Gene Mapping, Linked
& Unlinked Genes
Packet #40
Chapter #15
S
Wednesday, March 23, 2016
1
Linked Genes vs.
Unlinked Genes
S
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2
Linkage
S Each chromosome behaves genetically as if it consisted of
genes arranged in a linear order
S Linkage is the tendency for a group of genes, on the same
chromosome, to be inherited together via crossing over
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3
Linkage II
S
However, for linkage to occur,
the genes, located on the same
chromosome, must be close
enough to each other.
S
If the genes are close enough to
each other, and have a high
probability of crossing over
together, then they are
considered to be linked genes.
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4
Linkage III
S
If genes are linked together, then
Mendel’s law of independent
assortment does not apply.
S Genes, in this case, are not
passed on independent of
another gene.
S However, linked genes may
recombine during crossing
over.
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5
Determining Whether
Genes are Linked or
Unlinked
S
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6
Recombination of Genes
S Recombination of unlinked genes is achieved via
independent assortment of chromosomes
S Mendel’s Law
S Recombination of linked genes is only possible during
crossing over.
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7
Two-point Cross I
S The two-point test cross allows scientists to determine
whether genes are linked or unlinked.
S In order to perform a two-point test cross one parent must be
heterozygous while the other must be homozygous recessive.
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8
Two-point Cross II
S If the majority of the offspring have a genotype similar to
one of the parents, then the genes are linked.
S If the majority of the offspring have a recombinant
genotype, then the genes are unlinked.
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9
Two Point Cross Example
S
Parent #1
S BbVv
S
S
Grey with normal wings
Parent #2
S bbvv
S
Black with vestigial wings
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10
Two-Point Cross
BV
bv
Bv
bV
S
bv
BbVv bbvv
Expec 575
ted
Resul
ts
Bbvv bbVv
575
575
575
Actua 965
944
l
Resul
ts March 23, 2016
Wednesday,
206
185
11
Calculations
S Parental Genotypes
S
965 (42%) +944 (41%) = 1909
S
1909/2300 = 83%
S Recombinant Genotypes
S
206 (9%)+185 (8%) = 391
S
391/2300 = 17%
S If independent assortment was to
occur, the percentages would be 25% a
piece.
S Based on the data, the recombinants
arose because of crossing over
Gene Mapping
S
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12
Gene Mapping
S By measuring the frequency of recombination between
linked genes, it is possible to construct a linkage map of a
chromosome
S Scientists were able to develop a detailed genetic map of
Neurospora (fungus), fruit fly, the mouse, yeast and many
plants that are particularly important as crops via gene
mapping.
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13
Gene Mapping II
S
The recombinant frequencies
(percentages) are used as units
that separate two alleles.
S Recombinant = 58%
S
The genes are 58 units
apart.
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14
Gene Mapping III
Example
S Create a map using the information provided below.
S A & D = 2 units
S B & D = 10 units
S C & B = 3 units
S C & A = 5 units
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15
Sex-Linked Genes
S
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16
Introduction I
S
Sex is determined by sex
chromosomes
S X and Y
S
XX = female
S
XY = male
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17
The X Chromosome
S The X chromosome contains many important genes that are
unrelated to sex determination
S These genes are required for both males and females
S
A male receives ALL of his X-linked genes from his mother while a
female receives her X-linked genes from both parents.
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18
Dosage Compensation
S
Female mammals display dosage
compensation.
S Only one of the two
chromosomes is expressed in
each cell
S The other allele is inactive
S Seen as a dark-staining Barr
body at the edge of the
interphase nucleus.
S Equalizes the expression of xlinked genes for both genders.
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19
Dosage Compensation II
S
A random event that occurs in
each somatic cells
S A female that is heterozygous
expresses one of the alleles in
about half her cells and the
other allele in the other half
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20
Practice Homework
Assignments
S
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21
Practice Assignment #1
S
Determine if the genes are linked
together using the information
provided.
Wednesday, March 23, 2016
BV
bv
Bv
bV
BbVv
bbvv
Bbvv
bbVv
Expect 300
ed
Result
s
300
300
300
Actual 310
Result
s
315
287
288
bv
22
Practice Assignment #2
S
Determine if the genes are linked
together using the information
provided.
Wednesday, March 23, 2016
BV
bv
Bv
bV
BbVv
bbvv
Bbvv
bbVv
Expect 295
ed
Result
s
295
295
295
Actual 360
Result
s
380
230
230
bv
23