PUNNETT SQUARE

A Punnett square is a chart which shows/predicts all possible gene combinations in a cross of parents (whose genes are known). Punnett squares are named for an English geneticist, Reginald Punnett. He discovered some basic principles of genetics, including sex linkage and sex determination. He worked with the feather color traits of chickens in order to quickly separate male and female chickens. 

Genetic Problem using Punnett Squares - Example and Steps

Sample Problem

*In pea plants (which Gregor Mendel studied), tall pea plants are dominant over short pea plants. Using Punnett Squares, you can predict the genotypes and phenotypes of the offspring of a cross between a homozygous (purebred) tall pea plant and a homozygous (purebred) short pea plant. 

Step 1.

Designate letters which will represent the genes/traits. Capital letters represent dominant traits, and lowercase letters represent recessive traits. 

             T = tall t = short 

Step 2.

Write down the genotypes (genes) of each parent. These are often given to you or are possible to determine.

genotype = the genes of an organism; for one specific trait we use two letters to represent the genotype.

              TT X tt

                      (tall) (short) - both homozygous (same) or purebred 

Step 3.

List the genes that each parent can contribute.

                                                           Parent 1 Parent 2

Step 4.

Draw a Punnett square - 4 small squares in the shape of a window. 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.

Step 5.

Fill in each box of the Punnett square by transferring the letter above and in front of each box into each appropriate box. As a general rule, the capital letter goes first and a lowercase letter follows.

Step 6.

List the possible genotypes and phenotypes of the offspring for this cross.

The letters inside the boxes indicate probable genotypes (genetic makeup) of offspring resulting from the cross of these particular parents. There are 4 boxes, and the genotypic results can be written either as fractions or percents. In this case, all 4 boxes out of the 4 are showing the Tt genotype. Therefore, each of the offspring has a 4/4 or 100% chance of showing the Tt genotype.

We have also written the phenotype (physical appearance) in each box under the genotype. Remember, T = tall and t = short (see step #1 above). Since a capital letter indicates a dominant gene, T (tall) is dominant over t (short). Therefore, each of the offspring has a 4/4 or 100% chance of being tall. 

What are the possible genotype(s) of a tall plant? 

What are the possible genotype(s) of a short plant? 

What would be the phenotype of TT? 

What would be the phenotype of tt? 

Why is the phenotype of Tt tall and not medium/average?

Practice

Using the steps of the Punnett Squares  do some Punnett squares on your own. Grab your paper and a pencil. For each problem, be sure to do the work for each step and don't forget the genotypes and phenotypes of the offspring. We'll get you started with the first problem.

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.

*Now you're on your own to finish Steps 4 through 6.

Problem #2 - In pea plants, yellow peas are dominant over green peas.
Use a Punnett square to predict the phenotypic and genotypic outcome (offspring) of a cross between a plant heterozygous/hybrid for yellow (Yy) peas and a plant homozygous/purebred for green (yy) peas. 

Problem #3 - In pea plants, yellow peas are dominant over green peas.
Use a Punnett square to predict the phenotypic and genotypic outcome (offspring) of a cross between two plants heterozygous for yellow peas.