A geneticist discovers an obese mouse in his laboratory colony. He breeds this obese mouse with a normal mouse. All the F1 mice from this cross are normal in size. When he interbreeds two F1 mice, eight of the F2 mice are normal in size and two are obese. The geneticist then intercrosses two of his obese mice, and he finds that all of the progeny from this cross are obese. These results lead the geneticist to conclude that obesity in mice results from a recessive allele. Call this allele O1.

A second geneticist at a different university also discovers an obese mouse in her laboratory colony. She carries out the same crosses as the first geneticist did and obtains the same results. She also concludes that obesity in mice results from a recessive allele. Call this allele O2.

One day the two geneticists meet at a genetics conference, learn of each other's experiments, and decide to exchange mice. They both find that, when they cross two obese mice from the different laboratories, all the offspring are normal; however, when they cross two obese mice from the same laboratory, all the offspring are obese. Select the answer that gives the best explanation for these results.

a. The O1 allele is recessive, but the O2 allele is dominant.
b. The O2 allele is recessive, but the O2 allele is dominant.
c. Both alleles are recessive, but they must be located at different gene loci.
d. Both alleles are dominant, but they must be located at different gene loci