GOALS   |   BACKGROUND   |   SOLs   |   DEMOS   |   INTERACTIVE   |   EXAMPLES   |   RESOURCES

TITLE:  Genetics: Punnet Squares

GOALS
The purpose of this lesson is to demonstrate how Punnet Squares are used to investigate genetic crosses. This lesson also involves a review of probability and its importance in genetics, as well as a review of introductory genetics principles.

SPECIFIC OBJECTIVES

• The students will understand the role of probability in genetics.
• The students will be able to perform theoretical genetic crosses through the construction and use of Punnet Squares.
• The students will be able to apply probability principles to genetic crosses.
• The students will be able to determine phenotypes and genotypes using Punnet Square crosses.

BACKGROUND

SOLs COVERED
 LS 14 The student will investigate and understand that organisms change over time.  The key concept included in this lesson is that genetic variation can lead to diversity of organisms.

DEMONSTRATIONS AND EXHIBITS
Materials needed:  coin, chalkboard, chalk

1. Probability.  Discuss probability using coin toss example.  Since there are only two options - heads and tails - on a single toss, there is a 50% chance that the coin will come up heads and a 50% chance that it will come up tails.  The coin will not "remember" what happened last time, so the outcome of each flip is independent of all previous outcomes.  (Go to interactive exercise.)

2.  Punnet Squares.  Choose an example of constructing a Punnet Square to show the students.  The illustration shows one parent homozygous for the AA allele and another that is heterozygous (Aa).  Point out that there are two ways to get AA and two to get Aa.  Overall, the average set of offspring for these parents should be 50% AA and 50% Aa.  This is a great place to discuss genotypes and phenotypes again, since in the illustrated example, the offspring will only show one phenotype even though two genotypes are present.  (Go to interactive exercise.)

INTERACTIVE EXERCISES
Materials needed:  One coin for each pair of students
1. Simple Probability.

• Students form pairs and predict the number of "heads" and "tails" they would expect if they tossed a coin 20 times.  They then test their predictions by tossing a coin 20 times and calculating the percentage of each coin face they saw.  They then compare their predictions to their results.  The class can tally the results for all the pairs, and compare the totals to a 50:50 expectation.  This could be a great opportunity to discuss the importance of large sample sizes in experiments!
2. Constructing Punnet Squares.
• Assign students to use different combinations of  "parents" - either homozygous (TT or tt) or heterozygous (Tt) - and have them predict the possible genotypes of the offspring using a Punnet Square.  By observing how many times a genotype is shown in the Punnet Square, students can predict the probability that a single offspring will have a given genotype.  For instance, a cross of TT with tt will result in 100% Tt offspring, but a cross of Tt with Tt will on average yield 25% TT, 50% Tt, and 25% tt offspring.  Instruct students to summarize both genotypes and phenotypes for each of the assigned crosses.

INTERESTING EXAMPLES