Synaptic interrelationships between the segments of the heteromorphic bivalent in double heterozygotes for paracentric inversions in chromosome 1 of the house mouse.

Electron microscopic analysis of synaptonemal complexes in double heterozygotes for the partially overlapping inversions In(1)1Rk and In(1)12Rk in chromosome 1 of the house mouse was carried out. A great variety of synaptic configurations with complicated combinations of homologously and non-homologously paired segments was observed. Analysis of these configurations revealed at least five independent pairing regions in chromosome 1. Interrelationships between these regions with respect to their pairing ability were estimated. Pairings in the distal non-inverted segment and in inversions inhibit each other, while pairing in either inverted segment facilitates synapsis in the other. In other words, pairing initiations in different parts of the same bivalent are not independent events.