Eukaryotic Chromosome Mapping

1
Eukaryotic Chromosome Mapping

• Using Genetic Recombination to Estimate Distances
  Between Genes

2
Linked Genes
Mendels experiments Linked Genes
Gene location Genes on separate chromosomes Genes on the same chromosome
Gamete types Equal numbers of all possible allele combinations More parental combinations than recombinant combinations
3
Independent Assortment vs. Gene Linkage

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

4
Independent Assortment vs. Gene Linkage

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

5
Independent Assortment vs. Gene Linkage
F2 phenotype Number of Offspring Expected for Unlinked Genes
Red eyes Beige body 398 250
Pink eyes Ebony body 382 250
Red eyes Ebony body 108 250
Pink eyes Beige body 112 250
6
Independent Assortment vs. Gene Linkage

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

RB
Rb
rb
rB
RrBb Red Beige Rrbb Red Ebony rrBb Pink Beige rrbb Pink Ebony
rb
7
Independent Assortment vs. Gene Linkage
If genes are linked Red eyes, x
Pink eyes Beige body Ebony body
Coupling or Cis Configuration
8
Independent Assortment vs. Gene Linkage

• F1 Red eyes, Beige body

Four types of gametes are produced
Parental Recombinant
9
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
r b
r b
r b
r b
10
Independent Assortment vs. Gene Linkage
F2 phenotype Number of Offspring Chromosome arrangement
Red eyes Beige body 398 RB//rb Parental
Pink eyes Ebony body 382 rb//rb Parental
Red eyes Ebony body 108 Rb//rb Recombinant
Pink eyes Beige body 112 rB//rb Recombinant
11
Genetic Map Units

• 1 recombination 1 map unit 1
  centimorgan
• These genes are located ___ map units apart on
  the same chromosome.

12
Limits of Genetic Mapping

• Genes that are 50 map units apart will appear
  to assort independently.
• The calculated distance between any TWO genes
  on the same chromosome should be less than 50 map
  units.

13
Predicting Gamete Frequencies for Linked Genes
Red eyes, x Pink eyes Ebony body
Beige body
Repulsion or Trans Configuration
14
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
15
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.

16
Problem 1, Page 2-1

• In corn, a strain homozygous for the recessive
  alleles a (green), d (dwarf) and rg (normal
  leaves) was crossed to a strain homozygous for
  the dominant alleles of each of these genes,
  namely A (red), D (tall) and Rg (ragged leaves).
  Offspring of this cross were then crossed to
  plants that were green, dwarf and had normal
  leaves. The following phenotypic classes were
  observed.

17
Problem 1, Page 2-1
Offspring Resulting from Three-Point Testcross
18
Problem 1, Page 2-1
A Red
a Green
D Tall
d Dwarf
Rg Ragged leaves
rg Normal leaves
19
Problem 1, Page 2-1 With Arbitrary Gene Order
20
Problem 1, Page 2-1With Arbitrary Gene Order
21
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.

22
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
23
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.

24
Problem 1, Page 2-1
This shows why other gene orders are incorrect.
25
Problem 1, Page 2-1

• Assign genotypes to all classes
• Use correct gene order

Contribution of F1 parent
26
Problem 1, Page 2-1
Recombination between A and Rg
Single Crossovers
Double Crossovers
Recombination
27
Problem 1, Page 2-1
Recombination between Rg and D
Single Crossovers
Double Crossovers
Recombination
28
Problem 1, Page 2-1
Two maps are possible
29
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

30
Calculating Interference

• Coefficient of coincidence
• Observed double crossovers
• Expected double crossovers
• Interference
• 1(coefficient of coincidence)