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Chapter 5
Genetic Linkage
and
Chromosome
Mapping
Jones and Bartlett Publishers © 2005
Use of cytologically marked chromosomes shows
that crossing over involves breakage
and reunion of chromosomes
Recombination rates in males and females
• In Drosophila, males do not have any
recombination, so all syntenic genes (those
on one chromosome) always are completely
linked.
• How and why are not known.
• Human males have recombination rates
about 60% of that seen in females.
Unusual inheritance of X-linked genes in
crosses involving female Drosophila with
attached X-chromosomes
The gametes generated
by the 3 kinds of double crossovers
Consequences of a 2-strand double
crossover in a cross involving 3 genes
Chromatid interference
• Sometimes crossing over at one point on a
chromosome interferes with other crossing over
events on the same chromosome:
• Chromatid interference means that there will be
fewer double, triple, etc. crossing over events.
• Interference is greatest over short distances.
Three-point testcross (p. 195)
Parental
Single
Single
Double
Double
Single
Single
Parental
Three-point cross in corn
• Crossovers between lz and su:
Lz su gl
Lz Su Gl
Lz su Gl
lz Su gl
40
33
4
2
79 => 79/740=10.7%
• Between su and gl:
Lz Su gl
lz su Gl
Lz su Gl
lz Su gl
59
44
4
2
109 => 109/740=14.7%
Coefficient of Coincidence
• Chromosome interference is much more
common than chromatid interference.
• i=interference; 1-Coefficient of Coincidence
• CC=observed # double crossovers/predicted
• Predicted: P(single crossover between lz and
su)*P(single crossover between su and gl).
Coefficient of Coincidence
For the previous corn data,
R1=0.107 for lz and su
R2=0.147 for su and gl.
If independent, double crossovers would
occur (R1 x R2)x # of progeny:
0.107 x 0.147 X 740=11.6.
Only 6 double crossovers were observed.
CC=6/11.6=0.51, i=interference=1-0.51=0.49.
Coefficient of Coincidence
• With greater distance between genes,
interference usually disappears.
• In Drosophila, i=0 at about 10 cM
• For most organisms, interference
disappears at about 30 cM (CC=1).
A mapping function corrects for the loss of
detectable recombinants due to multiple crossovers
Three-Point Testcross
Another example (Morgan’s data)
Progeny
Progeny
Phenotype
Genotype
Scute echinus crossveinless
sc ec cv /sc ec cv
4
Wild type
+ + + / sc ec cv
1
Scute
sc + + / sc ec cv
997
Echinus crossveinless
+ ec cv /sc ec cv
1002
Scute echinus
sc ec + / sc ec cv
681
Crossveinless
+ + cv / sc ec cv
716
Scute crossveinless
sc + cv / sc ec cv
8808
Echinus
+ ec + / sc ec cv
8576
Total
Number
20,785
Morgan’s three-point results
• Distance between sc and ec:
(681 + 716 + 4 + 1)/20,785 x 100 = 6.74 cM
Distance between cv and ec:
(1002 + 997 + 4 + 1)/20,785 x 100 = 9.65 cM
Predicted DC=(0.0674 x 0.0965) x 20,785=135.2
CC=5/135.2 = 0.0369; i = 0.9630
Another example:
•
•
•
•
Three linked loci in tomato:
Mottled (m) vs. normal (M) leaf
Smooth (P) vs. pubescent (p) epidermis
Purple (Aw) vs. green (aw) stems
Three Linked Tomato Loci
Progeny phenotype
Normal smooth purple
Mottled pubescent green
Normal smooth green
Mottled pubescent purple
Normal pubescent purple
Mottled smooth green
Mottled smooth purple
Normal pubescent green
Total
Number
(M P Aw)
(m p aw)
(M P Aw)
(m p Aw)
(M p Aw)
(m P aw)
(m P Aw)
(M p aw)
18
15
180
187
1880
1903
400
417
5000
Questions: What are the genotypes of original parents, the gene
order, and map distance between these genes?
There is much less recombination
in heterochromatin compared to euchromatin
Genetic maps are based on % recombination.
Physical maps are based on other methods
such as gel electrophoresis or DNA sequencing