Genetic problem!!! help !!

A geneticist discovered 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. 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 form a recessive allele. 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, 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. Explain their results.

Let’s call obese gene o (recessive) and normal gene O (dominant). Then obese mouse has oo genotype and normal wt mouse has Oo genotype.

Cross of F0 normal wt to obese mouse: OO x oo
would result in all Oo (normal heterozygous) – F1 – consistent with observed results.

Now, cross Oo x Oo would result in 1OO (normal wt homozygous): 2Oo (normal heterozygous): 1oo (obese homozygous) – phenotypic ratio of 3 normal: 1 obese – observed ratio is 8 normal: 2 obese or 4:1 close enough to expected.

Cross between oo x oo would result in all oo (obese) – just as observed.

Now imagine that even though results are the same for 2 mice, genes are different. Let’s call obese gene from the 1st mouse o and normal O while obese gene from the 2nd mouse q and normal Q. Any mouse that has either oo or qq genotype will be obese, however, the obese mice from the 1st lab will be of ooQQ genotype, while obese mice from the 2nd lab will be OOqq genotype.

Cross between the two: ooQQ x OOqq will result in all OoQq normal heterozygous mice!

If you need to explain it in terms of the pathway, imagine this scenario:

Gene O codes for enzyme involves in fat degradation/metabolism so when it is non-functional (in oo genotype), stored fat cannot be broken down and mouse becomes obese.

Gene Q codes for transcription factor of O, then if Q is normal, gene O is transcribed and mouse is normal. However, when this gene is non-functional (in qq), gene product of O cannot be produced because of the absence of the TF and mouse becomes obese.

Good luck!

Thank you so much!

You are welcome!