Reciprocal exchange of material between two paired chromosomes during meiosis, resulting in the transfer of a block of genes from each chromosome to its homologue. In contrast to genetic recombination (2), which is a phenotypic phenomenon, crossing-over is genotypic. Any even number of crossing-overs between two loci will cancel out phenotypically and no recombination will occur.
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Examples: glitazone, GI cocktail, etc.