The quantitative analysis of chromosome pairing and chiasma formation based on the relative frequencies of M I configurations. IV. Interchange heterozygotes

The quadrivalent of an interchange heterozygote can be divided into six regions, each with its own probability of being bound by at least one chiasma. This gives rise to 2⁶ or 64 different possible combinations of bound and not bound regions and as many M I configurations, several of which are isomorph. Sixteen different types of configuration occur, 10 of which involve more than two chromosomes and can thus be distinguished from the non-interchange bivalents. As each combination of bound and not bound regions has a certain probability of occurring depending on the probability of each region to be bound or not, a frequency expectation value can be estimated for each configuration. Also, when the relative frequencies of the different configurations are known, the probabilities of being bound can be estimated for each of the six regions of a quadrivalent, and from these the crossing-over potentials. One translocation heterozygote was studied, involving the long arm of the satellite chromosome of rye (Secale cereale). Crossing-over was reduced, perhaps mainly by partner exchange. The terminal segments retained a relatively high chiasma frequency. The interstitial regions had low chiasma frequencies even when relatively long.