Objectives
Predict the outcomes of genetic crosses using Punnett squares
Explain what is meant by independent assortment
Key Words independent assortment probability Punnett square
Independent Assortment
Gregor Mendel did not stop experimenting with pea plants once he discovered the concepts of dominance and alleles. He continued his quest to understand inheritance patterns by adding more complexity to his investigations. Instead of concentrating on only one trait, he began to explore several traits at a time. He wondered if the presence of one trait had any effect on another trait. Were traits linked or did they pass from parent to offspring independently of one another?
Explore Mendel’s approach to answering this question. Create a table in your science notebook like the one shown below. Be sure to leave enough room in each section to take notes about each of the generations involved in Mendel’s experiment.
Generation
Notes
P1

F1 F2 1. How many different phenotypes were found in the F2 generation? Four
2. Did the traits have any effect on one another? No, the traits were independently.
3. What is the principle of independent assortment? Traits are sorted independently of another so that the one trait is not linked to another.
Probability and Punnett’s Square
If you flip a coin, you have a 50% chance that it will land heads up and a 50% chance that it will land tails up. That means that out of 100 coin tosses, you are likely to end up with heads 50 times and with tails 50 times.
Probability is the calculated frequency of the occurrence of an event. It is based on the number of different possibilities. If there are only two possible outcomes of an event, each will have a 50% probability of occurring at any one time.
The segregation of alleles is somewhat like flipping a coin. Since there are two copies of each gene and only one copy goes into a gamete, there is a 50% probability that a gamete will receive one copy and a 50% probability that it will receive the other copy.
Biologists use probability to make predictions about genetic crosses. Learn how a grid called a Punnett square is used for this purpose. Click on the link below to watch the “Punnett’s Square” Discovery Education™ streaming movie. Take notes so that you can answer the following questions.
1) What does the Punnett square look like for the mating of a purebred short-haired cat (SS) with a purebred long-haired cat (ss)? What is the probability that the cats