Clearly, if two genes occupy loci on opposite ends of a chromosome, then 100% of the crossover events would result in recombinations between the two genes. However, if the genes were exceedingly close together, then a very small percentage of the cross over events would result in recombinations between the two genes. Geneticists have utilized this fact to establish the positions of genes on chromosomes relative to each other. Some of the genes to be found on the 3rd chromosome pair of Drosophila are shown at the right. Mapping by means of crossing over events is not precise because there are localized regions of enhanced crossing over and, as you might expect, regions of restricted crossing over. Hence, the genes in those regions will show a tendency to be falsely spread or artificially condensed respectively. However, for establishing the relative positions of genes, the method is accessible, convenient and suitable. By the way, you might note that the dominant genes are capitalized, the recessive genes are in lower case. Flip to the next card to set up a mapping exercise.
