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A model of four-strand crossing-over is presented in which all non-randomness is attributed to the non-Poisson distribution of the number of exchanges. Nth order recombination fractions and the coefficient of coincidence are calculated in terms of the model. It is shown that coincidence may be greater or less than unity depending on the prior distribution of exchanges. An obligate exchange is a sufficient condition for coincidence to be less than unity if there is a maximum of four exchanges, but not necessarily if five or more are possible. It is also shown that marginal coincidence is an insufficient concept to explain the behaviour of coincidence over its entire two-dimensional domain.
Risch et al. (Sun,) studied this question.