Uniparental disomy

Another unusual mechanism causing human disease is that of
uniparental disomy (UPD), where both copies of a particular
chromosome are inherited from one parent and none from the
other. Usually, UPD arises by loss of a chromosome from a
conception that was initially trisomic (trisomy rescue). The
resulting zygote could contain one chromosome from each
parent (normal), two identical chromosomes from one parent
(isodisomy) or two different chromosomes from one parent
(heterodisomy). Occasionally UPD may arise by fertilisation of
a monosomic gamete followed by duplication of the
chromosome from the other gamete (monosomy rescue). This
mechanism results in uniparental isodisomy. Theoretically, UPD
could also arise by fertilisation of a momosomic gamete with a
disomic gamete, resulting in either isodisomy or heterodisomy.