Uniparental Disomy

An exploration of uniparental disomy, with tangents into genomic imprinting, meiotic nondisjunctions, and de novo mutations.
Clarification on heterodisomy and effects on imprinting: genomic imprints in this case refer to epigenetic (methyl) silencing marks that are placed on specific genes. These marks are placed in a biological sex-specific manner during gametogenesis in each parent (in females, a methyl will be placed on one gene when in males that same methyl will not be placed). Old inherited marks from these parents' parents are erased before the new marks are placed. Therefore, when zygotes inherit genes affected by imprinting from their parents, one copy is methylated and therefore silenced while the other is not.
In heterodisomy, zygotes received two homologous chromosomes from one parent. These chromosomes exhibit essentially identical patterns of genomic imprinting - they are methylated (or not) at all the same loci because they both were marked (or not) in the same fashion during the parent's gametogenesis. Therefore, you get under (or over) expression at these imprinted genes, which can be deleterious.