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NEL Beyond Mendel 639

Section 19.2

If gene C were the middle gene, then the sum ofthe distances between A and C and C

and B must equal the distance between A and B. These distances are equal. Therefore,

C is in the middle (Figure 7).

AB  BC  AC

12  7  5

Therefore, B is not in the middle.

A 12 B

B 7 C

AC 5

AC  CB  AB

5  7  12

Therefore, C is in the middle.

A 12 B

B 7 C

AC 5

Figure 6

Figure 7

Practice

1. A geneticist observes that the crossover frequency between gene A and gene
   B is 4 %, the crossover frequency between gene B and gene C is 14 %, and
   between gene A and gene C is 10 %. What is the order and relative distances
   of these three genes on the chromosome?

Mapping Chromosomes

A. H. Sturtevant, a student who worked with Thomas Morgan,

hypothesized that

• genes are located in a linear series along a
  chromosome, much like beads on a string,


• genes that are closer together will be separated
  less frequently than those that are far apart,


• and that crossover frequencies can be used to
  construct gene maps.

Sturtevant’s work with Drosophilahelped establish

techniques for chromosome maps.

Procedure

1. Examine the picture of a chromosome (Figure 8,
   next page). Crossing over takes place when breaks
   occur in the chromatids of homologous
   chromosomes during meiosis. The chromatids break
   and join with the chromatids of homologous
   chromosomes. This causes an exchange of alleles
   between chromosomes.

Purpose Design Analysis

Problem Materials Evaluation

Hypothesis Procedure Synthesis

Prediction Evidence

LAB EXERCISE 19.B Report Checklist