Oregon State University

Linkage and Mapping

Eukaryote chromosome mapping by recombination

Meiosis is the basis of transmission genetics. Recombination in a heterozygous individual is a useful tool for making linkage maps, and linkage maps help us understand evolution, synteny, and selection response.

I. Key terms

  1. Linkage refers to the association of two or more phenotypic characters in inheritance because the genes controlling these characters are located in the same chromosome. Genes carried by the same chromosome are members of the same linkage group; the number of linkage groups corresponds, therefore, to the basic number of chromosomes in the organism in question. The strength of linkage, or the amount of recombination, is dictated by the "distance" between genes in the same chromosome.

  2. Crossing over refers to the physical exchange between homologous chromosomes. As discussed in the previous class, meiotic crossing over is a potent source of genetic variability.

  3. Recombination is the genetic result of crossing over and is detected by new combinations of alleles at two or more loci.

  4. Linkage:
    1. Complete: Gene pairs are so close together that crossing over rarely occurs and recombinant types are generally not recovered.

    2. Partial: Gene pairs are sufficiently far apart that some recombinant types are recovered. The "distance" between genes ranges from a few percent recombination to 50% recombination.

  5. Terms describing the allelic condition at linked loci

    • cis/ coupling ___________________

    • trans/repulsion ___________________


II. More on the meiotic basis of recombination

At Pachynema, there can be breakage of chromatids, followed by their fusion with sister chromatids, or with non-sister chromatids. As long as orientation corresponds, reciprocal exchanges between sister chromatids will not be detected and will not lead to genetic variability. Keys points in non-sister chromatid exchange:

  • Crossing over usually does not involve loss or addition of chromatin.

  • Only two chromatids are involved in any single crossover event.

  • There may be multiple crossovers between non-sister chromatids.

  • Any combination of crossover configurations can occur, and the outcome of such configurations can be radically different.

  • Crossing over occurs after chromosome replication.

III. Factors affecting meiotic crossing over

  • Sex chromosomes _______________________________

  • Position on chromosome _______________________________

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