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## Revista chilena de historia natural

*Print version* ISSN 0716-078X

#### Abstract

VALENZUELA, CARLOS Y; PEREZ-ALZOLA, PATRICIA and SANTOS, MANUEL J. A critical view to explanations for the maximal value of 50 % of genetic recombination.* Rev. chil. hist. nat.* [online]. 2001, vol.74, n.1, pp.99-105.
ISSN 0716-078X. http://dx.doi.org/10.4067/S0716-078X2001000000015.

The maximal value of 50 % of genetic recombination (RF = recombination fraction) between two linked gene loci has been explained by two models: (i) the restriction of crossovers to only two of the four chromatids of a tetrad (tetrad model); (ii) the limit of the occurrence of an odd number of crossovers between two loci, when the expected number of crossovers tends to infinite (odd-even model). This article shows that (i) is wrong. If tetrads were responsible for the maximal 50 % RF, sister chromatid exchanges (SCE) should display a maximal 100 % RF. In mammalian CHO cells we found that SCE yield a maximal 50 % recombination. Also, we demonstrated that the maximal 50 % RF for tetrads occurs because the sum of odd order coefficients in the binomial distribution is equal to the sum of even order coefficients. At the end of meiosis the two chromatids of a chromosome that receives K crossovers are separated. Then K breaks should distribute into two chromatids. Thus, the expected number of breaks per chromatid distributes, when K is large, in 50 % of chromatids having an even number of breaks (RF = 0) and 50 % having an odd number (RF = 1), irrespective of tetrads, hexads, octads or more complex meiotic chromosome configurations

**Keywords
:
**50 % recombination; tetrads; odd-even crossovers; SCE.