Chiasma frequency and distribution were studied in male Mus musculus domesticus from the John O'Groats±standard chromosomal hybrid zone in northern Scotland. Individuals of the John O'Groats race (2n 32; homozygous for the Robertsonian fusions 4.10, 6.13, 9.12 and 11.14) and the standard race (2n 40, all telocentric), and hybrids with various karyotypes, were examined. Chiasma frequency was signi®cantly negatively correlated with the number of Robertsonian con®gurations in the meiotic cell. The decrease of chiasma frequency can be attributed to intrachromosomal eects that reduce the number of chiasmata in Robertsonian bivalents (formed in homozygotes for Robertsonian fusions) and trivalents (formed in heterozygotes). However, the reduction is more pronounced in Robertsonian bivalents and is related to a shift of chiasmata to the distal ends of the chromosome arms. A dierent type of repatterning occurs in trivalents where there is a signi®cant increase in proximal and interstitial chiasmata.
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