## Cracking the AP Biology Exam

# 15

# Laboratory

## LAB 7: GENETICS OF ORGANISMS

In this lab, the fruit fly, *Drosophila melanogaster*, was used to study genetic crosses in both F1 and F2 generations. Fruit flies that are homozygous for normal traits (also called wild-types) were crossed with mutant fruit flies bearing recessive traits. Mutations such as vestigial wings and white-colored eyes are examples of recessive traits that were studied.

In biology, the most common way to determine whether the observed frequencies markedly differ from the frequencies that we would expect by chance is to do a chi square analysis. The chi-square test (χ^{2}) is used to test the hypothesis that no association exists. Note: chi square is more likely to establish significance to the extent that (1) the relationship is strong, (2) the sample size is large, and/or (3) the number of values of the two associated variables is large. Generally, a probability of .05 or less is considered to be a significant difference and is usually a reason to reject the null hypothesis. However, high values of χ^{2} may be due to a number of factors such as deleterious mutations or crossing over, and must be taken into account when you evaluate your results.

Chi square (χ^{2}) is calculated by finding the difference between each observed and expected frequency for each possible outcome, squaring them, dividing each by the theoretical frequency and taking the sum of the results.

What are the general concepts you *really* need to know?

- Review the discussion of genetics in this book. Know how to use a Punnett square to determine the results in both the F1 and F2 generations.
- Know how to apply this information to design other genetic experiments.