Chapter 6:Genetic Linkage and Mapping in
Eukaryotes
6.1 Overview of Linkage
6.2 Relationship between Linkage and
Crossing Over
6.3 Genetic Mapping in Plants and Animals
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©Stanley K. Sessions
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HS-LS3-1. Develop and use a model to show how DNA in the form of
chromosomes is passed from parents to offspring through the processes of
meiosis and fertilization in sexual reproduction.
HS-LS3-2. Make and defend a claim based on evidence that genetic variations
(alleles) may result from (a) new genetic combinations via the processes of
crossing over and
random segregation of chromosomes during meiosis, (b) mutations that occur
during replication, and/or (c) mutations caused by environmental factors.
Recognize that mutations that occur in gametes can be passed to offspring.
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❖ Describe why genetic mapping is useful.
❖ Calculate the map distance between linked genes
using data from a testcross.
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What is genetic linkage?
How does the mutant gene differ from the wild
type gene?
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What do you think is the difference between
physical mapping and genetic mapping?
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Genetic mapping refers to the process of determining the order
and relative distance between genetic markers on a chromosome
from their pattern of inheritance.
Physical mapping refers to the technique used to find the order
and physical distance between DNA base pairs by DNA markers.
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Genetic Mapping in Plants and Animals
Each gene has its own unique locus
Genetic mapping (also known as gene mapping or
chromosome mapping) is performed to determine the linear
order of linked genes along the same chromosome
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Simplified Genetic Linkage Map of Drosophila melanogaster
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How do you map a genome?
What is genetic mapping?
list 5 reasons why genetic mapping are
useful?
What is the relationship between rate of
recombination and distance between two
genes?
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Uses of Genetic Maps
1. They allow us to understand the overall complexity
and genetic organization of a particular species
2. They can help molecular geneticists to clone genes
3. They improve our understanding of the evolutionary
relationships among different species
4. They can be used to diagnose, and perhaps,
someday to treat inherited human diseases
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Uses of Genetic Maps
5. They can help in predicting the likelihood that a
couple will produce children with certain
inherited diseases
6. They provide helpful information for improving
agriculturally important strains through
selective breeding programs
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Genetic Maps Allow Us to Estimate the Relative
Distances Between Linked Genes
The relative distance between linked genes is based on
the likelihood that a crossover will occur between them.
Experimentally, the percentage of recombinant
offspring is correlated with the distance between the
two genes
• Genes that are far apart result in many
recombinant offspring
• Close genes result in very few recombinant
offspring
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Crossing Over and Genetic Mapping
How does test cross help detect
linkage?
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How do you map a genome?
A testcross is one with one of the parents
being homozygous recessive. All the progeny
exhibits the possible combinations of traits in
equal ratio if the alleles are not linked and the other
parent f the original cross is heterozygous. Any
significant deviation from this denotes the
possibility of linkage
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Genetic Maps Allow Us to Estimate the Relative
Distances Between Linked Genes
The units of distance are called map units (mu); they
are also referred to as centiMorgans (cM)
One mu is equivalent to 1% recombination frequency
(RF)
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Example: testcross with two linked genes (s and e) affecting
bristle length and body color in fruit flies
s = short bristles and s+ = normal bristles
e = ebony body color and e+ = gray body color
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Determine the test cross??
Find recombinant and nonrecombinant?
Calculate map distance?
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Testcross and Genetic Linkage Map
Genetic mapping experiments are typically
accomplished by a conducting a testcross, where an
individual that is heterozygous for two or more genes is
crossed to one that is homozygous recessive for the
same genes
Example: testcross with two linked genes (s and e)
affecting bristle length and body color in fruit flies
s = short bristles and s+ = normal bristles
e = ebony body color and e+ = gray body color
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Testcross to Distinguish Recombinant and
Nonrecombinant Offspring
Access the text alternative for slide images.
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Testcross to Distinguish Recombinant and
Nonrecombinant Offspring
Testcross data can be used to estimate the
distance between the two genes
Map distance = ((75 + 76) / (542 + 537 + 76 + 75 )) ×
100 = 12.3 map units
The s and e genes are 12.3 map units apart from
each other along the same chromosome
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Recombination and Map Distance
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Three-Factor Crosses
Data from three-factor crosses can also yield additional information
about map distance and gene order
The following experiment outlines a common strategy for using
three-factor crosses to map genes
In this example, fruit flies that differ in body color, eye color and wing
shape are considered
• b = black body color
• b+ = gray body color
• pr = purple eye color
• pr+ = red eye color
• vg = vestigial wings
• vg+ = long wings
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Three-Factor Crosses
Data from three-factor crosses can also yield additional information
about map distance and gene order
The following experiment outlines a common strategy for using
three-factor crosses to map genes
In this example, fruit flies that differ in body color, eye color and wing
shape are considered
• b = black body color
• b+ = gray body color
• pr = purple eye color
• pr+ = red eye color
• vg = vestigial wings
• vg+ = long wings
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Step 1: Crossbreed Flies Homozygous for 3 Alleles
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Step 2: Perform Test Cross
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Step 2: Perform Test Cross
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Step 3: Collect Data from a Three-Factor Cross
Phenotype
Number of Observed
Offspring (Males and
Females)
Gray body, red eyes, long
wings
411
Gray body, red eyes,
vestigial wings
61
Gray body, purple eyes, long
wings
2
Gray body, purple eyes,
vestigial wings
30
Black body, red eyes, long
wings
28
Black body, red eyes,
vestigial wings
1
Black body, purple eyes, long
wings
60
Black body, purple eyes,
vestigial wings
412
Total
1005
Chromosome Inherited
from F1 Female
Access the text alternative for slide images.
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Map the Genes
Analysis of the F2 generation flies allows you to map
the three genes
The three genes exist as two alleles each
• Therefore, there are 23 = 8 possible combinations of
offspring
If the genes assorted independently, all eight
combinations would occur in equal proportions
• It is obvious from the data that they are far from
equal
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Map the Genes
In the offspring of crosses involving linked genes
• Nonrecombinant phenotypes occur most frequently
• Double crossover phenotypes occur least frequently
• Single crossover phenotypes occur with
“intermediate” frequency
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Step 4: Calculate the Map Distance Between Pairs of
Genes
One strategy is to regroup the data according to pairs of
genes
• From the P generation, we know that the dominant
alleles are linked, as are the recessive alleles
• Nonrecombinant F2 offspring have a pair of dominant or a pair
of recessive alleles
• Recombinant F2 offspring have one dominant and one
recessive allele
• The regrouped data will allow us to calculate the map
distance between the two genes
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• The regrouped data will allow us to calculate the map
distance between the two genes
Phenotype
Number of Observed
Offspring (Males and
Females)
Gray body, red eyes, long
wings
411
Gray body, red eyes,
vestigial wings
61
Gray body, purple eyes, long
wings
2
Gray body, purple eyes,
vestigial wings
30
Black body, red eyes, long
wings
28
Black body, red eyes,
vestigial wings
1
Black body, purple eyes, long
wings
60
Black body, purple eyes,
vestigial wings
412
Total
1005
Chromosome Inherited
from F1 Female
Access the text alternative for slide images.
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Step 4: Calculate the Map Distance Between Pairs of
Genes
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Step 4: Calculate the Map Distance Between Pairs of
Genes
Parental offspring
Gray body, red eyes
(411 + 61)
Black body, purple eyes
(412 + 60)
Total
Nonparental Offspring
Total
472
72
944
Total
Gray body, purple eyes
(30 + 2)
32
Black body, red eyes
(28 + 1)
29
Total
61
The map distance between body color and eye color is
Map distance = ( 61 / (944 + 61) ) × 100
= 6.1 map units
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Step 4: Calculate the Map Distance Between Pairs of
Genes
Nonrecombinant
offspring
Gray body, normal wings
(411 + 2)
Total
413
Black body, vestigial wings
(412 + 1)
413
Total
826
Recombinant Offspring
Total
Gray body, vestigial wings
(30 + 61)
91
Black body, normal wings
(28 + 60)
88
Total
179
The map distance between body color and eye color is
Map distance = ( 179 / (826 + 179) ) × 100
= 17.8 map units
Note: This value is a little low because it doesn’t take
double crossovers into consideration.
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Step 4: Calculate the Map Distance Between Pairs of
Genes
Parental offspring
Red eyes, normal wings
(411 + 28)
Total
439
Nonparental Offspring
Total
Red eyes, vestigial wings
(61 + 1)
62
62
Purple eyes, vestigial wings
(412 + 30)
442
Purple eyes, normal wings
(60 + 2)
Total
881
Total
124
The map distance between body color and eye color is
Map distance = (124 / (881 + 124) ) × 100
= 12.3 map units
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Step 5: Construct the Map
The body color and wing shape genes are farthest apart
based on the map unit calculation
• The eye color gene is in the middle
The data are consistent with the map being drawn as
b − pr − vg (from left to right)
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