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For the students of Veterinary Science
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Linkage and Crossing Over
AGB121
Dept. Of Animal Genetics & Breeding
Veterinary College, Hebbal, Bangalore
Terminologies to Know
• Linkage• Crossing over• Chiasma• Recombination• Gene mapping• Interference
Dr R Jayashree, Asst. Professor(AGB)
Linkage
• 2 genes on separate chromosomes assort independently at meiosis.
• 2 genes far apart on the same chromosome can also assort independently at meiosis.
• 2 genes close together on the same chromosome pair do not assort independently at meiosis.
• A recombination frequency << 50% between 2 genes shows that they are linked.
Dr R Jayashree, Asst. Professor(AGB)
Gene linkage was explained through fruit flies
• Morgan found that linked traits are on the same chromosome.
• Chromosomes, not genes, assort independently during meiosis.
Wild type Mutant
Dr R Jayashree, Asst. Professor(AGB)
Testcross to show that two genes are linked
When there is crossoverDr R Jayashree, Asst. Professor(AGB)
Complete Linkage: P1 Cross
In completelinkage onlyparentalgametes form
hvbw
hvbw
bwhv
bwhvP
:1
F1:bwhv
bwhv
Brown eyes –bwRed eyes –bw+Thin veins- hv+Heavy veins-hv
Dr R Jayashree, Asst. Professor(AGB)
Dr R Jayashree, Asst. Professor(AGB)
Crossing-over
Crossing-over is the reciprocal exchange of homologous chromatid segments of non-sister chromatid, involving the breaking and rejoining of DNA.
Dr R Jayashree, Asst. Professor(AGB)
Mechanism of Crossing-Over
Dr R Jayashree, Asst. Professor(AGB)
The relationship between crossing-over and map distanceDr R Jayashree, Asst. Professor(AGB)
Chiasma
A chiasma (plural chiasmata) represents the physical manifestation of a chromosomal crossover event
Dr R Jayashree, Asst. Professor(AGB)
Recombination
Crossing-over is the event leading to genetic recombination between linked genes in both prokaryotes and eukaryotes.
Recombinant gametes
Dr R Jayashree, Asst. Professor(AGB)
No Linkage-Independent Assortment
Linkage with Recombination
Linkage without Recombination
Creates only parental (noncrossover) gametes.
creates parental gametes and recombinant (crossover) gametes.
Dr R Jayashree, Asst. Professor(AGB)
Single Crossovers: Non-crossover (Parental) and Crossover (Recombinant) Gametes
Dr R Jayashree, Asst. Professor(AGB)
Progeny of single and double crossovers when two genes are linked
Dr R Jayashree, Asst. Professor(AGB)
Double Crossovers
Dr R Jayashree, Asst. Professor(AGB)
Types of Double Exchanges:Not All are Detectable
Dr R Jayashree, Asst. Professor(AGB)
Consequences of a double crossover in a triple heterozygote for three linked genes
Dr R Jayashree, Asst. Professor(AGB)
Rearrangement of the three genes to p j r
Dr R Jayashree, Asst. Professor(AGB)
Crosses of Two X-linked genes
Expect only parental types if no crossing over occursDr R Jayashree, Asst. Professor(AGB)
Dr R Jayashree, Asst. Professor(AGB)
Linkage Maps
Linkage maps/ Chromosome maps show the relative locations of genes on a chromosome determined from recombination rate
• Interlocus distance is proportional to the degree of crossing over between.
– Little or no crossing over in close genes.
– Frequent, even multiple crossovers between distant genes
Dr R Jayashree, Asst. Professor(AGB)
Morgan and Sturtevant (1913) used recombination frequencies to make a genetic map.
A 1% crossover rate is a genetic distance of 1 map unit (mu).
A map unit is also called a centimorgan (cM).
1% crossing over = 1 map unit (in humans 1mu = 1 million bp)
Dr R Jayashree, Asst. Professor(AGB)
Linkage maps estimate distances between genes
• The closer together two genes are, the more likely they will be inherited together.
• Cross-over frequencies are related to distances between genes.The higher the frequency, further the genes are apart
Dr R Jayashree, Asst. Professor(AGB)
Dr R Jayashree, Asst. Professor(AGB)
Three-point mapping, showing the testcross used and the resultant progeny
Dr R Jayashree, Asst. Professor(AGB)
Rewritten form of the testcross and testcross progeny based on the actual gene order p j r
c/o between j and r =22+22+4+2/500 =50/500 = 10%
c/o between p and j =52+46+4+2/500 =104/500 = 20.8%
Dr R Jayashree, Asst. Professor(AGB)
Interference
• Crossovers in adjacent chromosome regions are usually not independent. This interaction is called interference.
• A crossover in one region usually decreases the probability of a crossover in an adjacent region.
tsrecombinan double of # expected
tsrecombinan double of # observed1ce(I)Interferen
Dr R Jayashree, Asst. Professor(AGB)
Genetic Map of Drosophila melanogaster
Dr R Jayashree, Asst. Professor(AGB)
Morgan’s experimental crosses of white-eye and miniature-wing variants of Drosophila melanogaster
Dr R Jayashree, Asst. Professor(AGB)
In an individual heterozygous at two loci, there are two arrangements of alleles:
1. The cis (coupling) arrangement has both wild-type alleles on one homologous chromosome, and both mutants on the other
(e.g., w+ m+ and w m).w+ m+
w m
2. The trans arrangement has one mutant and one wild type
on each homolog (e.g., w+ m and w m+).
w+ m
w m+
Dr R Jayashree, Asst. Professor(AGB)
• Cross-over frequencies can be converted into map units.
• Ex: A 5% cross-over frequency equals 5 map units.
–gene A and gene B cross over 6.0 percent of the time
– gene B and gene C cross over 12.5 percent of the time
– gene A and gene C cross over 18.5 percent of the time