Rest%20of%20Chapter%205

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Rest of Chapter 5

• Human recombination studies
• Mapping by tetrad analysis in fungi
• Analysis of ordered tetrads
• Other features of recombination.

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Human pedigrees and mapping

• In humans, progeny numbers are small.
• Matings cannot be arranged for experiments.
• Coupling (AB/ab) vs. repulsion (Ab/aB)
  heterozygotes cannot be distinguished most of the
  time.
• Pedigrees can be pooled.

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Lod scores
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Fungi in genetic studies

• Haploid organisms can have genetic maps made by
  using spore analysis.
• No testcrosses are needed.
• (review of fungal biology)

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The life cycle of the bakers yeast
(Saccharomyces cerevisiae)
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Types of tetrads
PD parental ditype NPD Nonparental
ditype (no linkage, PDNPD) (with linkage,
PDgtNPD) TT Tetratype tetrad
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Tetrad analysis of unlinked genes using unordered
asci
For unlinked genes, parental ditype (PD) (having
2 kinds of spores) and nonparental ditype (NPD)
asci are produced in equal proportion
For unlinked genes, recombination between one of
the genes and its centromere produces tetratype
asci (TT) having 4 kinds of spores
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Tetrad analysis results for linked genesin
unordered tetrads-1
No crossovers or 2-strand double crossovers
result in parental ditype (PD) asci. One
recombination between the genes results in
tetratype (TT) asci
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Tetrad analysis results for linked genesin
unordered tetrads-2
Three-strand double crossovers give the same
result as a single crossover, tetratype asci
(TT). Four-strand double crossovers give
non-parental ditype (NPD) asci. As a result, for
linked genes, PD gtgt NPD.
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Mapping with unordered tetrads

• Map distance(½)TT-2NPD4NPD/total
• (½)TT3NPD/total of tetrads x 100

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A branch diagram for analyzing unordered tetrads
data
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The life cycle of an ascomycete fungus with
ordered tetrads
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Analysis of ordered tetrad data
Ordered tetrads allow one to map the distance
between a gene and its centromere. No crossover
between a gene and its centromere gives first
division segregation. A crossover between a gene
and its centromere gives a second division
segregation.
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Other types of recombination studies

• Recombination within genes
• Mitotic recombination

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Fine structure mapping allows mapping the
internal structure of a locus. Cistron defines
one genetic function mutations that fail to
complement The term comes from cis and trans.
Lozenge gene in Drosophila
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• X lzBS lzg v X
• X ___________ x _______________gt Y
• ct lzg
• ct-lz7.7 cM
• lz-v5.3 cM
• 134 out of 16,000 progeny had normal eyes
• Frequency of recombination0.008
• What happened?
• X ct in all nonlozenge females and
• X lzBSlzg v in 5 males with new lz
  phenotype.

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Somatic recombination (mitotic crossover) can
lead to twin spots