Uniparental disomy usually starts out as a trisomy with one of the gametes having 2 copies of the same chromosome (due to non-disjunction; discussed in detail in the Cytogenetics: Chromosomal aberrations article) fuses with another normal gamete having just one copy of the chromosome.
The organism will attempt to 'rescue' the zygote by losing one of the three chromosomes is most trisomies are incompatible with life (exceptions being trisomies 13, 18 and 21; discussed in detail in the Cytogenetics: Chromosomal aberrations lesson). If one of the two chromosomes from the same parent is lost, it will result in a healthy zygote with one chromosome per parent. But if the chromosome lost is from the normal gamete (the only copy ), it will result in the zygote having two chromosomes from the same parent, hence Uniparental disomy.
Examples of disorders caused by