The best way to understand the power of Punnett squares in genetic research is to use them. We will take the following example and follow it through the process of determining the outcome of a cross.

Freckles? Or not?
A homozygous freckled man marries a non-freckled woman. Will their children be freckled-faced?

Given: Freckles are dominant

Step 1. Determine the parental genotypes and phenotypes.

Therefore the couple is:

Man: genotype = FF, phenotype = freckles

Woman: genotype = ff, phenotype = no freckles

Step 2. Figure out what possible gametes can be produced from the genotypes.

According to Mendel’s law of segregation, the alleles in each parent separate when the gametes form. So FF will separate into an F allele within a gamete and a second Fallele in a gamete. The ff will do the same, creating anf allele gamete and a second f allele gamete.

Step 3. Set up the Punnett square.

The possible offspring combinations then will be on the inside of the square. As long as you keep the possible gametes from each parent on the same side of the square it does not really matter whether you put them on the horizontal or the vertical side of the square. In other words our maternal gamete possibilities are either f or f.They can go either on the horizontal outside or the vertical they just need to stay together.

Once the gametes are listed, fill in the boxes to show the genotypic combinations possible in the offspring.

Step 4. Determine the genotypic and phenotypic possible outcomes of the cross. In our example all of the offspring options are the same Ff. The frequency that these genotypes occur in the offspring is expressed as a probability.

The phenotype of the offspring would be freckled because all possible offspring carry a dominant F for freckles.