Meiosis & Genetic Diversity (College Board AP Biology)

A research team has genetically engineered a strain of fruit flies to eliminate errors during DNA replication. The team claims that this will eliminate genetic variation in the engineered flies. A second research team claims that eliminating errors during DNA replication will not entirely eliminate genetic variation in the engineered flies.

Provide ONE piece of evidence that would indicate new genetic variation has occurred in the engineered flies.

Describe ONE mechanism that could lead to genetic variation in the engineered strain of flies.

During meiosis, double-strand breaks occur in chromatids. The breaks are either repaired by the exchange of genetic material between homologous nonsister chromatids, which is the process known as crossing over (Figure 1A), or they are simply repaired without any crossing over (Figure 1B). Plant breeders developing new varieties of corn are interested in determining whether, in corn, a correlation exists between the number of meiotic double-strand chromatid breaks and the number of crossovers.

Figure 1 Double-strand breaks in chromatids are repaired with crossing over (A) or without crossing over (B)

Using specialized staining and microscopy techniques, scientists counted the number of double-strand chromatid breaks and the number of crossovers in the same number of meiotic gamete-forming cells of six inbred strains of corn (Table 1).

TABLE 1. NUMBER OF CHROMATID DOUBLE-STRAND BREAKS AND AVERAGE NUMBER OF CROSSOVERS IN INBRED STRAINS OF CORN

Strain of Corn	Number of Double-Strand Breaks	Average Number of Crossovers ()
I	710	19.5  0.5
II	650	18.0  0.7
III	600	17.5  1.0
IV	510	16.0  1.0
V	425	14.0  0.5
VI	325	11.0  1.5

The double-strand breaks occur along the DNA backbone. Describe the process by which the breaks occur.

Using the template in the space provided for your response, construct an appropriately labeled graph that represents the data in Table 1 and allows examination of a possible correlation between double-strand breaks and crossovers. Based on the data, determine whether corn strains I, II, and Ill differ in their average number of crossovers.

Based on the data, describe the relationship between the average number of double-strand breaks and the average number of crossovers in the strains of corn analyzed in the experiment.

Crossing over (Figure 1A) creates physical connections that are required for proper separation of homologous chromosomes during meiosis. A diploid cell with four pairs of homologous chromosomes undergoes meiosis to produce four haploid cells. Crossing over occurs between only three of the pairs. Predict the number of chromosomes most likely present in each of the four haploid cells. Provide reasoning to justify your prediction. Explain how plant breeders can use the information in Table 1 to help develop new varieties of corn.