A geneticist from an alien planet that prohibits genetic research brought with him to Earth two pure-breeding lines of frogs. One line croaks by uttering “rib-it rib-it” and has purple eyes. The other line croaks more softly by muttering “knee-deep knee-deep” and has green eyes. With a newfound freedom of inquiry, the geneticist mated the two types of frogs, producing F1 frogs that were all utterers and had blue eyes. A large F2 generation then yielded the following ratios:
27/64 | blue-eyed, “rib-it” utterer |
12/64 | green-eyed, “rib-it” utterer |
9/64 | blue-eyed, “knee-deep” mutterer |
9/64 | purple-eyed, “rib-it” utterer |
4/64 | green-eyed, “knee-deep” mutterer |
3/64 | purple-eyed, “knee-deep” mutterer |
(a) How many total gene pairs are involved in the inheritance of both traits? Support your answer.
(b) Of these, how many are controlling eye color? How can you tell? How many are controlling croaking?
(c) Assign gene symbols for all phenotypes and indicate the genotypes of the P1 and F1 frogs.
(d) Indicate the genotypes of the six F2 phenotypes.
(e) After years of experiments, the geneticist isolated pure-breeding strains of all six F2 phenotypes. Indicate the F1 and F2 phenotypic ratios of the following cross using these pure-breeding strains:
blue-eyed, “knee-deep” mutterer × purple-eyed, “rib-it” utterer
(f) One set of crosses with his true-breeding lines initially caused the geneticist some confusion. When he crossed true-breeding purple-eyed, “knee-deep” mutterers with true-breeding green-eyed, “knee-deep” mutterers, he often got different results. In some matings, all offspring were blue-eyed, “knee-deep” mutterers, but in other matings all offspring were purple-eyed, “knee-deep” mutterers. In still a third mating, 1/2 blue-eyed, “knee-deep” mutterers and 1/2 purple-eyed, “knee-deep” mutterers were observed. Explain why the results differed.
(g) In another experiment, the geneticist crossed two purpleeyed, “rib-it” utterers together with the results shown here:
9/16
purple-eyed, “rib-it” utterer
3/16
purple-eyed, “knee-deep” mutterer
3/16
green-eyed, “rib-it” utterer
1/16
green-eyed, “knee-deep” mutterer
What were the genotypes of the two parents?
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