Meiotic studies of Robertsonian heterozygotes from natural populations of the common shrew, Sorex araneus L.

Adult male common shrews, both Robertsonian heterozygotes and homozygotes, were collected from Oxford and elsewhere in Britain. In both simple Robertsonian heterozygotes and Robertsonian heterozygotes with monobrachial homology, regular chain configurations were observed at meiosis I; only 1-2% were incomplete such that univalents were observed. On the average, there was one chiasma per chromosome arm among those that displayed Robertsonian variation, including both chain configurations and bivalents. According to one hypothesis, a single chiasma per chromosome arm may facilitate proper disjunction of chain trivalents of simple Robertsonian heterozygotes. Based on metaphase II counts, anaphase I nondisjunction frequency can be estimated as 1.0% per heterozygous individual and 0.7% per heterozygous arm combination.