Tetrad Analysis - Fungal Genetics
Ascomycetes - meiotic spores in ascus
Ex. Sordaria, Neurospora, Saccharomyces
perithecium
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Tetrad Analysis - Fungal Genetics
Haploid mycelia or
single cells
Tetrad
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Life Cycle of Saccharomyces cerevisiae
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Unordered Tetrads
Saccharomyces cerevisiae
From Al Kapp’s comic ‘Li’l Abner’ 1943
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Unordered Tetrads
Saccharomyces cerevisiae
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Ordered Tetrads
Sordaria, Neurospora
Each chromatid
- one spore
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Ordered Tetrads
Sordaria, Neurospora
Each cell from meiosis is
represented by two identical spores
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Tetrad Analysis
Evaluate by direct examination or tetrad dissection
Sordaria fimicola
tan, black gray, white
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Classify Ascus (ordered, unordered) - Spore Phenotypes
Parents crossed: tan x gray
t g zygote
Parental ditype (PD)
also t g t g, g t t g, etc.
Nonparental ditype (NPD)
also b w w b, b w b w, etc.
Tetratype (TT)
also b w g t, b t w g, etc.
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Determining Number of Genes Involved
Parents crossed: tan x gray
# genes for color?
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Ascus Types Expected - Independent Assortment
If Unlinked,
PD = NPD
> TT
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Ascus Types Expected - Linked Genes
More frequent, fewer COs
PD
TT
PD
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Ascus Types Expected - Linked Genes
If linked,
Less frequent, more COs
PD >> NPD
TT
TT
NPD
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Determining MU Between Linked Genes
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Distance Between Genes - Sample Problem
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Distance Between Gene and Centromere
Only possible to determine with Ordered Tetrads
MI segregation
MI ascus
MII segregation
MII ascus
No CO
CO between
Gene and Cen
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Distance Between Gene and Centromere
Only possible to determine with Ordered Tetrads
Distance Gene to Cen =
1/2 (# MII asci) x 100
total
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Distance Gene and CEN - Sample Problem
Where did the
crossovers occur?
Distance:
Aa to Cen?
Bb to Cen?
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Fungal Genetics - Sample Problem
Classify
PD, TT, NPD
Linked?
PD>>NPD?
Distance:
Kk - Ll
Kk and Cen?
Ll and Cen?
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Determining Gene Order
MII means CO has occurred between gene and centromere
Relative frequency: Lowest DCO; Intermediate SCO
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Determining Gene Order
Possible Orders
COs required for
MII both genes
2 CO
1 CO
1 CO
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Determining Gene Order
When MII for both is lowest frequency, genes are on opposite sides.
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Determining Gene Order
When MII for both is intermediate, genes are on the same side.
Which gene is farthest away?
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Determining Gene Order
MII for both genes
Intermediate frequency
MI for inner, MII for outer
Intermediate frequency
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Determining Gene Order - Sample Problem
Number of CO?
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Ordered Tetrad Analysis - Sample Problem
1. Classify as PD, NPD, TT.
2. Classify as MI, MII for each gene.
3. Determine linkage, gene order.
4. Calculate distances between genes, each gene and centromere.
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Ordered Tetrad Analysis - Tracking Crossovers
Where did
crossovers
occur?
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Mechanism - Homologous Recombination
Point to point pairing, breakage and rejoining
How does this occur?
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Mechanism - Homologous Recombination
Model must explain:
1) Conversion Asci
2) CO intermediates observed
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Holliday Model - Homologous Recombination
First step,
nicks in backbones
of two ds DNAs
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Holliday Model - Homologous Recombination
Exchange
strands
Strand
migration
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Holliday Model - Resolution of Chiasma
Nick Outer Strands
Cross Over Obvious
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Holliday Model - Resolution of Chiasma
Nick Inner Strands
No Cross Over Observed
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Holliday Model - Resolution of Heteroduplexes
Mispairing within heteroduplexes (hybrid regions)
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Holliday Model - Resolution of Heteroduplexes
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Holliday Model - Overview in Text
Box 15.1
Russell, 2008
Available On Line
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