Cytogenetic analysis by R-banding and T-banding techniques demonstrated a pericentric inversion of chromosome No. 10 in a woman and a duplication-deficiency in her abnormal son. This “aneusomy by recombination,” which was compatible with a brief survival, is explained by the relatively distal position of the break points. Crossingover inside the large inverted segment produced an important duplication and a small deficiency.