Passage II

During prophase I of meiosis, homologous chromosomes frequently exchange segment in a process called crossing over. As a result, genes on homologous chromosomes recombine, forming new allele combinations along chromosomes (see Figure 1).

Because the frequency of recombination (RF) increases as the map distance (distance along a chromosome, in map units [mu]) between 2 genes increases, RF can be used to estimate the map distance between genes on a chromosome. However, as the map distance between 2 genes increases, the probability of multiple crossovers increases: Multiple crossovers decrease the apparent RF between 2 genes, resulting in RF values that underestimate map distance. To compensate for this effect, researchers use a mapping function to better estimate the map distance between 2 genes based on their RF (see Figure 2).

Four researchers performed a series of experiments to determine the RF for various pairs of genes on a chromosome. They then used the mapping function to determine the map distance between each pair. The results appear in Table 1.

Each of the 4 researchers then proposed a model that is consistent with the results in Table 1. Each model shows how the genes might be located along the chromosome (see Figure 3). Each model correctly assumes the lengths of the genes are short enough that they can be ignored when calculating the map distance between genes.

Figure 3

Later, a fifth researcher working with the same chromosome and the same genes determined that the RF for Genes A and C is 0.091.