Stickleback Genetics Lab!

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.

• The null statistical hypothesis is that the observed outcome of 351 fish with pelvic spines and 97 fish without pelvic spines is not significantly different from the expected 3:1 ratio; any deviation from the expected 3:1 ratio likely occurred by chance alone.

• The null statistical hypothesis is that the observed outcome of 351 fish with pelvic spines and 97 fish without pelvic spines is significantly different from the expected 3:1 ratio; deviation from the expected 3:1 ratio likely occurred due to a factor unaccounted for.

Which of the family crosses that you did, REJECTS the hypothesis that pelvic or no pelvic spine is controlled by one gene with two alleles or the offspring will show a 3:1 ratio?

• the total population of fish with a chi-square value of 2.679

• family 7 with a chi-square value of 4.158

• family 8 with a chi-square value of 3.947

• family 1 with a chi-square value of 0.34