Partial trisomy 13 presumably due to recombination in an inversion heterozygote and by unequal crossing‐over

Abstract

Two unrelated infants with partial trisomy 13 for the distal half of the long arm are described. In one, a familial pericentric inversion is present in 3 generations and crossing-over in the inversion loop is considered as cause of partial trisomy 13. The other showed a tandem duplication of the distal half of the long arm of chromosome 13 beyond 13q14. This is interpreted to have arisen by unequal crossing-over in mispaired synapsis. Recombination rather than breaks is apparently a distinctive although rare cause of human chromosomal imbalance.