Carrying out a genetic cross

Monohybrid crosses

Genetic crosses involving single genes (monohybrid inheritance) can be shown and examined using Punnett squares.

This shows the possible offspring combinations that could be produced. 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 a Punnett square like the one shown below to show the possible allele combinations of the offspring produced when two pea plants are bred.

Genotype of female = TT

Phenotype of female = tall

Genotype of male = tt

Phenotype of male - short

TT
tTtTt
tTtTt

You see that all the possible offspring have the same genotype - 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 genotype of the female is Dd
  • the genotype of the male is dd
Dd
dDddd
dDddd

Half of the possible offspring have the genotype Dd and the other half have the genotype dd.

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

Maths - use probability, direct proportion and simple ratios - Higher

You should be able to express the outcome of a genetic cross using probability, direct proportion or ratios.

Worked example 3

In this genetic cross:

  • the genotype of the female is Aa
  • the genotype of the male is aa
Aa
aAaaa
aAaaa

Direct proportion

Half the offspring have the combination Aa and half have aa.

or:

Out of four offspring, two have the combination Aa and two 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 that during the process of fertilisation, the allele combinations are created in a random process, which 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.

Worked example 4

The inheritance of fur colour in mice is controlled by a single gene. White fur is recessive. Grey fur is dominant. Two mice that are heterozygous for the fur colour gene mate.

Determine the probability that the offspring will have grey fur.

Step 1:

Female: Aa

Male: Aa

Step 2:

Aa
A
a

Step 3:

Aa
AAAAa
aAaaa

There is a 75% probability of the offspring having grey fur, and a 25% probability that offspring will have white fur.

The expected ratio of the offspring will be 3:1, grey fur to white fur.