Origin of the adjacent 1 disjunction

Abstract

To investigate individuals heterozygous for reciprocal translocations is one object of prenatal diagnosis because of the high risks inherent in chromosomally unbalanced offspring (Bou6 and Gallano, 1984;Daniel et al., 1988). In genetic counseling, it is important to estimate the percentage risk of having such offspring (Fuhrmann and Vogel, 1982;Harper, 1988)~ The risks may depend on events including various ratios of cells with an unbalanced chromosome constitution resulting from segregation from quadrivalents, the lack of unbalanced gamete participation in fertilization, and selection of unbalanced embryos during the period from early embryogenesis to later development.Segregation ratios from quadrivalents in tanslocation heterozygotes have often been discussed on the basis of cytogenetic data from offspring and fetuses of translocation carriers (Petrosky and Borgankar, 1984;Stene and Stengel-Rutkowski, 1988). The chromosome segregation in male translocation heterozygotes has also been studied by sperm chromosome analysis using the hamster oocyte-human sperm fusion technique (Martin, 1988; Spriggs et aI., 1992). Several authors have described that, when the genotype of offspring and sperm are normal or balanced, the gamete derived from alternate disjunction in the translocation heterozygote. When the genotype is unbalanced, on the other hand, they assume that the gamete originated from the adjacent 1, 2, or 3:1 disjunction. In this really the case?In studies of offspring or sperm in translocation heterozygotes, however, no one has obtained data of meiotic crossing-over, except in a few individuals with specific karyotypes from adjacent 2 and 3:1 disjunctions accompanying crossingover, which are usually rare (Lindenbaum, 1990;Spriggs et al., 1992). A meiotic crossing-over can not usually be detected as a specific change of the karyotype in gametes and zygotes. In particular, when the alternate disjunction occurs accompanying meiotic crossing-over at the "interstitial" (in this letter, a segment between the centromere and a break point in a derivative chromosome) segment in translocation heterozygotes, the derived karyotypes seen in gametes and zygotes might be indistinguishable from those derived from the adjacent 1 disjunction accompany-