Download Gene Linkage Genetics

Gene Mapping, Linked
& Unlinked Genes
Packet #40
Chapter #15
S
Tuesday, May 23, 2017
1
Linked Genes vs.
Unlinked Genes
S
Tuesday, May 23, 2017
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
Tuesday, May 23, 2017
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.
Tuesday, May 23, 2017
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
Bbvv bbVv
Expec 575
ted
Resul
ts
575
575
575
Actua 965
l
Resul
ts May 23, 2017
Tuesday,
944
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.
Tuesday, May 23, 2017
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.
Tuesday, May 23, 2017
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.
Tuesday, May 23, 2017
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
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21
Practice Assignment #1
S
Determine if the genes are linked
together using the information
provided.
Tuesday, May 23, 2017
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.
Tuesday, May 23, 2017
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