Chromosome banding studies and synaptonemal complex analyses of four reciprocal translocations in the domestic pig

Three new reciprocal translocations in the domestic pig were identified in boars deemed responsible for decreased litter size though having a normal semen picture. The translocations were described by the use of handing techniques and involved both one‐ and biarmed chromosomes. The translocated segments were of variable size. The same translocations, with the addition of a fourth one earlier described, were investigated by synaptonemal complex analysis of spermatocytes. Measurements of paired segments were carried out in early and late pachytene and compared with rough measurements of the translocated segments observable in the mitotic chromosomes. The translocated chromosomes and their normal homologues formed almost exclusively a quadrivalent, and very few pairings with other chromosomes were seen. In early pachytene, there was a pairing of homologous segments with the formation of a cross‐shaped configuration, most often having a buckle centrally, due to unpaired chromosome segments, or showing complete pairing. One translocation, however, demonstrated, in late zygotene and early pachytene, a quite unexpected shape of the quadrivalent because of nonhomologous pairing. In late pachytene, the quadrivalent demonstrated heterologous pairing and there was a tendency for the largest and the smallest one of the four chromosomes involved in the translocation not to pair hetcrologously. In diakinesis‐metaphase 1 the translocation chromosomes formed ring‐ or both ring‐ and chain‐shaped configurations.