How to Set up a Punnett Square

How to set up a Punnett Square

In pea plants, round seeds (R) are dominant to wrinkled seeds (r). In a genetic cross of two plants that are heterozygous for the seed shape trait, what fraction of the offspring should have spherical seeds?
heterozygous – two different alleles for the same gene (ex: Rr)
- also known as “hybrid”
homozygous – two same alleles for the same gene (ex: RR or rr)
- also known as “true-breeding” or “purebred” /
The figure above represents a cross of F1-hybrid plants. Both plants are heterozygous (Rr) for an allele that determines seed shape. Presence of the dominant allele (R) in homozygous dominant (RR) or heterozygous (Rr) plants results in round seeds. Homozygous recessive (rr) plants have wrinkled seeds.

Setting up a Punnett square

1. / Set up a 2 by 2 Punnett square. /
2. / Write the alleles for parent 1 on the left side of the Punnett square.
Each gamete will have one of the two alleles of the parent. In this particular cross, half of the gametes will have the dominant (R) allele, and half will have the recessive (r) allele. We will use blue and brown to keep track of the alleles of each parent. /
3. / Write the alleles from parent 2 above the Punnett square.
For this heterozygous parent (Rr), half of the gametes will have the dominant (R) allele, and half will have the recessive (r) allele. /
4. / Fill the squares for parent 1.
Fill each square with the allele from Parent 1 that lines up with the row. /
5. / Fill the squares for parent 2.
Fill each square with the allele from Parent 2 that lines up with the column. /

Interpreting the results of a Punnett square

We now have the information for predicting the outcome of the cross. The genotypes in the four boxes of the Punnett square are each equally likely to occur among the offspring of this cross. We may now tabulate the results.
Genotype – genetic makeup (what you don’t see) Ex: Rr
Phenotype – physical characteristics (what you see) Ex: Round seeds
Genotypes that resulted from this monohybrid cross (Rrx Rr)

25%
homozygous
dominant /
50%
heterozygous /
25%
homozygous
recessive
Ratio of genotypes: 1 RR : 2 Rr : 1 rr
Phenotypes that resulted from this monohybrid cross (Ss x Ss)

Ratio of phenotypes: 3 Round: 1 Wrinkled

Practice problems for Punnett Squares

Problem #1 - Use a Punnett square to predict the genotypic and phenotypic outcome (offspring) of a cross between two heterozygous/hybrid tall (Tt) pea plants.

Step 1 - Designate letters which will represent the genes/traits.
T = tall t = short (and they do not always have to be the letter T!)

Step 2 - Write down the genotypes of each parent. It is helpful if you write the phenotype in parenthesis under the genotypes.
Tt X Tt
(tall) (tall)

Step 3 - List the genes that each parent can contribute.

Step 4 - Write the possible gene(s) of one parent across the top and the gene(s) of the other parent along the side of the Punnett square.

What is the ratio of the genotypes?