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 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.

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.

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, 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.

Maths - Probability, direct proportion and simple ratios

You can express the outcome of a genetic cross using probability, direct proportion or ratios.

Worked example 3

aa
AAaAa
aaaaa

Direct proportion

Half the offspring have the combination Aa and half have aa

or:

out of 4 offspring 2 have the combination Aa and 2 have aa

Probability

The probability of the offspring being Aa is 50%

The probability of the offspring being aa is 50%

Ratio

The ratio of Aa to aa is 1:1

It is important to remember during the process of fertilisation, the allele combinations created are a random process, and that is why probability is used, as nothing is guaranteed.

Each of the four possible offspring combinations is as likely to happen during every fertilisation event.