Experiment 3: The Third Map Distance

A third map distance needs to be estimated in order to determine the linkage map of these three loci. Since we’re uncertain of the location of the W loci with respect to the S loci, we need to make a cross that gives us the opportunity to measure crossing over frequency between the S,s and W,w loci (Table 4). One such cross is shown below.

  • Parent: ssWW (Shrunken, Normal) x SSww (Plump, Waxy)
  • F1: sW / Sw x sW / Sw
Table 4. Offspring data from a sW / Sw x sW / Sw cross.


Number of progeny

Plump, Normal:


Plump, Waxy:


Shrunken, Normal:


Shrunken, Waxy:




  • Recombinant gametes - 646 + 630 = 1276
  • Map distance S,s to W,w: 1276/7000 = 18.2 map units


Based on the third map distance our best map for all three loci is the second alternative (Fig. 12). Conducting more mapping experiments that involve other genes with loci in this part of maize chromosome 3 can confirm the correctness of our gene or linkage map. To map all three loci using this two-point test cross data, the geneticist needed to conduct three different crossing experiments. This is how gene maps in organisms such as corn have been generated. As new genes controlling traits that control an observable phenotype are discovered, crosses can be made to test if these genes are independent or linked to other genes. Once linkage is determined, additional crosses can be analyzed to position the gene on the linkage map.