Carrying out a genetic cross

Monohybrid crosses

Genetic crosses of single gene combinations (monohybrid inheritance) can be shown and examined using Punnett squares. This shows 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 these Punnett squares the female alleles are shown in the top row and the male alleles are shown in the left-hand column.

TT
t
t

The female is TT - tall

The male 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.

curriculum-key-fact
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, the female alleles are Dd and the male alleles are dd.

Dd
dDddd
dDddd

Half of the possible offspring have the same allele combination Dd and the other half have the dd combination.

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