| crossing over | <genetics, molecular biology> The breaking during meiosis of one maternal and one paternal chromosome, the reciprocal exchange of corresponding sections of DNA along pairs of homologous chromosomes by symmetrical breakage and crosswise rejoining of the chromosomes. This results in the transfer of a block of genes from each chromosome to its homologue. In contrast to genetic recombination, 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. Recombination can result from DNA exchange of alleles between between homologous chromatids in meiosis, giving rise to chiasmata. Compare: recombination. (10 Nov 1998) |
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| Gunn's crossing sign | <clinical sign> Retinal arteriovenous crossing with venous compression in hypertensive disease. (05 Mar 2000) |
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| somatic crossing-over | Crossing-over that occurs during the mitosis of somatic cells, in contrast to that which occurs in meiosis. (05 Mar 2000) |
| unequal crossing over | <cell biology> Crossing over between homologous chromosomes that are not precisely paired, resulting in nonreciprocal exchange of material and chromosomes of unequal length. Favoured in regions containing tandemly repeated sequences. (18 Nov 1997) |
| uneven crossing-over | Unequal crossing-over, crossing-over that happens when the breaks do not occur at precisely homologous points in two chromatid strands, and hence results in localised duplication of genetic material in one chromatid and complementary deletion in the other. (05 Mar 2000) |
| four-strand crossing over | Crossing over with both strands of DNA from each sister chromatid, so that both strands from one chromatid are exchanged for both strands on the other. This is different from the usual case where only one of the two DNA strands on one sister chromatid is exchanged for the one matching strand (of the two strands) on the other sister chromatid. (09 Oct 1997) |