Two Point Test Cross Mapping

The results of a testcross study different from Dr. Osterman’s is given below 

Red, Shrunken CCss X White, Plump ccSS 

F1: all Red, Plump (CcSs) 

F1 (CcSs) crossed with White Shrunken (ccss) 


Red, plump:.......     ..120  Red, shrunken:...  ..3420  White, plump:.. ....  3334  White, shrunkend: .. 126 

Total offspring scored were 7000 

Obviously, the geneticist who did this experiment made a lot of test crosses to generate these numbers. 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. With this information all we can do is estimate the map unit distance. Map units are a measure of the tendency for crossovers to occur between two loci. Because genes that are farther apart will have a higher likelihood of crossovers, the higher the crossover frequency, the farther apart the genes are on the chromosome. 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 individual. 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: 

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. 

Therefore, the frequency of recombinant gametes was 246 (120 + 126) 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 = 3.5 Map units. 

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? Lets look at another data set. 

Red, plump CCSS X White, shrunken ccss 

F1: all Red, Plump (CcSs or CS / cs) 

F1 (CS / cs) crossed with White Shrunken (ccss) 


Red, plump:..    .......192  Red, shrunken:..    ......5  White, plump:....     .....3  White, shrunkend:.. 200 

Total offspring scored were 400 

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 measurement reliable? Yes it is, to a certain degree. It is important to recognize the differences between the two test cross experiments.