2015 03-13 Linkage and Crossover
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Transcript of 2015 03-13 Linkage and Crossover
LINKAGE AND CROSS OVER
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Polytene Chromosome
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LINKAGE
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F
eE
cC
bBaA
f
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F
eE
cC
f
B
A
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Thomas Hunt Morgan
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Red - dominant (R)Purple - recessive (r)
Normal wings - dominant(N)Vestigial wing - recessive (n)
Eye colour
Type of wing
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Normal Vestigial
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RrNn x rrnn
RRNN X rrnn
RrNn
RN rn
Parent
Gamete
F1
Test cross
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Expected results were:25% Red Normal25% Red vestigial25% purple Normal25% purple vestigial
The actual results were: 47% Red Normal43% Purple vestigial5% Red vestigial5% purple normal
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Types of linkage
1.Complete linkage
Closely associated
Pass to 2/ more generations
2.Incomplete linkage
Widely located
High chance of separation by cross over
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CROSS OVER
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Crossover is the process by which two chromosomes paired up duringprophase I of meiosis exchange a distal portion of their DNA
Crossover occurs when two chromosomes, normally two homologousinstances of the same chromosome, break and connect to each other'sends
If they break at the same locus, this merely results in an exchange ofgenes
This is the normal way in which crossover occurs
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If they break at different loci, the result is a duplication of genes on onechromosome and a deletion on the other
If they break on opposite sides of the centromere, this results in onechromosome being lost during cell division
Any pair of homologous chromosomes may be expected to cross over three orfour times during meiosis
This aids evolution by increasing independent assortment, and reducing thegenetic linkage between genes on the same chromosome
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Dihybrid cross – one point
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Trihybrid cross – 2 point
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Types of cross over product
1. Parental/ Non cross over type
Coupling phase
2. Recombinant/ Cross over type
Repulsion phase
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Genetic mapAny method used for determining the location of and relativedistances between genes on a chromosome
Map distance =Number of recombinant offspring
Total number of offspring× 100
Map unit – centiMorgan (cM)
1 Map unit = 1% recombination frequency
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Parental phenotypes – most frequently
Double cross over – least frequently
Single cross over – Intermediate frequently
Variation in the number of chromosomes
Variation in the arrangement of chromosomes
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Why it is not possible to use a homozygous dominant organism
(such as TT) in a test cross experiment to determine the
genotype of an organism showing the dominant phenotype.
Illustrate your answer with fully using appropriate genetic
symbol?
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In maize the following cross between pure breeding plants were made,coloured shrunken starchy × colourless full waxy. The F1 plants were allfound to be coloured full starchy. The F1 plants were then test crossedand the following progenies were obtained. Explain the results andobtain the necessary information regarding the inheritance of thesecharacters.
Coloured shrunken starchy 2538
Colourless full waxy 2708
Coloured full waxy 116
Colurless shrunken starchy 113
Colured shrunken waxy 601
Colourless full starchy 626
Colured full starchy 4
Colourless shrunken waxy 2