Purple petals, long pollen (Rr,Ss). F 1. Review Problems. 1905 William Bateson and R.C. Punnett. X. Red petals, round pollen (rr,ss). Purple petals, long pollen (RR,SS). Question. If two genes are tightly linked, such that no crossing over occurs between them:
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Purple petals, long pollen (Rr,Ss)
F1
Review Problems
1905 William Bateson and R.C. Punnett
X
Red petals, round pollen
(rr,ss)
Purple petals, long pollen
(RR,SS)
o
o
+
X
yellow (Gg), round (Ww)
yellow (Gg), round (Ww)
GW
GW
gw
gw
F2
GW
GgWw
GGWW
GGWW
GgWw
Generation
GW
GGWW
GgWw
GGWW
GgWw
gw
GgWw
GgWw
ggww
ggww
gw
GgWw
ggww
GgWw
ggww
24
red, round
71
red, long
215
Purple, long
71
Purple, round
If they assort independently (they are not linked)
F1 selfed (Rr,Ss) X (Rr,Ss)
Expected F2
9 3 3 1
o
o
+
GgWW
GGWw
GGWW
GgWw
GGWw
GgWw
GGww
Ggww
GgWW
GgWw
ggWW
ggWw
GgWw
ggWw
Ggww
ggww
X
yellow (Gg), round (Ww)
yellow (Gg), round (Ww)
GW
gW
Gw
gw
F2
GW
Generation
Gw
gW
gw
Results
24
red, round
71
red, long
55
red, round
21
red, long
215
Purple, long
71
Purple, round
284
Purple, long
21
Purple, round
F1 selfed (Rr,Ss) X (Rr,Ss)
Expected F2
Gene linkage, Recombination and Mapping
Chapter 4
Why map the genome ?
Two types of maps ?
Purple petals, long pollen (Rr,Ss)
F1
The observation
1905 William Bateson and R.C. Punnett
X
Red petals, round pollen
(rr,ss)
Purple petals, long pollen
(RR,SS)
Expected F2
24
red, round
72
red, long
216
Purple, long
72
Purple, round
F1 selfed (Rr,Ss) X (Rr,Ss)
Results
55
red, round
21
red, long
284
Purple, long
21
Purple, round
Symbols and terminology
ABalleles on the same homolog, no punctuation
A/aalleles on different homologs, slash
A/a; B/b genes known to be on different chromosomes, semicolon
A/a . B/bgenes of unknown linkage, use a period
CisAB/ab or ++/ab
TransAb/aB or +b/a+
Red eyes, normal wings (pr+/pr . vg+/vg)
F1
Thomas Hunt Morgan & Drosophilia
X
Red eyes, normal
(pr+/pr+. vg+/vg+)
Purple eyes, vestigal
(pr/pr . vg/vg)
Instead of selfing the population, he did a test cross.
Test cross
X
Red eyes, normal
(pr+/pr . vg+/vg)
Purple eyes, vestigal
(pr/pr . vg/vg)
1339 Red eyes, normal wings (pr+ . vg+)
1195 Purple eyes, vestigal (pr . vg)
151 Red eyes, vestigal (pr+. vg)
154 Purple eyes, normal wings (pr . vg+)
Test cross
1339 Red eyes, normal wings (pr+ . vg+)
1195 Purple eyes, vestigal (pr . vg)
151 Red eyes, vestigal (pr+. vg)
154 Purple eyes, normal wings (pr . vg+)
pr+
vg+
305/2839 = 10.7 percent
vg
pr
cis or trans ?
Test cross with pr/pr . vg/vg
Red eyes, normal wings (pr+/pr . vg+/vg)
F1
157 Red eyes, normal wings (pr+ . vg+)
146 Purple eyes, vestigal (pr . vg)
965 Red eyes, vestigal (pr+. vg)
1067 Purple eyes, normal wings (pr . vg+)
pr+
vg
304/2335 = 12.9 percent
vg+
pr
Initial cross
X
Red eyes, vestigal
(pr+/pr+. vg/vg)
Purple eyes, normal
(pr/pr . vg+/vg+)
Morgan proposes
Linkage and Crossing Over
Fig. 4-3
Occurs at Prophase I (tetrad stage)
Crossing-over of the
chromosomes.
A chiasma is formed.
Genetic recombination.
Microscopic evidence for chromosome breakage and gene recombination
Harriet Creighton and Barbara McClintock, 1931
Wx
C
Wx
c
c
wx
wx
C
For linked genes, recombinant frequencies are less than 50% in a testcross.
Fig. 4-8
Mapping by Recombinant Frequency
Morgan set his student Alfred Sturtevant to the project.
“In the latter part of 1911, in conversation with Morgan, I suddenly realized that the variations in strength of linkage, already attributed by Morgan to differences in the spatial separation of genes, offered the possibility of determining sequence in the linear dimension of a chromosome. I went home and spent most of the night (to neglect of my undergraduate homework) in producing the first chromosome map.” Sturtevant
Frequency of crossing over,
indicates the distance between two genes on the chromosome.
Fig. 4-9
Morgan proposes
Linkage and Crossing Over
Fig. 4-3
For linked genes, recombinant frequencies are less than 50% in a testcross.
Fig. 4-8
Fig. 4-9
Review Problems
1. A plant of genotype
is test crossed.
A B
a b
If the two loci are 14 m.u. apart, what proportion of progeny will be AB/ab ?
43% AB, 43% ab, 7% Ab, 7% aB
Review Problems
2. A plant of genotype A/a . B/b is test crossed.
The progeny are 74 A/a . B/b
76 a/a . b/b
678 A/a . b/b
672 a/a . B/b
Explain.
A and B are linked in trans and are 10 m.u. apart.
Review Problems
3. You have analyzed the progeny of a test cross to a tetrahybrid.
The results indicate that
10% of the progeny are recombinant for A and B
14% for B and C
24% for A and C
4% for B and D
10% for C and D
14% for A and D
Provide a linear map for the chromosome.
Review Problems
3. You have analyzed the progeny of a test cross to a tetrahybrid.
The results indicate that
10% of the progeny are recombinant for A and B
14% for B and C
24% for A and C
4% for B and D
10% for C and D
14% for A and D
Provide a linear map for the chromosome.
|----------|----|----------|
A 10 B 4 D 10 C
Mapping with Molecular Markers
Chapter 4, continued.
What is a molecular marker
AAGGCTCAT
TTCCGAGTA
AAGACTCAT
TTCTGAGTA
RFLPs
EcoR1 site
GAATTC
CTTAAG
GGATTC
CCTAAG
digest with EcoR1
RFLP analysis
Fig 4-15a
RFLP analysis
Fig 4-15b
RFLP analysis
Fig 4-15c
Using combinations of SNPs
A haplotype is a chromosomal segment defined by a specific array of SNP alleles.
Using haplotypes to deduce gene position
Fig. 4-16
Simple sequence length polymorphisms
(SSLPs)
VNTRs (variable number tandem repeats)
Repeats of DNA sequence, with different numbers of repeats occurring in different individuals.
Minisatellites (DNA fingerprints)
– Repeating units of 15-100 nucleotides
Microsatellites – repeat of 2-3 nucleotides
ACACACACACACAC
Minisatellites
Fig. 4-18
CACACACACACACA
GTGTGTGTGTGTGT
Microsatellites
Amplified by polymerase chain reaction.
primer 1
CACACACACA
GTGTGTGTGT
primer 2
St.
M
M’
Fig. 4-19
Molecular markers can be used instead of phenotype to map genes.
Chi-square
A/A . B/B X a/a . b/b
A/a . B/b
Test cross to a/a . b/b
Observed
Expected
Total 500
Using recombinant maps with physical maps