Biologists use Punnett squares to predict the genotype (genetic makeup) and phenotype (observable characteristics) of offspring produced by a parental generation. Punnett squares typically take no longer than 10 minutes to put together and use to solve a basic genetics question.
What you need:
- Pencil or pen
- Basic knowledge of algebra
Problem: In pea plants, round seeds are dominant over wrinkled seeds. You cross one heterozygous seed with a homozygous recessive seed. What is the probability of having a round seed as a result of the cross?
Step 1: Identify the Genotypes That Are Being Crossed
In order to identify the genotypes of that are being crossed, look to the problem to determine whether the parents are homozygous recessive or dominant or heterozygous. Homozygous recessive parents are symbolized with two lowercase letters (for example, tt). Homozygous dominant parents can be symbolized with two capital letters (such as, TT). Using the same letters as in the previous examples, a heterozygous parent would be symbolized with one capital and one lowercase letter (Tt).
As indicated in the sample problem, a heterozygous seed is crossed with a homozygous recessive seed. R can be used to symbolize the dominant allele and r can symbolize the recessive allele. So, Rr is being crossed with with rr.
Step 2: Draw a Punnett Square
The Punnett square is a large square divided into 4 boxes. It is important to set aside space on the top and left side of the Punnett square to be used in a later step.
Step 3: Label the Punnett Square
To label the Punnett square, assign each parental genotype a side of the square (one on top and one on the left side). Each allele of each parental genotype needs to be placed either above or beside a box depending on which side you have chosen for its particular parent.
For the sample problem, place the heterozygous parent on top and the homozygous recessive plant on the left. The top boxes will be labeled as R and r. The side boxes will be labeled as r and r.
Step 4: Determine Possible Offspring by Filling in the Punnett Square
To fill in the Punnett square, take the letters on top and put them into the two boxes beneath them and then take the letters to the left of the square and place them into the boxes to the right. The resulting boxes with two alleles are representative of the possible offspring.
An example of how this would be done with the sample problem is listed in the graphic labeled "Step 4".
Step 5: Determine the Possible Phenotypes of the Offspring
When doing simple Punnett squares, there are three possible outcomes for offspring: heterozygous, homozygous dominant, and homozygous recessive. By looking at these outcomes, determine the phenotypes. Heterozygous and homozygous dominant offspring will both exhibit the dominant phenotype. Homozygous recessive offspring will exhibit the recessive phenotype.
The resulting Punnett square for the sample problem has two boxes with Rr and two with rr. Since round seeds are dominant to wrinkled seeds, the heterozygous offspring will have round seeds while the homozygous recessive will have wrinkled seeds. Thus, only two possible phenotypes have resulted from this cross: round or wrinkled.
Step 6: Answer Question Asked in the Prompt
The last step is to use the Punnett square to answer the question asked in the prompt.
What is the probability of having a round seed as a result of the cross?
Since 2 possible phenotypes have round seeds and 2 possible phenotypes have wrinkled seeds, there is a 50% chance of having a round seed as an offspring.