Crosses were made between all possible combinations of the single factor lines, and the F1 ears were self-pollinated to produce F2 ears. Intercrosses among three of the single factor lines yielded only nonmutant ears in the F2, but crosses of these three single factor lines with the fourth single factor line gave all F2 ears segregating 15:1. Thus, we succeeded in isolating the two duplicate factors as single factor lines. I name these two factors brn1 and brn2. brn1 is the factor that was located to the short arm of chromosome 3; brn2 remains unmapped.
The question remains as to why when brn1 was originally isolated, it was thought to be only a single factor. brn1 originally arose in a Robertsonís Mutator population that had been propagated by crossing an active Mutator line alternate generations to the two non-Mutator hybrids Standard B70 (B77 X B79) and Standard Q60 (Q66 X Q67). Apparently, these two hybrid lines are already homozygous mutant for the other factor, brn2, so when the brn1 mutation occurred, it segregated 3:1 on a self-pollinated ear. Outcrosses of brn1 to Standard B70, Standard Q60, and M14/W22 all give F2ís segregating 3:1, proving that they are homozygous for brn2. Similarly, the TB-3Sb line used in the arm-locating cross as well as the linkage stocks used for mapping brn1 must also have been homozgyous for brn2. Crosses to inbred B73 gave F2ís segregating 15:1; therefore B73 is homozygous nonmutant for both factors. Crosses are being planned to confirm the brn2 status of the Robertson hybrid lines.
brn1 brn2 is not the first duplicate
factor pair discovered upon crossing a mutant into B73. Two other mutants
previously considered to be single factor traits, su3 (now su3
su4; Stinard. 1997. MNL 71:83) and ns1 (now ns1 ns2;
Scanlon and Freeling. 1995. MNL 69:23), were also shown to have duplicate
factor inheritance upon crossing into B73.
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