FORTUITOUS DETECTION OF UNIPARENTAL ISODISOMY OF CHROMOSOME-6

Uniparental isodisomy is defined as the inheritance of two copies of t he same parental chromosome and can result in defects when it produces homozygosity for a recessive mutation or in the presence of imprintin g. We describe the detection of a chromosome 6 uniparental isodisomy i n a 9 year old girl, discovered during a search for an HLA identical s ib. HLA typing, erythrocyte phenotyping, and genotypes of microsatelli te polymorphisms were compatible with a paternal isodisomy of chromoso me 6, with normal biparental origin of the other chromosomes. Paternal cells were not responsive to the patient's cells in mixed lymphocyte cultures. This fortuitous detection of a chromosome 6 isodisomy sugges ts that cases of chromosome 6 UPD may not be deleterious and may there fore go undetected.