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1 LECTURE 5: LINKAGE
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LECTURE 5: LINKAGE
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Linked genes, recombination, and chromosomal mapping
Mendel's Law of Independent Assortment is a consequence of the fact that chromosomes segregate independently in meiosis
Take two individualsOne heterozygous and one homozygous
AaBb x aabb
ab
AB
aB
Ab
ab
AaBb
aaBb
Aabb
aabb
25%
25%
25%
25%
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Chromosome alignment in MeiosisI
These results are readily explained by the two alternative ways the chromosomes can line up on the metaphase plate during meiosis I:
A a
B b
OR
A a
b B
AB ab Ab aB
Because the A and B genes assort independently, AaBb dihybrids constructed from different parental genotypes will behave the same.
AABBxaabb AAbbxaaBB
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AaBb x aabb
ab
AB
aB
Ab
ab
AaBb
aaBb
Aabb
aabb
25%
25%
25%
25%
ab
AB
aB
Ab
ab
AaBb
aaBb
Aabb
aabb
25%
25%
25%
25%
AABB x aabb AAbb x aaBB
AaBb x aabb
Aa
Bb
Aa
bB
AaBb
OR
Test Cross
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Why 50:50Why not 25:25:25:25
A hypothetical dihybrid cross involving the genes A and C produced the following results:
· A= Tall a= short
· C= Cream c = white
Cross I: Cross II:
Tall, Cream x short, white Tall, white x short, Cream AACC aacc AAcc aaCC
Tall, Cream AaCc Tall, Cream AaCc
X X
short, white aacc short, white aacc
50% Tall, Cream 50% Tall, white
50% short, white 50% short, Cream
Why 50:50? Why not 25:25:25:25?
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In these crosses, independent assortment is not occurring.
For example in the first cross, the alleles Tall and Cream behave as if they are linked to one another.
Similarly in the second cross the alleles Tall and white appear as if they are linked to one another.
These results are readily explained if the genes A and C lie next to one another on the same chromosome:
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Cross I:
A= Tall a= short
C= Cream c = white
A-C
A-CTall cream
a-c
a-cShort white
A-C
a-c
a-c
a-c
a-c
A-C
a-c
A-C
a-cTall cream
a-ca-c Short white
X
X
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Cross II:
A= Tall a= short
C= Cream c = white
A-c
A-cTall white
a-C
a-CShort cream
A-c
a-C
a-c
a-c
a-c
A-c
a-C
A-c
a-cTall white
a-Ca-c
Short cream
X
X
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Purple vestigial
Morgan performed the following experiments in Drosophila to determine if the genes pr and vg were linked.
PR+ = normal red eyes pr = purple eyes
VG+ = normal wings vg = vestigial wings
P PR+PR+ VG+VG+ x prpr vgvg
F1 PR+pr VG+vg x prpr vgvg
If they are on different chromosomes they should assort independently
If they are next to one another on the same chromosome they should not assort independently
pr vg
PR+ VG+
PR+ vg
pr VG+
pr vg
25%
25%
25%
25%
PR+ VG+pr vg
PR+ vgpr vg
pr VG+pr vg
pr vgpr vg
pr vg
~50%
~0%
~0%
~50%
PR+ VG+
PR+ vg
pr VG+
pr vg
PR+ VG+pr vg
PR+ vgpr vg
pr VG+pr vg
pr vgpr vg
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When Morgan performed this cross, he obtained the following result:
pr vg
PR+ VG+
PR+ vg
Pr VG+
pr vg
~44%
~6%
~6%
~44%
PR+ VG+pr vg
PR+ vgpr vg
Pr VG+pr vg
pr vgpr vg
1005
968
153
143
Although the non- parental classes are present, their frequencies are dramatically reduced from that expected from independent assortment.
The two loci are linked !!!
How do we explain the presence of non-parental classes?
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The chromosomes that have gone through this crossover are known as crossover products or recombinants. The original chromosomes and those that have not undergone a crossover are known as parental.
Evidence for the model that chromosomes physically exchange during meiosis is found in meiotic structures known as chiasmata.
During meiosisI when homologs pair, non-sister chromatids appear to cross with each other. The resulting cross-shaped structure is known as a chiasmata.
pr vg
PR+ VG+
PR+ VG+ pr vg
pr vg
pr vg
PR+ VG+
PR+ VG+
pr VG+
pr vg
PR+ vg
PR+ VG+
Crossoverchromosome
Morgan suggested that when homologous chromosomes pair during meiosis I, the chromosomes occasionally exchange parts
P
F1
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Crossing-over through the microscope
Duplicated homologous chromosomes
Synapsis in meiosis I
Crossing over betweenNon-sister chromatids
Anaphase in meiosisISegregation of homologouschromosomes
Haploid gametes in meiosis II
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Answer:
To explain this we need to define the terms parental and recombinant:
Parents: AB/AB x ab/ab
Gametes:
AB
ab
F1: AB/ab
Meiosis produces the following gametes:
ABAbaBab
Recombinant gametes are those with different allelic combinations than those gametes of the previous generation.
How does one determine whether two genes reside on different chromosomes or reside on the same chromosome as linked genes?
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Coupling/repulsion
Genes located on the same pair of homologous chromosomes are called LINKED GENES
Therefore when the A and C alleles are introduced from one parent they are physically located on the same chromosome and they do not assort independently. We say that they are linked.
In the above cross we say that the A and C genes are linked.
Therefore when we write the genotype of a dihybrid for two linked genes, there are two possible conformations:
AaCc
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Coupling/repulsion- PHASE
Results like these led Morgan to suggest that the A and C genes are located on the same pair of homologous chromosomes.
Therefore when the A and C alleles are introduced from one parent they are physically located on the same chromosome and they do not assort independently. We say that they are linked.
In the above cross we say that the A and C genes are linked.
Therefore when we write the genotype of a dihybrid for two linked genes, there are two possible conformations:
AaCc
----A----C------o-----
----a----c------o----- Coupling conformation
(linkage of two dominant
or two recessive alleles)
---A----c-------o-----
---a----C-------o----- Repulsion conformation
(linkage of a dominant and recessive allele)
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If genes A and B are on different chromosomes:
25% Parental
25% Parental
25% Recombinant
25% Recombinant
Test crossprogeny
P
Gamete
F1 diploid
(tester)
A
A
B
B a
a
b
b
A B a b
A
a
B
b
a
a
b
b
a
A B
b
a b
a b
A b
a b
a B
a b
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Genes A and B are linked on the same chromosome
> 25% Parental
> 25% Parental
< 25% Recombinant
< 25% Recombinant
Test crossprogeny
P
Gamete
F1 diploid
(tester)
A-B
A-B
a-b
a-b
a-bA-B
A-B
a-b
a-b
a-b
A-B
a-b
a-b
a-b
A-b
a-b
a-B
a-b
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Recombination frequency
IF a crossover occurred between linked genes each time homologs paired, the recombinant frequency would be 50%
This is because crossing-over involves only two of the four chromatids on the metaphase pair (each of the paired homologs consists of two sister chromatids).
For example, the frequency of recombinant gametes between linked genes A and B is 50% if crossing-over occurred each time the homologs paired.
A B
A Ba b
a b
A B
A b
a B
a b
parental
parental
recombinant
recombinant
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However there are many instances in which the homologs pair and crossing over does not occur between genes A and B.
It occurs somewhere else
Consequently the overall frequency of recombinants is significantly reduced from 50%
A B
A Ba b
a b
A B
A B
a b
a b
parentalparental
parental
parental
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A B
A Ba b
a b
A B
A B
a b
a b
A B
A b
a B
a b
A B
A Ba b
a b
A B
A Ba b
a b
A B
A B
a b
a b
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Distance
The larger the distance between two genes residing on the same chromosome, the higher the probability there is that a crossover event will occur between them.
That is for any chromosome, there is a fixed probability per given distance on the chromosome that a crossover event will event.
Sturtevant realized that this property could be used to map genes with respect to one another. For each pair of genes on a chromosome a recombination frequency can be determined.
By determining the recombination frequency between many pairs of genes on a chromosome, the relative distance between genes and the order of the genes on the chromosome can be determined.
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Distance
The larger the distance between two genes residing on the same chromosome, the higher the probability there is that a crossover event will occur between them.
That is for any chromosome, there is a fixed probability per a given distance on the chromosome that a crossover event will event.
Sturtevant realized that this property could be used to map genes with respect to one another. For each pair of genes on a chromosome a recombination frequency can be determined.
By determining the recombination frequency between many pairs of genes on a chromosome, the relative distance between genes and the relative order of the genes on the chromosome can be determined.
On average a car breaks down every 40 miles
Santa Cruz to San Francisco is 80 miles= 2 breakdowns
Santa Cruz to Monterrey is 45 miles = 1 breakdown
There is greater probability that your car will break down between Santa Cruz and San Francisco.
If crossing-over occurs once every 50 kb of DNA, then there is greater probability of a crossover between two genes 100 kb apart than two genes 50 kb apart.
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For example Sturtevant identified three recessive mutations that reside on the X chromosome of Drosophila
W+ red eyes w- white eyes
CV+ normal wings cv- crossveinless
SN+ normal bristle sn- singed bristle
By calculating recombination frequencies between each pair of
genes we can begin to establish where these three genes reside
on the X chromosome with respect to one another
X chromosome
CenTel
w cvsn
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To determine the distance between the w gene and the sn gene
P w sn/w sn x W+ SN+/Y
F1 w sn/W+ SN+ x w sn/Y
F2 w sn y
w sn
W+ SN+
w SN+
W+ sn
White eyeSinged bristle88
White eyeSinged bristle92
Red eyeNormal bristle102
Red eyeNormal bristle96
White eyeNormal bristle24
White eyeNormal bristle23
Red eyeSinged bristle24
Red eyeSinged bristle23
Parental
Recomb
Fill out the phenotypes- recombinants can be determined by phenotype analysis
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Recombination frequency equals the number of recombinants over total number of progeny
white eye red eye
# recombinant progeny = normal bristle + singed bristle
# total progeny # total progeny
= 24+24+23+23=94/472
1 map unit (m.u.) = 1% recombination frequency
Therefore _19.92% or ~20cM or ~20 m.u. separate the W+ and SN+ genes.
This is a relative distance- depends upon recombination between two genes. Not an absolute distance like bp
In the above cross, we could have determined recombination frequency by counting only males (or only females)
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w -------20--------sn
The next issue is where does cv map with respect to w and sn:
By crosses similar to those described above, we find that there are 7 m.u. between cv and sn
This means cv can map to either one of two positions:
A)
w ____________14?______________cv_________7___ sn
OR
B)
w______________20____________ sn_________7____ cv
These models can be distinguished by determining the map distance between w and cv. Recombination analysis indicates 14 m.u. between w and cv.
By determining the map distance between w and cv and the map distance between cv and sn, we can determine the distances and order of all three genes.
Which map is consistent with this distance?
Internal inconsistency- 14+7=21 not 20
Map gives you order of genes but not PRECISE distance
