Detection and localization of an extra HLA locus in a karyotypically normal male by chromosomal in situ hybridization

The codominant expression of three HLA haplotypes was found in a healthy 21-year-old Black male, whose prometaphase karyotype was normal by light microscopy. He was the sibling of an antenatally diagnosed female fetus with a partial duplication of 6p. The duplication arose from a complex presumably balanced maternal chromosome rearrangement: 46,XX,dir ins(14;6)(14pter----14p11::6p22----6p21.1::14 p11----14qter; 6pter----6p22::6p21.1----6qter). Chromosomal in situ hybridization using a tritium-labeled genomic clone corresponding to a class I HLA gene revealed two sites of hybridization: at 6p21.3, the band to which this probe has been assigned in normal individuals (Morton et al. 1984a) and a second site at 6p11. We postulate that a recombinational event during meiotic pairing in the mother led to the reintroduction into the normal chromosome 6 homolog of a small segment of the original insertion in chromosome 14 which contained the HLA-A and -B determinants.