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  Section: Genetics » Linkage and Crossing Over in Diploid Organisms (Higher Eukaryotes)
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Linkage maps

Linkage and Crossing Over in Diploid Organisms (Higher Eukaryotes)
Coupling and repulsion hypothesis
A testcross in maize
Crossing over and meiosis 
Crossing over and chiasma formation
Mechanism of genetic recombination
Crossing over and linkage maps 
Recombination frequencies from a test-cross
Recombination frequencies from F2 data
Interference and coincidence
Linkage maps
Mapping function and poisson distribution
Linkage groups
Chi-square test 
Cytological basis of crossing over
Creighton and McClintock's experiment in corn
Meselson and Weigle's experiment using lambda (λ) phage
Crossing over at four strand stage

Linkage maps have been prepared in large number of animals and plants with the help of recombination frequencies as discussed above. By testing different groups of genes having atleast one gene in common, additional genes can be added in a linkage map. It is important to notice that recombination frequencies and map distances do not correspond, because over longer distances, although the map distance will exceed even 100, but recombination frequency between any two genes will never exceed 50, as earlier discussed, since double and multiple crossovers will not be discovered. For illustration, a part of linkage-map of X-chromosomes in Drosophila, as initially worked out, is given is Figure 10.11. The map distances are given by recombination frequencies and are sometimes represented in map units (m.u.) also called centi Morgan (1 centi Morgan = 1 cM = 1% recombination).

Linkage maps of four different chromosomes of Drosophila are shown in Figure 10.12. X-chromosome has 66 crossover units, with genes yellow and bobbed at two extreme ends of the map. But this does not mean that recombination frequency between yellow and bobbed will be 66 units. In fact, we know that recombination frequency will never exceed 50% between any two loci. These 66 units will be actually obtained by making use of a mapping function.

It is thus evident that we should not confuse map distances with recombination frequencies. Based on data from Drosophila, corn and Neurospora it became clear that relationship between map distances and recombination frequencies assumes a curve shown in Figure 10.15. The curve suggests that at lower values there is a linear relationship, but as recombination value approaches towards 50%, the linear relationship is gradually lost and recombination frequency is always less than the map distance, so much so that at 50% recombination, further increase in map distance does not cause any increase in recombination value.
A part of linkage of X-chromosome in Drosophila.
Fig. 10.11. A part of linkage of X-chromosome in Drosophila.
Linkage maps of four different chromosomes of Drosophila.
Fig. 10.12. Linkage maps of four different chromosomes of Drosophila.

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