Page 173 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

5.6 Mitotic Recombination and Genetic Mosaics 165

Recombination can also occur during mitosis. Unlike what containing tissues of different genotypes. Individual yel-
happens in meiosis, however, mitotic crossovers are initi- low or singed patches could arise from chromosome loss or
ated by mistakes in chromosome replication or by chance by mitotic nondisjunction. These errors in mitosis would
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exposures to radiation that break DNA molecules, rather yield XO cells containing only y (but not y ) or sn (but not
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than by a well-defined cellular program. As a result, mi- sn ) alleles; such cells would show one of the recessive
totic recombination is a rare event, occurring no more often phenotypes.
than once in a million somatic cell divisions. Nonetheless, The twin spots must have a different origin. Stern
the growth of a colony of yeast cells or the development of reasoned that they represented the reciprocal products of
a complex multicellular organism involves so many cell mitotic crossing-over between the sn gene and the cen-
divisions that geneticists can detect these rare mitotic tromere. The mechanism is as follows: During mitosis in
events routinely. a diploid cell, after chromosome duplication, homolo-
gous chromosomes occasionally—but rarely—pair up
with each other. While the chromosomes are paired, non-
Twin Spots Indicate Mosaicism Caused by sister chromatids can exchange parts by crossing-over.
The pairing is transient, and the homologous chromo-
Mitotic Recombination somes soon resume their independent positions on the
In 1936, the Drosophila geneticist Curt Stern inferred the mitotic metaphase plate. There, the two chromosomes
existence of mitotic recombination from observations of can line up relative to each other in either of two ways
twin spots in fruit flies. Twin spots are adjacent islands of (Fig. 5.29a). One of these orientations would yield two
tissue that differ both from each other and from the tissue daughter cells that remain heterozygous for both genes
surrounding them. The distinctive patches arise from and are thus indistinguishable from the surrounding
homozygous cells with a recessive phenotype growing wild-type cells. The other orientation, however, will gen-
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amid a generally heterozygous cell population displaying erate two homozygous daughter cells: one y sn / y sn ,
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the dominant phenotype. In Drosophila, the yellow (y) mu- the other y sn / y sn. Because the two daughter cells
tation changes body color from normal brown to yellow, would lie next to each other, subsequent mitotic divisions
while the singed bristles (sn) mutation causes body bristles would produce adjacent patches of y and sn tissue (that
to be short and curled rather than long and straight. Both of is, twin spots). Note that if crossing-over occurs between
these genes are X-linked. sn and y, single spots of yellow tissue can form, but a
In his experiments, Stern examined Drosophila XX fe- reciprocal singed spot cannot be generated in this fashion
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males of genotype y sn / y sn. These double heterozy- (Fig. 5.29b).
gotes were generally wild type in appearance, but Stern
noticed that some flies carried patches of yellow body
color, others had small areas of singed bristles, and still Sectored Yeast Colonies Can Arise
others displayed twin spots: adjacent patches of yellow from Mitotic Recombination
cells and cells with singed bristles (Fig. 5.28). He assumed
that mistakes in the mitotic divisions accompanying fly Diploid yeast cells that are heterozygous for one or more
development could have led to these mosaic animals genes exhibit mitotic recombination in the form of
sectors: portions of a growing colony that have a differ-
ent genotype than the remainder of the colony. If a dip-
loid yeast cell of genotype ADE2 / ade2 is placed on a
petri plate, its mitotic descendents will grow into a col-
Figure 5.28 Twin spots: A form of genetic mosaicism. In ony. Usually, such colonies will appear white because the
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a y sn / y sn Drosophila female, most of the body is wild type, but
aberrant patches showing either yellow color or singed bristles dominant wild-type ADE2 allele specifies that color.
sometimes occur. In some cases, yellow and singed patches are However, many colonies will contain red sectors of dip-
adjacent to each other, a configuration known as twin spots. loid ade2 / ade2 cells (Fig. 5.30). These cells are red
because a block in the adenine biosynthesis pathway
causes them to accumulate red pigment. The red sectors
arose as a result of mitotic recombination between the
ADE2 gene and its centromere. (Homozygous ADE2 /
ADE2 cells will also be produced by the same event, but
they cannot be distinguished from heterozygotes because
both types of cells are white.)
The size of a red sector relative to the size of the col-
ony as a whole indicates when mitotic recombination took
Single yellow spot Twin spot Single singed spot place. If a red sector is relatively large, mitotic recombination