The possibility that the distributive pairing mechanism which operates in Drosophila melanogaster meiosis may also operate in mammals and be a causal factor in the production of aneuploidy was tested in the mouse. This was done by determining the segregation of the single maternal X-chromosome and the T6 translocation among the offspring of XO females heterozygous for the translocation. Although the appearance of animals partially trisomic for the chromosomal material of the T6 marker chromosome substantiated existing cytological evidence that the T6 marker chromosome is frequently present as a univalent in the meiotic divisions of translocation heterozygotes, no evidence for the distributive pairing of the univalent X with the univalent marker was obtained. The results do not, therefore, support the concept that distributive pairing may occur between non-homologous chromosomes in the mouse.
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