Experiment 2: Mapping Another Seed Trait Gene Pair (Waxy Starches)

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. Let’s look at the following test cross data (Table 3):

Parent: Red, Normal CCWW x White, Waxy ccww

F₁: Red, Normal CcWw x White, Waxy ccww

• 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 is because the loci are further 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 use map unit distances to create a gene map (or linkage map) the same way we use miles to create road mapping. We can draft our two possible maps (Fig. 12). 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 calculated, the best estimate is 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?