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Burdick, A.B. 1999. Trihybrid with duplicate independent factors.
Dros. Inf. Serv. 82: 129. View PDF
Burdick, A.B. 1999. Effect of environment on segregation results.
Dros. Inf. Serv. 82: 130. View PDF
We have made up a stock in which v (1-33.0), cn (2-57.5), and st (3-44.0) are all homozygous and which has eye color indistinguishable from either v, cn, or st, that is, a right vermilion color. We refer to this stock as “Bright” and use it in the student laboratory as an unknown mutant type.
If a virgin Bright (i.e., v;cn;st) female is mated with a wild-type male, the F1 males are Bright and the F1 females are wild type. This leads to the tentative conclusion that Bright is a sex-linked recessive. However, when an F2 is produced from F1 X F1 mating, it is in a ratio of about 2.5 Bright to 1 wild type; or when an F1 female is testcrossed to a Bright male parent, the ratio is 7 Bright to 1 wild type. The Bright trait, which in F1 looked like a sex-linked recessive, now looks somewhat like a dominant.
On the basis of the above data the student may conclude either (1) three duplicate independent genes, one sex-linked, or (2) two duplicate sex-linked genes with about 26% recombination. Test matings of F2 Bright types with Bright parents are interesting if time permits. They yield 1:1, 3:1, and 7:1 ratios and indicate that conclusion (1) is correct.
| Phenotype | At 70° F | At 80° F |
|---|---|---|
| Vg | 50 | 4 |
| + | 48 | 46 |
| C | 4 | 50 |
We have a stock with vg (2-67.0) and c (2-75.5) both homozygous. The flies appear vg at room temperature, but if they are reared at 80°-85°F the vg wings become almost normal and the stock appears c. If the stock is outcrossed to wild type, and F1 females testcrossed to vg c, the following testcross proportion results (Table 1).
Results may vary depending on the temperature used and the ability of the student to detect an “almost normal” vg wing. However, the experiment provides an impressive illustration of gene-environment interaction and, used late in the semester, moderates a student’s faith in genetic ratios as such.