We have used data from chromosomally unbalanced offspring observed at birth, as well as data from sperm chromosome analysis, to study the meiotic segregation of reciprocal translocations. Using data from a total of 1,597 unbalanced children, we have observed an excess in maternal origin for all modes of imbalance. This excess is particularly marked for the 3:1 unbalanced mode, for which we have also observed a maternal age effect, indicating a close relationship with autosomal trisomies. In addition, a statistical analysis of data from 34 different published studies using sperm chromosome analysis has demonstrated that factors which, for reasons of viability, produce a predisposition for a particular mode of imbalance at birth also appear to favor meiotic production of this type of imbalance. Thus the production of unbalanced gametes of a particular type is influenced by the size of the imbalance.

From a data base of 1,590 independent families with autosomal reciprocal translocations, 1,159 viable unbalances were studied and the lengths of their trisomy/monosomy segments measured according to the method proposed by Daniel. About 5% of cases were found not to comply with Daniel viability criteria. The thresholds of viability vary with the mode of unbalance and with the sex of the carrier. Thus, new viability criteria are proposed as a guide for genetic counseling and prenatal diagnosis.

Reciprocal translocations (rcp) are among the most common constitutional chromosomal aberrations in man. Using a European database of 1574 families carrying autosomal rcp, a cartographic study was done on the breakpoints involved. The breakpoints are non-randomly distributed along the different chromosomes, indicating "hot spots". Breakpoints of rcp that result in descendants that are unbalanced chromosomally at birth are more frequent in a distal position on chromosomal arms, and 65% of them are localised in R-bands. Among the R-bands, bands rich in GC islands and poor in Alu repetitive sequences are more frequently the site of breakpoints, as well as bands that include a fragile site. This result suggests that the variation in degree of methylation in GC islands could be involved in chromosomal breakage and hence in chromosomal rearrangements. The heterogeneity of the human chromosomal structure has been demonstrable by metaphase banding techniques since 1970. In contrast to G-bands, R-bands are sites of high gene concentration (Korenberg et al. 1978), are relatively rich in cytosine plus guanine (GC), and in Alu repetitive DNA sequences (Korenberg and Rykowski 1988). More recently Holmquist (1992) has proposed four types of R-bands, depending on their relative richness in GC and Alu DNA sequences. R-bands rich in GC correspond almost exactly to T-bands (Dutrillaux 1977). They contain 65% of all genes while they represent only 15% of the genome (Holmquist 1992). The aim of this study is to analyse the distribution of the breakpoints along chromosomes from a European database of autosomal rcp in order to relate it to the specificity of different chromosomal regions.

The human genome with its 23 pairs of chromosomes, is the result of evolution. This evolution has been ruled by the mutation process and also by the physiological and pathological reorganization of the genomic material inside or between the chromosomes, which are conditioning the genomic variability. This reorganization is starting at singular points on the short or long chromosomic arms, called crossing-over, or translocations, insertions, break points. In this paper, we will show that these points, also called weak points or hot spots of the genome are correlated, independently of their origin. In addition, we will give some properties of the genetic interaction matrices in terms of attractors of the genetic expression dynamics.