How to construct Punnett squares

Follow this method to construct Punnett squares.

Determine the parental genotypes – the allele combinations for the male and female. You can use any letter you like, but select one that has a clearly different lower case, for example: Aa, Bb, Dd.
Split the alleles for each parental type and add them into your Punnett square around the edges.
Work out the new possible genetic combinations inside the Punnett square.
Use this to answer the question. 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.

Example

The inheritance of fur colour in mice is controlled by a single gene. White fur is recessive - a. Grey fur is dominant - A. Two mice that are heterozygous for the fur colour gene mate. Determine the probability that the offspring will have grey fur.

Step one

Female – Aa

Male – Aa

Step two

Table with the rows labelled as Male alleles (A,a) and the columns labelled as Female alleles (A,a)

Step three

Table with the rows labelled as Male alleles (A,a) and the columns labelled as Female alleles (A,a). The cells of the table contain AA, Aa, Aa and aa

Because the genotypes Aa and AA both result in the grey fur phenotype, the probability that the offspring will have grey fur is \(\frac{3}{4}\) (or 75%), and the probability that the offspring will have white fur is \(\frac{1}{4}\) (or 25%).

More examples of genetic crosses

Here are more possible examples of single gene combinations.

Example one

Aa x AA, where Aa = female alleles, and AA = male alleles.

Half of the possible offspring has the same allele combination AA (homozygous dominant). The other half has the combination Aa (heterozygous).

The calculation for this example shows that two out of four possible combinations create a 50% chance for AA, and a 50% chance for Aa alleles.

There is a ratio of 1:1.

Example two

Ee x Ee, where Ee = female alleles, and Ee = male alleles.

Table with the rows labelled as Male alleles (E,e) and the columns labelled as Female alleles (E,e). The cells of the table contain EE, Ee, Ee and ee

There are three different offspring combinations. One is EE (homozygous dominant), two are Ee (heterozygous) and one is ee (homozygous recessive).

Probability calculations for this example show that one out of four possible combinations create a 25% chance for EE, two offspring create a 50% chance for the Ee alleles, and finally a 25% chance for the ee alleles.

This can be shown simply as:

Table with rows labelled as Offspring and Probability: EE, 25%. Ee, 50%. Ee, 25%

The probability values must add up to 100% (25 + 50 + 25 = 100).

The ratio can be summarised as follows.

Table with rows labelled as Offspring, Probability and Ratio EE, 25%, 1. Ee, 50%, 2. Ee, 25%, 1

Question

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.

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

Table with the rows labelled as Male alleles (D,d) and the columns labelled as Female alleles (D,d)

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

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

Question

Higher tier only

Flower colour in pea plants is inherited genetically. The white allele is recessive, and the pink allele is dominant. A plant that is homozygous recessive is bred with a plant that is heterozygous.

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

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DNA and inheritance – WJECHow to construct Punnett squares