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The nature of the 1;29 translocation in cattle as revealed by synaptonemal complex analysis using electron microscopy.

Synaptonemal complex analyses were carried out by electron microscopy on surface-spread spermatocytes of one normal bull and two bulls that were heterozygous for the so-called 1;29 translocation. The autosomal bivalents of the normal karyotype, which could be arranged by size in a series, demonstrated kinetochores at the terminally located attachment plaques. One autosomal bivalent was clearly larger than the rest and apparently consisted of the long arm of the 1;29 translocation. The 1;29 translocation was the longest autosome in the set and had a kinetochore in a subtelocentric position. Some of the autosome pairs had nucleolus organizer regions in telomeric regions. The X and Y chromosomes, which were not paired at zygotene, demonstrated association in a very short segment at early pachytene; in no cells could a synaptonemal complex be seen between the X and Y. Very often the sex chromosomes were dissociated. At zygotene, a few, usually large, bivalents were unpaired proximally. This always also involved the proximal parts of the arms of the 1;29 translocation and their normal homologs. At early pachytene, the 1;29 trivalent, although to a less extensive degree, was also unpaired in the pericentric region. Configurations in which one chromosome, either 1 or 29, was completely paired with its corresponding arm in the 1;29 translocation chromosome also occurred. When unpaired proximally, the size of chromosome 1 agreed fairly well with the size of its corresponding arm, but the size of chromosome 29 was considerably larger than the corresponding arm of the 1;29 translocation chromosome. During late zygotene and early pachytene, the percent difference between chromosome 29 and its corresponding arm decreased, and at mid and late pachytene there had been a complete synaptic adjustment. The size difference and pairing behavior indicated that a deletion of the kinetochore and the most proximal segment of chromosome 29 had preceded the fusion with chromosome 1 into the 1;29 translocation. The unique structural appearance of the 1;29 translocation chromosome compared to that of other centric fusion translocations in cattle lends support to the theory of a monophyletic origin of the 1;29 translocation. The importance of the pairing behavior observed in governing recombination and chromosome disjunction is briefly discussed.