Question

A geneticist discovers an obese mouse in his ca 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 laboratory in ny also discovers an obese mouse in her laboratory colony. she carries out the same crosses as those done by the first geneticist and obtains the same results. she also concludes that obesity in mice results from a recessive allele. the geneticists meet and share their mice with each other. they both find that when they cross an obese ca mouse with an obese ny mouse, all the offspring are normal. explain their results.

Answer 1

The first geneticist has identified an obese allele that he or she believes to be recessive. We will define his or her allele as o1 and the normal allele as O1. The obese allele appears to be recessive based on the series of crosses he or she performed.

Cross 1 with possible genotype:  

 Obese (o1o1) × Normal (O1O1)  F1 All normal (O1o1)

Cross 2 with possible genotypes:

 F1 normal (Oo1) × F1 normal (O1o1)  F2 8 normal (O1O1 and O1o1)

       2 obese (o1o1)  

Cross 3 with possible genotypes:

 Obese (o1o1) × Obese (o1o1)  All Obese (o1o1)

 A second geneticist also finds an obese mouse in her colony and performs the same types of crosses, which indicate to her that the obese allele is recessive. We will define her obese allele as o2 and the normal allele as O2.

 The cross of obese mice between the two different laboratories produced only normal mice. These different alleles are both recessive. However, they are located at different gene loci. Essentially, the obese mice from the different labs have separate obesity genes that are independent of one another.

 The likely genotypes of the obese mice are as follows:

Obese mouse 1 (o1o1O2O2) × Obese mouse 2 (O1O1o2o2)  

 F1 All normal (O1o1O2o2)