Page 174 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

166 Chapter 5 Linkage, Recombination, and the Mapping of Genes on Chromosomes

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Figure 5.29 Mitotic crossing-over. (a) In a y sn / y sn Drosophila female, a mitotic crossover between the centromere and sn can produce
two daughter cells, one homozygous for y and the other homozygous for sn, that can develop into adjacent aberrant patches (twin spots). This
outcome depends on a particular distribution of chromatids at anaphase (top). If the chromatids are arranged in the equally likely opposite
orientation, only phenotypically normal cells will result (bottom). (b) Crossovers between sn and y can generate single yellow patches. However, a
single mitotic crossover in these females cannot produce a single singed spot if the sn gene is closer to the centromere than the y gene.
Transient pairing
during mitosis Mitotic metaphase Daughter cells

(a) Crossing-over between sn and the centromere

sn + y Yellow sn + y
sn + y sn + y
Twin
sn y + sn y + spot
sn + y sn y Singed sn y +
+
or

sn y + sn + y Wild type sn y +
sn + y sn y +
Normal
sn y + sn + y tissue
sn y + Wild type sn + y

(b) Crossing-over between sn and y

sn + y Yellow sn y Yellow
sn + y sn y spot

sn + y + sn y + Normal
sn + y + + +
sn y Wild type sn y tissue
or

sn y + sn + y Wild type sn y +
sn + y sn y +
Normal
sn + y + sn y tissue
+
sn + y Wild type sn y

Figure 5.30 Mitotic recombination during the growth of happened during a cell division early in the growth of the
diploid yeast colonies can create sectors. Arrows point to large, colony, giving the resulting daughter cells a long time to
red ade2 / ade2 sectors formed from ADE2 / ade2 heterozygotes. proliferate. If a red sector is small, the recombination hap-
Image courtesy of B.A. Montelone, Ph.D. and T.R. Manney, Ph.D pened later.

Mitotic Recombination Has Significant
Consequences

Problem 51 at the end of this chapter illustrates how ge-
neticists use mitotic recombination to obtain information
about the locations of genes relative to each other and to the
centromere. Mitotic crossing-over has also been of great
value in the study of development because it can generate
animals in which different cells have different genotypes
(see Problem 52 and also Chapter 19). Finally, as the
Genetics and Society Box Mitotic Recombination and
Cancer Formation explains, mitotic recombination can
have major repercussions for human health.