Name______Date______Hour______Table #____

Punnett Squares

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. He worked with the feather color traits of chickens in order to quickly separate male and female chickens.

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 homozygous (pure) tall pea plant and a homozygous (pure) short pea plant.

Step 1: Designate letters which will represent the gene/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 easy to determine. TT x tt or (tall) (short)—both homozygous pure

Step 3: List the genes that each parent can contribute

Parent 1 / Parent 2
TT / tt
T / t

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.

Genotype / Phenotype
TT / Tall
Tt / Tall
tt / short

Step 5: Fill in each box of the Punnett square by transferring the letter above and at the side of each box into the 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. Since the capital letter indicates a dominant gene, T (tall) is dominant over t (short). Therefore, each of the offspring in this case will be tall.

Use Punnett squares to determine the answers to the following samples.

1. One cat carries heterozygous, long-haired traits (Ss), and its mate carries homozygous short-haired traits (ss). Use the Punnett square to determine the probability of one of their offspring having long hair.

A. 100%

B. 25%

C. 75%

D. 50%

2. One flower is heterozygous red (Rr) and it is crossed with a homozygous white (rr) plant. Use the Punnett square to determine the probability of one of their offspring having a red color.

A. 75%

B. 25%

C. 50 %

D. 100%

3. In certain species of plant, the color purple (P) is dominant to the color white (p). Use the Punnett square to determine the probability of the offspring being purple if a pure purple plant is crossed with a pure white plant.

A. 25%

B. 0 %

C. 100%

D. 75%


Trait Determination Practice

Should This Dog Be Called Spot?

Imagine this microscopic drama. A sex cell from a male dog joins with a sex cell from a female dog. Each dog’s sex cell carries 39 chromosomes. The zygote which results contains 78 chromosomes. It receives a set of chromosomes from each parent. Suppose you could look at one pair of the zygote’s chromosomes.

From the female dog

From the male dog

Each chromosome of the pair contains genes for the same traits. But one chromosome may have a dominant gene and the other a recessive gene. Use the drawing and the chart to answer the questions.

Trait / Dominant Gene / Recessive Gene
Hair Length / Long (L) / Short (l)
Hair Texture / Bristly (B) / Silky (b)
Hair curliness / Curly (C) / Straight (c)
Coat pattern / Spotted (S) / Solid(s)

1. Would the new puppy have a spotted coat? ______

2. What would the puppy’s hair texture be? ______

3. Would the puppy have curly or straight hair? ______

4. Would the puppy be a hybrid or purebred for hair

length? ______

What color is the Pod?

Green (G) is the dominant color for pods of pea plants. Yellow (g) is recessive.

1. What are the phenotypes of the following genotypes?

a. GG______

b. Gg ______

c. gg ______


2. Fill in the Punnett squares below to show the outcomes of the crosses. Next to each genotype write the correct phenotype.

a. GG x Gg b. GG x gg

c. Gg x Gg d. Gg x gg

A Visit to Solaron

On an imaginary planet called Solaron, green skin is dominant over red skin.

1. a. If a hybrid green-skinned male married a hybrid green-skinned female, what is the probability that they will have children with green skin?

b. What is the probability that their children will have red skin?

2. a. If a hybrid green-skinned male married a homozygous red-skinned female. What is the probability that they will have children with green skin?

b. What is the probability that their children will have red skin?