Mapping Another Seed Trait Gene Pair
A third seed trait in corn is normal vs. waxy starch. Waxy starch has a different chemistry than normal starch and can be scored by staining with iodine. This gene is commercially important because some waxy corn is produced for specialty markets. Lets look at the following test cross data:
| Parent: Red, Normal CCWW | X | White, waxy ccww |
| F1: Red, Normal CcWw | X | White, Waxy ccww |
| Testcross progeny: | |
|---|---|
| Red, normal: | 2781 |
| Red, waxy: | 759 |
| White, normal: | 749 |
| White, waxy: | 2711 |
| Total: | 7000 |
| Parental gametes made by the F1: | CW and cw |
| Recombinant gametes made by the F1: | Cw and cW 759 + 749 = 1508 |
| Map unit distance between the C,c and the W,w loci: | 1508/7000 = 21.5 Map Units |
It is clear from the testcross data that the genes at the C,c and W,w loci do not have as great a tendency to be inherited together compared to the genes at the C,c and S,s loci. This would be because the loci are farther apart and our 21.5 map units is an indicator of the relative distance.
From the two distances calculated, how far apart are the W,w and S,s loci? Here is where we can try to use map unit distances to do gene mapping the same way we use miles to do road mapping. We can pencil our two possible maps (Fig. 8) In one map the C,c locus is in between the W,w and S,s loci. Using the 21.5 plus 3.5 map units we have calculated, the best estimate will be that 25 map units separate the W,w and S,s loci. The other map that fits our data is having the S,s locus in the middle with the W,w and S,s loci 18 map units apart (21.5 - 3.5). If the genes occupy a fixed position on the chromosome, only one map will be correct. How can we determine which is the correct map?
Fig. 8. Two possible linkage maps from two point data. (Image credit: D. Lee)