Two Point Test Cross Mapping

The results of a potential testcross experiment as we described is given below.

  • Red, Plump CCSS X White, Shrunken ccss
  • F1: all Red, Plump (CcSs)
  • F1 (CcSs) crossed with White, Shrunken (ccss)


Table 1. Testcross offspring data from CcSs x ccss cross.
Phenotype Number of progeny

Red, Plump


Red, Shrunken


White, Plump


White, Shrunken





With a total number of offspring at 7,000 in Table 1, we know the geneticist who did this experiment made a lot of test crosses to generate this data. How can we use the numbers to map these genes? With this information we can answer one question: 'What is the distance between the C,c and S,s loci?' We cannot get out a fancy microscopic ruler and physically measure the distance on the corn chromosome. However, we can use this information and data to estimate the map unit distance between the loci that give rise to our traits. Map units are a measure of the tendency for crossovers to occur between two loci. Because genes that are further apart will have a higher likelihood of crossovers, the genes that are far apart will have a higher crossover frequency and therefore have a higher map unit distance between them. Let’s apply this idea to our test cross data.

Test cross data allows us to indirectly measure the frequency of gametes made by an individuals All of the testcross progeny inherited a gamete with the recessive 'c' and 's' alleles from the White, Shrunken parent. Therefore, the alleles that the F1 dihybrid parent has passed on determine the traits in the seed. We need to be able to measure how often crossovers occurred between the C,c and S,s loci when these dihybrids made gametes. From the data we have the following knowledge:

  • Gametes that were passed to the F1 from the parent lines were CS and cs.
  • Gametes made from crossing over in the F1 (recombinant gametes) were Cs and cS.
  • Gametes with the original parent combination (parental gametes) were CS and cs.

Looking at Table 1, we identify that the offspring with a recombinant gamete are the Red, Shrunken and the White, Plump kernels. This is because their phenotype does not match either parent, so this new trait combination is the result of a crossover. Map units are the measure of how frequently crossovers occur between two loci, so we count up the number of kernels with recombination of the two traits (120 Red, Shrunken + 126 White, Plump = 246 total recombinant kernels) divided by the total gametes we have information on (7000) or 246/7000 = 3.5%

Map unit distance between the C,c and S,s loci is equal to 3.5 map units. It reflects that in this data set, we got a recombination of our gametes between the C,c and S,s loci in about 3.5% of our gametes.

One map unit is equal to 1% recombinant gametes. Again, this is not a physical measurement. It is a relative measure of how often crossovers occurred between these loci. How reliable is this measurement? Let’s look at another data set (Table 2).

  • Red, Shrunken CCss X White, Plump ccSS
  • F1: all Red, Plump (CcSs or Cs / cS)
  • F1 (Cs / cS) crossed with White, Shrunken (ccss)


Table 2. Test cross offspring data from a Cs / cS X ccss cross.


# of progeny

Red, Plump


Red, Shrunken


White, Plump


White, Shrunken




Recombinant gamete frequency: 5 + 3 / 400 = 2%; 2 map units from C,c to S,s

Based on this result is the map unit distance calculation reliable? Yes, it is, to a certain degree. It is important to recognize the differences between these two test cross experiments.