Structural differences in pericentric inversions. Application to a model of risk of recombinants.

The potential chromosomal imbalance in offspring of pericentric inversion heterozygotes can be evaluated by measuring (% of haploid autosomal length, % HAL) the chromosomal segments distal to the breakpoints in the inversion. These distal segments were measured in presently reported and published cases of pericentric inversions, divided into two ascertainment groups: (I) those ascertained through recombinant offspring and (II) those ascertained through balanced heterozygotes. The distal segments in group II inversions were significantly larger than those of group I, i.e., the potentially larger chromosomal imbalances were not observed in full-term offspring. These results are discussed in relation to the model of risk of abnormal offspring in the progeny of heterozygotes for structural rearrangements (the chromosome imbalance size--viability model). The mean distal segment sizes for group I and group II pericentric inversions were respectively not significantly different from the mean interchange segment size for a sample of reciprocal translocations divided into the same two ascertainment groups. It was concluded that the restrictions on the size (% HAL) of chromosomal imbalance in offspring surviving until term are similar whether this imbalance arises from reciprocal translocations or pericentric inversions.