How to construct Punnett squares

  1. Determine the parental genotypes. You can use any letter you like but select one that has a clearly different lower case, for example: Aa, Bb, Dd.
  2. Split the alleles for each parent and add them into your Punnett square around the edges.
  3. Work out the new possible genetic combinations inside the Punnett square.

You may be asked to comment on the proportion of different allele combinations in the offspring, calculate a probability, ratio or just determine the phenotypes of the offspring.

Worked example

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

Parent 1 - Aa

Parent 2 - Aa

Step 2

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

Aa
A
a

Step 3

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

Aa
AAA greyAa grey
aAa greyaa white

The genotypes AA and Aa both result in the grey fur phenotype. The genotype aa results in the phenotype white. The grey and the white are known as 'offspring phenotypes'.

There is a 75% probability that the offspring will have grey fur.

We also write this as a ratio:

The ratio of grey to white fur is 3:1

Question 1

Coat type in dogs is determined by a single gene with two alleles.

The straight coat allele is dominant (D) and the curly coat allele is recessive(d).

Two dogs with the alleles Dd breed.

Question

Complete this Punnett square to show the allele combinations of the possible offspring produced.

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

Dd
D
d
Dd
DDDDd
dDddd
Question

Calculate the probability that an offspring will have the combination DD.

25%

Question

Determine the ratio of straight hair offspring to curly hair offspring.

3 straight : 1 curly

Question 2 - Higher

Flower colour in pea plants is inherited genetically.

The white allele is recessive and the pink allele is dominant.

Question

A plant that is homozygous recessive is bred with a plant that is heterozygous.

If 48 seeds are produced, estimate how many will have pink flowers.

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

aa
AAaAa
aaaaa

Offspring that are Aa are pink so there is a 50% probability of a plant being pink.

48 × (50/100) = 24 plants