The site distribution parameter for radiation induced chromosome exchanges

Abstract

The distribution between cells of radiation induced chromosome exchange aberrations in a number of species conforms to the terms of a binomial [ p + (1 − p)] n . The current explanation for this observation is that there are only a few places (termed “sites”) within the nucleus, where the participating chromosomal threads come close enough together for exchange to take place. p is taken as representing the probability of obtaining an exchange in a site, and n the number of “sites” in the cell available for exchange. Mathematical consideration is given to the case where not all cells in the population have the same number of “sites”, and it is concluded that n is a distribution parameter indicating the way in which cells with different numbers of sites are distributed in the population, and that it does not indicate directly the mean number of “sites” available. This conclusion is illustrated by calculated exchange distributions for a number of hypothetical populations having cells in the different site categories distributed in a known manner. It follows from this conclusion that: 1. (1) different types of aberration may have different mean numbers of “sites”, but the same distribution parameter; 2. (2) a Poissonian distribution of aberrations does not necessarily indicate that the mean number of “sites” per cell is unlimited or even large.