Download Independent Assortment and Meiosis

Independent Assortment and Meiosis
A
B
B
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B
A
25% AB
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Linkage and Meiosis
25% Ab
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Meiosis
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50% AB
50% ab
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Meiosis
25% aB
0% aB
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B
A
25% ab
0% Ab
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Partial Linkage and Meiosis
Crossingover and Recombinantion
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?% AB
?% ab
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Meiosis
?% aB
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?% Ab
Which are the Parental types and which the recombinant types?
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Genetic Distance
• The likelihood of recombination is roughly
proportional to the distance two genes are apart.
• If two genes are close together on a
chromosome there will be few recombinants.
Genetic Distance
Genetic
Distance
=
Frequency of
Recombination
=
(Rf)
of Recombinants
= #total
# of progeny
In a Testcross
An underestimate
of true Rf!!!
• If two genes are farther apart on a chromosome
there will be more recombinants.
•Units for genetic distance – centimorgans = map units
1 map unit = 1%Rf
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Outcome of Double Crossing-Over
Effect of Double Crossing-over
b
a
B
A
b
a
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Double Crossing Over Mask Recombination
Therefore analysis of recombinant genotype will
underestimate the frequency of crossing over
Practice Problem
In Drosophila, the droopy gene is 5 map
units away from the curved gene. A
heterozygous female was generated by
mating a wildtype female with a double
recessive male. If this heterozygote was
testcrossed and 1000 progeny were
examined, how many would you expect to
be wildtype for both traits?
Example
Generate a dihybrid and testcross
Observed frequency (%) of
Recombinant between loci
A
Actual frequency (%) of Recombinant
between two loci
Detecting and Estimating Linkage
• Why determine linkage? Help determine the chromosomal position
of a locus. Remember chromosomal position defines a gene.
• To detect linkage generate a dihybrid and testcross the dihybrid.
• First test for independent assortment (Chi square) in testcross
progeny
– If you fail to reject ind assort, then conclude the genes are on
different chromosome or are on the same chromosome but far
apart >45 mu.
– If you reject independent assortment conclude the genes are on
the same chromosome and closer than 50 map units.
Genetic Mapping
1. Wildtype female X black curled male
2. F1 Female X black curled male
Testcross progeny
784 wildtype
762 black curled
234 curled
210 black
Are these genes independently assorting?
Only test for linkage if reject independently assortment
How far apart are the genes on the chromosome?
Rf = # rec/total Progeny
Why is this an underestimate of genetic distance?
What results would you predict if you testcross a male F1 (remember in drosophila there is no crossing
over in males)
• Analyze the relative position on a
chromosome.
• Example
– Black and Curly are 22.3mu
– Black and Pink are 30.6mu
– Curly and Pink are 46.6
• Rough map
• Effects of double crossovers
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Three Point Mapping
• Analyze results of a testcross of a trihybrid.
• Better than two point mapping for determining
gene position on a chromosome
• Better for determining position on a
chromosome
• Trick: Analyze two genes at a time
Example
•
In nematode the dumpy body, uncoordinated movement, and knobby body
are all recessive to wildtype. A trihybrid female was generated by mating a
homozygous dumpy and a homozygous uncoordinated knobby. The
trihybrid was testcrossed and the following progeny were obtained.
Dpy Unc Knb
Unc Knb
Knb
Unc
3
392
34
61
Dpy Unc
Dpy Knb
Dpy
Wildtype
40
65
410
3
How would you test for Ind Assort of knb and dpy?
What is the linkage relationship between the three genes?
Draw a genetic map indicating the relative position of the genes on
a chromosome.
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