Making Lots of Gametes
Sexually reproducing organisms tend to make a large number of gametes to insure reproductive success. Therefore we need to think about crossing over events happening in many cells. Geneticists do not understand all the details of crossing over but in general, crossing over is a random process that occurs during prophase I. We also know that the occurrence of one crossover along a chromosome can reduce the chance of a second crossing over. As you look at Fig. 4, you can see that crossing over could occur at any point between where the C,c genes (the C,c locus) and the S,s locus reside on the chromosome causing recombinant gamete formation. If a crossover does not occur between the C,c and S,s loci (plural for locus), then only the parental gamete combination of CS and cs will be made. Considering the size of the chromosome and the relative positions of the C,c and S,s loci; how often would cells have crossovers occur between the genes compared to somewhere else along the chromosome? If crossing over is random, we would not expect a cross over to occur in very many cells between the C,c and S,s loci. Genes that are on the same chromosome tend to stay together at a rate that is proportional to how far apart they are on the chromosome. Since the C,c and S,s loci are close to each other, crossovers are rare. The 'C' gene tends to stay with the 'S' gene as it is passed from the red, plump line to the F1 and then to the testcross generation. The same is true of the 'c' and 's' parental combination.
How does this discussion of crossing over help us understand what we see from our test cross result? The parental combinations of red, plump and white shrunken were almost always made. The new combinations of red, shrunken and white, plump were rarely observed. It would take a recombinant gamete to make one of these rare combinations. While we never see the genes in the gametes or directly observe the genes on the chromosomes, the outcome suggests that these loci are linked close together on the chromosome. Making the connection between the results of crossing studies such as these has allowed geneticists to map genes or determine their relative positions on a chromosome. Let’s go through a mapping study to see how it is done.