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 â&#x20AC;˘ Morgan found that linked traits are on the same chromosome. â&#x20AC;˘ Chromosomes, not genes, assort independently during meiosis. Mutant
Wild type
Dr R Jayashree, Asst. Professor(AGB)
Testcross to show that two genes are linked
When there is crossover Dr R Jayashree, Asst. Professor(AGB)
Complete Linkage: P1 Cross bwhv bw hv P1 : bwhv bw hv Brown eyes –bw Red eyes –bw+ Thin veins- hv+ Heavy veins-hv
In complete linkage only parental gametes form
bwhv F1: bw 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 distance Dr 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
Recombinant gametes
Crossing-over is the event leading to genetic recombination between linked genes in both prokaryotes and eukaryotes. Dr R Jayashree, Asst. Professor(AGB)
No LinkageIndependent Assortment
Linkage without Recombination
Linkage with Recombination
Creates only parental (noncrossover) gametes. Dr R Jayashree, Asst. Professor(AGB)
creates parental gametes and recombinant (crossover) gametes.
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 occurs Dr 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 â&#x20AC;˘ The closer together two genes are, the more likely they will be inherited together. â&#x20AC;˘ 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 p and j = 52+46+4+2/500 = 104/500 = 20.8%
c/o between j and r = 22+22+4+2/500 = 50/500 = 10% 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.
observed # of double recombinan ts Interference(I) 1 expected # of double recombinan ts Dr R Jayashree, Asst. Professor(AGB)
Genetic Map of Drosophila melanogaster
Dr R Jayashree, Asst. Professor(AGB)
Morganâ&#x20AC;&#x2122;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 ww+ m). 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