The reciprocal translocation T(5;12)31H, induced by irradiation of oocytes, causes sterility in most heterozygous male carriers, a tenfold reduction in mean sperm count, and a high frequency of abnormal sperm. Chromosome breakpoints are in bands 5B and 12F1, leading to a long (125) and a short (512) marker chromosome. Ninety five percent of germ cells showed chain quadrivalents or trivalents at meiotic metaphase I (MI). The order of loci on chromosome 5 is centromere—T31HRwgo with a recombination frequency of 31 + 7% between T31H and Rw. No linkage was found between T31H and linkage group XVI genes previously assigned to chromosome 12; it was concluded that they are located elsewhere. Viable tertiary monosomic and trisomic young (lacking the 512 region or with it in excess) were generated with post-natal frequencies of 4.5% and 14.6%, respectively, from outcrosses of heterozygous females. A deficiency of monosomics was also found in 11½ to 14½ day foetuses, when abnormal phenotypes or severe retardation produced by other unbalanced karyotypes were found. There was also a marked deficiency of monosomic offspring from outcrossed monosomic mothers, only 16.1% being found, whereas trisomic mothers gave 52.8% trisomic young. Surviving monosomics of both sexes were smaller than trisomics at birth and tended to have unusual skeletal fusions. Both types of aneuploid male were sterile, but tertiary monosomics were less severely affected, as judged by testis mass, sperm counts, and the appearance of testis sections. Both tertiary monosomic and trisomic females have smaller litters than normal females. Possible reasons for these findings are discussed, as well as recent observations throwing light on mechanisms of sterility in this type of translocation.

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