Punnett Square Tutorial
Complete the following steps each time you complete a Punnett square problem.
1. Figure out the gene types of the parents from the problem; establish the dominant and recessive trait.
2. Set up a Punnett square diagram.
3. Fill in the Punnett square.
4. Calculate the genotype and phenotype ratio.
Let’s take a look at an example
A heterozygous black rat is being crossed with a homozygous white rat. What would be the possible coloring of the offspring?
1. Figure out the gene types of the parents from the problem; establish the dominant and recessive trait.
Parent 1 has one dominant and one recessive gene (Aa) because they are heterozygous, we also know black is the dominant color
because the dominant trait shows up in a heterozygous gene pair (A overpowers a.)
Parent 2 has two of the same gene (AA or aa) because they are homozygous. In this case we know it is aa because the parent shows
the recessive color, white.
Aa x aa
We now have the gene pairs of the parents
2. Set up a Punnett square diagram.
To set up the Punnett square take each of the genes from one parent and put one above each column of the Punnett square. Next,
take each of the genes from the other parent and put one to the left of each row of the square.
A a x a a
a
a
A
a
3. Fill in the Punnett square.
To fill in the Punnett square drop the letters above the square straight down into the boxes below and move the letters from the left
straight across into the boxes to the right.
a
a
A
Aa
Aa
a
aa
aa
4. Calculate the genotype and phenotype ratio.
Finally you need to calculate the genotype and phenotype ratios. For the genotype you are explain what the chances of each gene
possibility are. We always write the ratio in the following order Homozygous dominant: Heterozygous: Homozygous recessive.
For this example the genotype ratio would be G= 0 AA: 2 Aa: 2 aa
Phenotype ratio explains what the offspring will look like. It is always written in the order dominant trait: recessive trait.
For this example the phenotype ratio would be
P= 2 Black : 2 White