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
Step three
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 probabilityThe extent to which something is likely to be the case. 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.
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:
The probability values must add up to 100% (25 + 50 + 25 = 100).
The ratio can be summarised as follows.
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.
b. Calculate the probability that an offspring will have the combination DD.
c. Determine the ratio of straight hair offspring to curly hair offspring.
a. The Punnet square should look like this.
b. 25%
c. 3 straight: 1 curly
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.
Offspring that are Aa are pink, so the probability of a plant being pink is \(\frac{1}{2}\) (or 50%).
48 × (\(\frac{50}{100}\)) = 24 plants with pink flowers.
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