I. Genes found on the same chromosome = linked genes

Genetic recombination in
Eukaryotes: crossing over, part 1
I. <strong>Genes</strong> <strong>found</strong> on the same
chromosome = linked genes
II. Linkage and crossing over
III. Crossing over &
chromosome mapping
I. <strong>Genes</strong> <strong>found</strong> on the same
chromosome = linked genes
Conflicting cytological evidence, only a
few dozen chromosomes/individual – so
must be several genes per chromosome
Testcross experiments revealed gene
linkage – observed deviations from the
expected 1:1:1:1 ratio based upon
independent assortment
If a testcross is done
and the genes are on
separate
chromosomes:
Chromosome is the unit of
transmission, not the gene
Aa/Bb x aa/bb
Aa/Bb
aa/bb
Aa/bb
aa/Bb
2 genes, located
on different
chromosomes,
will segregate
independently.
• Instead, the alleles at all loci of one
chromosome, should in theory, be
transmitted as a unit during gamete
formation.
1:1:1:1 observed
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II. Linkage and crossing over
When two genes are compeletely linked, no crossing over
occurs therefore, each gamete receives the alleles present
on one chromatid or the other:
AB or ab
A. Crossing over – breakage
and rejoining process
between 2 NONSISTER
chromatids
• Crossing over produces
recombinants
• The % of recombinant
gametes varies,
dependent upon location
of the loci. The closer the
genes are, the less likely
recombination will occur
RECOMBINANT
Breakage and rejoining process between two
homologous non-sister chromatids
-there can be one or more cross-overs
-the cross over can occur anywhere
RECOMBINANT
Recombination Frequency = the # of
recombinants/total progeny
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B. Recombination Frequency, unlinked
genes v. linked genes
1). In the case of unlinked genes, independent
assortment holds true…
Testcross: Heterozygous x homozygous mutant
AaBb x aabb
Offspring:
the # of recombinants = the # of parental types
2). In the case of linked genes, no
independent assortment
• Offspring:
• the # of recombinants < the # of parental
types
‣ (RF) < 1/2 or 50%
Recombination Frequency =
‣We can compare the RF to what one would expect
with independent assortment…
RF Range – 0% to 50%
RF significantly < 50% - Linkage
RF = 50% - not linked
•Crossing between adjacent non sister chromatids generates
recombinants
•The two chromatids not involved in the exchange result in nonparental
gametes
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Recombination by Crossing Over – points
to keep in mind:
1. CO’s can occur between any two nonsister
chromatids
2. If there is NO crossing over, only parental
types will be observed
3. If there IS crossing over, RF will increase up
to 50%
4. when the loci of two linked genes are very
far apart, the RF approaches 50%, 1:1:1:1
ratio observed, thus transmission of the
linked genes is indistinguishable from that of
two unlinked genes
Morgan noted the proportion of recombinant
progeny varied depending on which linked genes
were being examined…
Testcross F1 results:
pr + pr vg + vg x pr pr vg vg
pr + vg + 1339
pr vg 1195
pr + vg 151
pr vg + 154
RF = 11%
y + y w + w x yy ww
y w 43
y + w 2146
y w + 2302
y + w + 22
RF = 1.4%
As Morgan studied
more linked genes, he
saw that the proportion
of recombinant
progeny varied
considerably.
III. Crossing over & chromosome mapping
• Morgan thought the variations in RF might
indicate the actual distances separating genes
on the chromosomes.
• Sturtevant (Morgan’s student) compiled data on
recombination between genes in Drosophila test
crosses
‣ The closer the two linked genes, the lower the
recombination frequency- thus RF may be
correlated with the map distance between two
loci on a chromosome
A. Linkage Maps
• Linkage of genes can be represented in the
form of a genetic map, which shows the
linear order of genes along a chromosome.
Can also determine the distance between the
genes. The % recombinant offspring is
correlated w/the distance between the two
genes
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B. Map Units
• Map Unit (m.u.) = the distance between
genes for which one product of meiosis
out of 100 is recombinant
• [RF of 1% = 1 m.u. or 1 cM]
• e.g. if RF 12% between A & B, and
28% between B & C:
A B C
12 mu 28 mu
RF – 1.3%, therefore y is 1.3mu from w
w 37.2 mu from m
In the garden pea, orange pods (orp) are recessive to normal
pods (Orp), and sensitivity to pea mosaic virus (mo) is
recessive to resistance to the virus (Mo). A plant with
orange pods and sensitivity is crossed to a true-breeding
plant with normal pods and resistance. The F1 plants
were then test-crossed to plants with orange pods and
sensitivity. The following results were obtained:
160 orange pods/sensitive
165 normal pods/resistant
36 orange pods/resistant
39 normal pods/sensitive
calculate the map distance between the two genes
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C. Mapping multiple genes – Threepoint
mapping & Alfred’s research
• Hypothesis = when
multiple genes are
located on the same
chromosome, the
distance between the
genes can be estimated
from the proportion of
recombinant offspring.
• Sturtevant’s First Genetic
Map
A. Sturtevant’s First Genetic Map
• The linear order of these genes can be determined using
testcross data
Examined 5 different genes: y, w, v, m, r
• All alleles were <strong>found</strong> to be recessive and X linked.
• Crossed the double heterozygote female with
hemizygous male recessive for the same alleles.
Example: y+y w+w x yw w+w r+r x wr
y+w+
yw
y+w
yw+
w+r+
wr
w+r
wr+
RF = 214/21,736 = 0.0098
RF = 2,062/6116 = 0.337
1 mu between y & w, 33.7 mu between w & r
genes are arranged on the chromosome in a linear
order- which can be determined…
The Complete Data:
Alleles # R./total#
y and w 214/21,736
y and v 1,464/4,551
y and r 115/324
y and m 260/693
w and v 471/1,584
w and r 2,062/6116
w and m 406/898
v and r 17/573
w and m 109/405
RF
1%
32.2%
35.5%
37.5%
29.7%
33.7%
45.2%
3%
26.9%
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y-w = 1 m.u.
v-r = 3 m.u.
y-m = 37.5 m.u.
w-r = 33.7 m.u.
w-v = 29.7 m.u.
Suggesting that v is between r
& w, but closer to r
Map distances more accurate
between genes that are closer
together, as the RF approaches
50%, the value becomes more
inaccurate as a measure of map
distance…
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