Supernumerary marker chromosomes (SMC) and uniparental disomy (UPD): coincidence or consequence?

### Key points Uniparental disomy (UPD) describes the inheritance of a pair of chromosomes from only one parent.1 Mechanisms of formation are trisomy rescue, monosomy rescue, post-fertilisation errors, and gamete complementation. Problems associated with UPD include trisomy mosaicism, genomic imprinting, homozygosity of autosomal recessive mutations, or even a combination of these.2 Supernumerary marker chromosomes (SMCs) are predominant in only a proportion of cells leading to a specific kind of trisomy mosaicism. The presence of a cell line …