Chapter 12, Problem 21P

Semisterility in corn, as seen by unfilled ears with gaps due to abortion of approximately half the ovules, is an indication that the strain is a translocation heterozygote. The chromosomes involved in the translocation can be identified by crossing the translocation heterozygote to a strain homozygous recessive for a gene on the chromosome being tested. The ratio of phenotypic classes produced from crossing semisterile F₁ progeny back to a homozygous recessive plant indicates whether the gene is on one of the chromosomes involved in the translocation. For example, a semisterile strain could be crossed to a strain homozygous for the yg mutation on chromosome 9. (The mutant has yellow-green leaves instead of the wild-type green leaves.) The semisterile F₁ progeny would then be backcrossed to the homozygous yg mutant.

a.	What types of progeny (fertile or semisterile, green or yellow-green) would you predict from the backcross of the F1 to the homozygous yg mutant if the gene was not on one of the two chromosomes involved in the translocation?
b.	What types of progeny (fertile or semisterile, green or yellow-green) would you predict from the backcross of the F1 to the homozygous mutant if the yg gene is on one of the two chromosomes involved in the translocation?
c.	If the yg gene is located on one of the chromosomes involved in the translocation, a few fertile, green progeny and a few semisterile, yellow-green progeny are produced. How could these relatively rare progeny classes arise? What genetic distance could you determine from the frequency of these rare progeny?