Chapter 22, Problem 15PS

Interpreting Data The Process of Science section in this chapter describes an experiment involving mice with normal and mutant versions of a gene called obese. To determine the role of a specific gene, geneticists often work with two groups of research subjects that differ only in the gene in question (normal version of the gene versus a mutant version of the gene). In this case, researchers compared mice that had two normal copies of the obese gene (called “ob+”) with mice that had two defective copies (called “ob”). The researchers found that mice with two copies of the mutant obese gene (ob/ob) gained significantly more weight than normal (ob+/ob+) mice. Next, researchers sought to determine whether this difference was due to the production of a hormone. To find out, researchers surgically linked the circulatory systems of mice, so that any factor circulating in the blood of one mouse would circulate in the blood of the other. The table below summarizes the results. How do these data support the hypothesis that the obese gene controls the production of a hormone?

Experiment	Genotypes of mice	Average weight gain per mouse (in grams)
(a)	ob+/ob+ paired with ob+/ob+	8.3
(b)	ob/ob paired with ob/ob	38.7
(c)	ob/ob+ paired with ob/ob	8.2