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638 Chapter 19 NEL

Map Units

long feelers short feelers

long wings dumpy wings

grey body

long legs short legs

black body

red eyes purple eyes

long wings

straight wings curved wings

red eyes brown eyes

vestigial wings

Normal Characteristics Mutant Characteristics

0

13

48.5

54.5

67

75.5

104.5

31

Figure 4
Gene mapping of chromosome 2 for
Drosophila melanogaster. Note that
many genes are located on one
chromosome.

From crosses between different Drosophila, a geneticist finds that the crossover frequency

between gene Aand gene Bis 12 %, the crossover frequency between gene Band gene C

is 7 %, and between gene Aand gene Cis 5 %. What is the order and relative distances of

these three genes on the chromosome?

Solution

If gene A were in the middle, then the sum of the distances between B and A and A

and C must equal the distance between B and C. These distances are not equal, so A

is not in the middle (Figure 5).

SAMPLE exercise 1

BA  AC  BC

12  5  7

Therefore, A is not in the middle.

B 12 A

C

5

B 7

AC

Figure 5

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

between B and C must equal the distance between A and C. These distances are not

equal, so B is not in the middle (Figure 6, next page).