Question

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 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 others 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.

Answer 1

Option C, Both alleles are recessive, but they must be located at different gene loci.

Step-by-step explanation:

It is given in the paragraph that when obese mice from two laboratories are crossed with another obese mice of the same group, then all the offspring are obese. This clearly indicates that mice would be having recessive allele.

For instance let obese gene be represented by O1 for laboratory 1 and as O2 for laboratory 2. Let the normal gene be N1 and N2 for laboratory 1 and 2 respectively.

Thus, cross between tow homozygous recessive species will produce recessive offspring

O1 O1 * O1O1

O1O1, O1O1, O1O1, O1O1

O2O2* O2O2

O2O2, O2O2, O2O2, O2O2

However, when two obese mice from different laboratories are crossed, then all the offspring are normal

i.e

O1O1 * O2O2

O1O2, O1O2, O1O2, O1O2

Being at two different loci, these obese genes could not express themselves and hence all the offspring are normal.

Hence, option C is correct