An alternative model of recombination and interference

Summary 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 triarginal coincidence is a11 insufficient concept to explain the behaviour of coincidence over it entire tn-o-climensioiial domain.