Figure 1.

Subtypes of UPD with Example Mechanisms and Detectable Genomic Features for Each UPD Subtype

There are three subtypes of UPD: heterodisomy (hetUPD), isodisomy (isoUPD), and partial isodisomy (partial isoUPD). HetUPD is caused by nondisjunction in meiosis I, and an affected individual will inherit both homologs of a chromosome from the same parent. IsoUPD is caused by nondisjunction in meiosis II, and an affected individual will inherit two identical copies of one homolog from one parent. Partial isoUPD is caused by nondisjunction in either meiosis I or meiosis II after crossing over has happened, resulting in sections of isodisomy and heterodisomy on the UPD chromosome. Given genomic data from a parent-child pair, all UPD subtypes can be detected in the same way based on identity-by-descent (IBD): a parent-child pair will be missing IBD across an entire chromosome. Lastly, isoUPD and some types of partial isoUPD will show large runs of homozygosity (ROHs), which can be detected computationally without the need for parental genotype data.