring have the allele combination Tt and will be tall and the other half have the combination tt and will be short.

These examples are single gene combinations, but remember that most phenotypes are controlled by multiple genes.