How to construct Punnett squares - Higher
Method to use:
- 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.
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:
Female: Aa Male: Aa
Step 2:
| A | a | |
| A | ||
| a |
| A | |
|---|---|
| A | |
| a | |
| a | |
|---|---|
| A | |
| a | |
Step 3:
| A | a | |
| A | AA | Aa |
| a | Aa | aa |
| A | |
|---|---|
| A | AA |
| a | Aa |
| a | |
|---|---|
| A | Aa |
| a | aa |
Because the genotypes Aa and AA both result in the grey fur phenotype, there is a 75% probability that the offspring will have grey fur. Because the genotype aa results in the white fur phenotype, there is a 25% probability that the offspring will have white fur.