Introduction to Punnett Squares
A Punnett square is a diagram that predicts the outcomes of the phenotypes and genotypes of genetic crosses.
We can use a Punnett square to represent the inheritance of traits in monohybrid crosses (which means one trait
being studied).
Worked Example:
In pea plants, tall is dominant to short. Predict the genotype and phenotype ratios of a cross between a
homozygous tall plant (TT) and a homozygous short plant (tt).
● The genotype ratio is how often the genotypes are predicted to appear in the offspring.
● The phenotype ratio is how often the phenotypes are predicted to appear in the offspring.
Step 1. Write out the parents of the cross.
Parents: _______________ x ________________
Step 2. Write out the possible alleles from each of the parents.
Alleles: __________ __________ x __________ __________
Step 3. Draw a box and divide it into 4 squares. Above the top squares, write the alleles from one parent. On the
left side of the squares, write the alleles from the other parent.
Step 4. Fill in the box
Step 5. Determine the number of different genotypes from the box and express each as a ratio and percentage.
This represents the expected frequency of offspring with that genotype for that cross.
Step 6. Determine the number of different phenotypes from the box and express each as a ratio and percentage.
This represents the expected frequency of offspring with the phenotype for that cross.
Example:
In humans, having dimples is the dominant trait to not having dimples. Predict the genotype and phenotype ratios
of a cross between a man heterozygous (Dd) for dimples and a woman without dimples (dd).
D = dimple, d = no dimples
Try this:
Straight fingers is the dominant trait to having a crooked finger. Predict the genotype and phenotype ratios of a
cross between a man heterozygous for a straight finger (Nn) and a woman homozygous for a straight finger (NN).
N = straight nose, n = crooked nose