3 FUNDAMENTALS OF LINKAGE ANALYSIS
13 Nondisjunction and recombination during mitosis generate mosaics
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- A mosaic is an organism that contains cells of
different genotypes.
- A twin spot is a pair of mutant sectors within
wild-type tissue. It results from mitotic crossing-over in an ancestral heterozygous
cell.
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- Chromosome rearrangements including
nondisjunction and recombination can occur during mitosis.
- These result in mosaic organisms.
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Figure 3.21
Nondisjunction can occur in mitosis, generating a mosaic in which some cells have
extra chromosomes and others are missing chromosomes.
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The same rearrangements and segregation defects that occur during meiosis can also
occur in mitotic cells. For example, nondisjunction in the equational division of
mitosis produces two aneuploid daughter cells (Figure 3.21). One cell is functionally
monosomic, while the other is trisomic. Although these are usually inviable if they
occur in autosomes during early development, again chromosome 21 provides an exception.
Mitotic nondisjunction early in embryogenesis can lead to trisomy 21 in a fraction
of the cells, generating a genetic mosaic for Down syndrome. The symptoms may be
less pronounced than for individuals who have a complete trisomy 21. However, mitotic
nondisjunction late in development or in the adult may give rise to only small patches
of cells that are mosaic and the phenotype may be mild.
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Figure 3.22
Mitotic recombination occurs between a marker and its centromere and results in
homozygosis of the chromosome arm distal to the crossover.
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Homologous recombination can also occur during mitosis, although it is much less
common than in meiosis. In 1936, Curt Stern examined female flies that were heterozygous
for the X-linked genes y (yellow body) and sn (singed bristles).
These flies of genotype y sn+/ y+ sn had wild-type
brown bodies and normal bristles. However, some individuals showed patches of yellow
or singed bristles on their bodies, and in some cases, there was a twin spot of
yellow body adjacent to singed bristles (Figure 3.22). These mosaics resulted from
mitotic recombination between homologous chromosomes. This is relatively uncommon
in most species, but happens at a discernable frequency in Drosophila.
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In order to observe a mitotic recombination event, the individual must be heterozygous,
and the crossover must occur between the gene and the centromere on the same chromosome.
Crossovers distal to the heterozygous gene will not affect its segregation. For
a single crossover, the net effect of mitotic recombination is to render the alleles
of all genes distal to the crossover point homozygous in the daughter cells. As
in meiotic recombination, mitotic recombination is sensitive to relative distance.
The closer two genes are to one another, the less likely it is that a crossover
will occur between them, so that they will tend to segregate together. Similarly,
the closer a gene is to its centromere, the less likely a mitotic crossover will
take place in that interval.
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The probability of a double crossover in a restricted region in mitotic recombination
is less than the probability of a single crossover, just as the case in meiotic
recombination. For example, the only way to get a spot that is singed without also
getting an adjacent yellow spot is if two crossovers occur: one between sn
and the centromere, and the second between sn and y on the
same pair of sister chromatids. In the original experiment, Stern observed a very
high frequency of twinspots, a slightly lower frequency of yellow- only
spots, and very few singed spots. This showed that the order of markers
was cen-sn-y. Thus, mitotic recombination can also be used to generate
a genetic map.
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Last Revised on April 12, 2004
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©Jones and Bartlett Publishers (2007)
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