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

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

As a result of linkage studies, genes linked together and thus not showing independent assortment are grouped together in a linkage group. However, number of such linkage groups in a particular species was found to correspond to the haploid number of chromosomes, indicating that genes linked together are situated on the same chromosome. Drosophila melanogaster has four linkage groups which correspond to n = 4. In maize, there are 10 linkage groups (n = 10) and in barley there are only seven linkage groups (n = 7).

Complete linkage in Drosophila males
While using Drosophila flies in linkage studies, Bridges discovered that no crossing over took place in male individuals. For instance, if F1 (AB/ab) is used as male and testcrossed with ab/ab as female, only two types of progenies, i.e., AB/ab and ab/ab, will be obtained in the next generation.
For this reason, in preparation of linkage maps, particularly in Drosophila melanogaster, F1 hybrid is always used as a female parent. On the other hand, if only linkage groups are to be established, the presence of complete linkage in Drosophila melanogaster males and in the heterogametic sex in a few other species (e.g. female silkworm) offers a special advantage. For instance, if F1 is used as male, linkage will be demonstrated by simple absence of crossovers in the testcross progeny. However, there is no suitable explanation available for the absence of crossing over in male Drosophila flies. However, this phenomenon is not observed in all species of Drosophila.

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