Monosomy refers to the condition in which only one chromosome from a pair is present in cells rather than the two copies usually found in diploid cells. When cells have one chromosome from a pair plus a portion of the second chromosome, this is referred to as partial monosomy. Monosomy, or partial monosomy, causes certain human diseases such as Turner syndrome and Cri du chat syndrome.
Monosomy is used to refer to a status of an autosomal gene, when normally two copies are supposed to be present and instead only a single copy of a gene is present. This word can also refer to multiple genes or segments, or even an entire chromosome, where an individual is supposed to have two copies of this gene or chromosome, and they only have a single copy. The loss of one of two copies of an autosomal gene or segment of genes, or an entire chromosome, is a cause of human genetic disease.