In Part 3 of this activity, you evaluated whether the ratio of fish with spines to fish with no spines suggests that the inheritance of the trait follows the inheritance of a phenotype controlled by a single gene with a dominant and recessive allele. That question is easy to answer if the F2 ratio is a perfect 3:1, but what if the ratio is 2.6:1 or 3.4:1? Are such differences from a 3:1 ratio a true variation from the expected 3:1 ratio, which would mean a different pattern of inheritance, or could these differences simply have occurred by chance? The chi-square test can help us answer this question. Remember that with our stickleback experiment the experimental hypothesis we were testing was that the pelvic spine trait follows the inheritance of a phenotype encoded by a single gene with two alleles. A statistical test does not test the experimental hypothesis but rather what is called the null hypothesis. In the case of the chi-square test, the null hypothesis is that the observed and expected outcomes are the same and that any deviations between them occurred by chance.
State the null hypothesis. Use the numbers of the total F2 generation results in Table 3 in your response.