Eukaryotic Chromosome Mapping

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Eukaryotic Chromosome Mapping

• Using Genetic Recombination to Estimate Distances
  Between Genes

2
Linked Genes
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Independent Assortment vs. Gene Linkage

• Example from Drosophila
• Red eyes, x Pink eyes
• Beige body Ebony body
• RRBB rrbb

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Independent Assortment vs. Gene Linkage

• Testcross cross to individual of
  known genotype
• F1Red eyes X Pink eyes
• Beige body Ebony body
• RrBb rrbb

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Independent Assortment vs. Gene Linkage
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Independent Assortment vs. Gene Linkage

• F1Red eyes X Pink eyes
• Beige body Ebony body
• RrBb rrbb

RB
Rb
rb
rB
rb
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Independent Assortment vs. Gene Linkage
If genes are linked Red eyes, x
Pink eyes Beige body Ebony body
Coupling or Cis Configuration
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Independent Assortment vs. Gene Linkage

• F1 Red eyes, Beige body

Four types of gametes are produced
Parental Recombinant
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Independent Assortment vs. Gene Linkage

• F1Red eyes X Pink eyes
• Beige body Ebony body
• R B r b
• r b r b

r B
R B
r b
R b
r b
r b
r b
r b
r b
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Independent Assortment vs. Gene Linkage
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Genetic Map Units

• 1 recombination 1 map unit 1
  centimorgan
• 108 112 x 100 22
• 1000
• These genes are located 22 map units apart on
  the same chromosome.

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Limits of Genetic Mapping

• Genes that are 50 map units apart will appear
  to assort independently.

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Predicting Gamete Frequencies for Linked Genes
Red eyes, x Pink eyes Ebony body
Beige body
Repulsion or Trans Configuration
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Predicting Gamete Frequencies for Linked Genes

• The genes are 22 map units apart, therefore we
  expect 22 recombinant gametes and 78 parental
  gametes.

0.22 recombinants
0.78 parentals
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Using a Three-point Testcross to Determine
Genetic Distance

• A cross between two parental strains is used to
  produce a tri-hybrid (heterozygous for three
  genes).
• The tri-hybrid is crossed to an organism that is
  homozygous recessive for all three genes.
• Eight classes of offspring are analyzed to
  determine recombination frequencies.

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Problem 1, Page 2-1
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Problem 1, Page 2-1 With Arbitrary Gene Order
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Problem 1, Page 2-1With Arbitrary Gene Order
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Problem 1, Page 2-1
Offspring Resulting from Three-Point Testcross
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Problem 1, Page 2-1

• Determine which classes are parentals
• Determine which classes are double
  recombinants
• Determine the gene order
• Assign genotypes to all classes
• Calculate recombination frequencies

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Problem 1, Page 2-1

• Determine which classes are parentals
• The two parental classes will represent
• the largest number of offspring in the
• F2 generation.
• Information on the parents may be
• given in the problem description itself.

Parentals red, tall, ragged
green, dwarf, normal
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Problem 1, Page 2-1

• Determine which classes are double
• recombinants
• Double recombinants have two
• crossovers one between the first and
• middle gene and one between the middle
• and third gene
• These will be the two smallest classes.

Double Recombinants red, tall, normal
green, dwarf, ragged
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Problem 1, Page 2-1

• Determine the gene order
• The middle gene is the one that changes
• places in the double recombinants when
• compared to the parental combinations.

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Problem 1, Page 2-1
This shows why other gene orders are incorrect.
A d Rg
A D Rg
Red, dwarf, ragged
X
X
a d rg
a D rg
Green, tall, normal
D a Rg
D A Rg
Green, tall, ragged
X
X
d A rg
d a rg
Red, dwarf, normal
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Problem 1, Page 2-1

• Assign genotypes to all classes
• Use correct gene order

P
A Rg D/a rg d
a rg d/a rg d
P
A rg D/a rg d
DC
a Rg d/a rg d
DC
A? Rg
A rg d/a rg d
A? Rg
a Rg D/a rg d
Rg? D
A Rg d/a rg d
Rg? D
a rg D/a rg d
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Problem 1, Page 2-1
Recombination between A and Rg
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Problem 1, Page 2-1
Recombination between A and Rg
A rg d 90
Single Crossovers
a Rg D 70
Double Crossovers
A rg D 24
a Rg d 16
Total 200
recombination (200/1000) x 100 20
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Problem 1, Page 2-1
Recombination between Rg and D
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Problem 1, Page 2-1
Recombination between Rg and D
A Rg d 120
Single Crossovers
a rg D 140
Double Crossovers
A rg D 24
a Rg d 16
Total 300
recombination (300/1000) x 100 30
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Problem 1, Page 2-1
Two maps are possible
A Rg
D 20 map units 30 map units
or
D Rg
A 30 map units 20
map units
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Interference

• Interference crossover in one region inhibits
  crossover in an adjacent region
• Interference 1 (coefficient of coincidence)
• Coefficient of coincidence
• Observed double crossovers
• Expected double crossovers

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Calculating Interference

• Coefficient of coincidence
• Observed double crossovers
• Expected double crossovers
• 24 16 40 0.667
• 0.2 x 0.3 x 1000 60
• Interference
• 1(coefficient of coincidence)
• 1- 0.667 0.333