chapter 5 linkage, recombination, and the mapping of genes on chromosomes
TRANSCRIPT
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Chapter 5
Linkage, recombination, and the
mapping of genes on chromosomes
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Fig. 5.1
linkage
recombination
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wY+
W+y
wY+
W+y
wY+
wY+
wY+
W+y
W+Y+wy
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When genes are linked, parental
combinations outnumber
recombination types.
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W+Y+
wy
W+Y+
W+Y+
wy
W+Y+
Two parental types
Two recombination types
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W+Y+
wy
W+Y+
W+Y+
wy
W+Y+
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Fig. 5.5
Autosomal genes can also exhibit linkage
bb: black cc: curved
bc+
bc+
b+c
bc+
b+c
b+c
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The Chi square test pinpoints the probability that experimental results are
evident for linkage
Chi test measures the “goodness of fit”:
How often an experimentally observed deviation from the prediction of a particular hypothesis will occur solely by chance.
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Fig. 5.6
Assume A and B genes are not linked.
F1
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Chi Square – Experiment 1 & 2
2 = (observed – expected)2
number expected
2 = (31 – 25)2 + (19 – 25)2 25 25
= 2.88
2 = (62 – 50)2 + (38 – 50)2 50 50
= 5.76
Experiment 1
Experiment 2
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Table 5.1
A and B are not linked A and B are linked
difference is significantdifference is non-significant
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Recombination results when crossing-over during meiosisseparates linked genes
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Fig. 5.7
Evidence that recombination results from reciprocal exchangebetween homologous chromosomes
X chromosome
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Fig. 5.8
Recombination through the light microscope
(synaptonemal complex)
anaphase
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Fig. 5.9
Recombination frequencies are the basis of genetic map
RF: recombination frequency;
1% RF= 1 Centimorgan (cM)=1 map unit (m.u.)
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Fig. 5.10
Unlinked genes show a recombination frequency of 50%
ry ry+
tkv tkv+
ry
tkv
ry
tkv+
ry+
tkv
ry+
tkv+
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Fig. 5.10
Unlinked genes show a recombination frequency of 50%
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Locus: chromosomal position of a gene
Mapping: the process of determining that locus
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Fig. 5.11
Mapping genes by comparison of two-point crosses
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The limitation of two-point cross
1. Gene order is difficult to determine if they are very close.
2. Actually distance do not always add up.
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Fig. 5.12
Vestigial wingsBlack bodyPurple eye color
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Vg to b: (252+ 241+131+118)/4197=0.177, 17.7%Vg to pr:(252+241+13+9)/4197=0.123, 12.3%B to pr:(131+118+13+9)/4197=0.064, 6.4%
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Fig. 5.13
Three point-crosses allow correction for double cross-over
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Vg to b (three-point cross):(252+241+131+118+13+13+9+9)/4197=0.187, 18.7%
Vg to b (two-point cross): (252+ 241+131+118)/4197=0.177, 17.7%
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For greatest accuracy, it is always best to constructa map using many genes separated by relative short
distance.
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Mapping genes at X chromosome by two-point cross
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Fig. 5.14
RF between Y and W: 49+41+1+2/6823 X100=1.3 m.u.
RF between m and W: 1203+1092+2+1/6823 X100=33.7 m.u
RF between m and y: 1203+1029+49+41+2+2+1+1/6823 X100=35 m.u.
y w m
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The actual physical distance between genes does notalways show a direct correspondence to genetic map
distance
1. Recombination is not uniform over the length of a single chromosome,
Hot spot.
• The existence of double, triple, or even more cross-overs.
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Rates of recombination differ from species to species
In human, 1 m.u. is = 1 million baseIn yeast, 1 m.u. is 1500 base pairs
In Drosophila, meiotic recombination only occurs in female.
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Fig. 5.15
Linkage groups:
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Fig. 5.16a
The life cycle of the yeast Saccharomyces cerevisiae
stress
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Fig. 5.16b
The life cycle of the bread mold Neurospora crassa
Bread mold
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Fig. 5.17ab
How meiosis can generate three kinds of tetrads
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Fig. 5.17cde
When PD=NPD, two genes are unlinked
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Four types of gametes when genes on different chromosome
H
HHtt hhTT
Ht hT
H
h
h
T
T
t
t
(h)
(H)
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Fig. 5.18
When genes are linked, PDs exceed NPDs
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Fig. 5.19abc
How crossovers between linked genesgenerate different tetrads
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Fig. 5.19def
Rare!
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How to calculate the recombination frequency between
two linked genes in the tetrad analyses?
RF=(NPD+1/2 T)/total tetrads x100
RF= 3+(1/2)(70)/200 x100=19 m.u. (tetrads) = (4X3)+ (2X70)/800 x100=19 m.u. (spores)
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Fig. 5.20
Tetrad analyses confirms that recombination occurs at the four-strand stage
(A mistake model!) (a lot)
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Fig. 5.22
How ordered tetrads form
Arrangement of the four chromatids of each homologous chromosome pair
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Fig. 5.23
(Cross-over between gene and centromere)
Ordered Tetrads help locate genes in relation to the centromere
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Fig. 5.24
Genetic mapping by ordered-tetrad analysis
Thr-centromere: (1/2) (16+2+2+1)/105 x100=10 m.u.
Arg-centromere:(1/2) (11+2+2+1)/105 x100=7.6 m.u.
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Thr-Arg linkage: 3+(1/2)(16+11+2)/105 X100=16.7
PD PDT T T NPD NPDDouble
cross-over
NPD Tetrad
Double cross-over
Double cross-over
Double cross-over
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Fig. 5.25
Mitotic recombinationWild-type tissue: y sn+/Y+ sn: wild-type color and bristle
y sn+/y sn+ Y+ sn /Y+ sny sn+/y sn+
Y+ sn /Y+ sn
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Fig. 5.26
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Fig. 5.27
Mitotic recombination during growth of diploid yeastcolonies can create sectors
ADE2/ade2
ade2/ade2
Recombinatioinbetweem ade2and centromere
AA
aa
A
Aa
aMitotic recombination
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Fig. 5.28
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Fig. 5a.p131
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Fig. 5a.p143
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Fig. 5.21
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TABLES
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ART & PHOTOS
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CO 05