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, but intercrosses between his obese mice produced only obese mice.
Can you predict the nature of this mutation (recessive or dominant), which causes obesity in mice? Call this allele O1.
A second geneticist at UT Austin discovers the same phenomena by carrying out the same crosses as those done by the first geneticist. 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.