Uniparental disomy


Uniparental disomy is the situation where a person inherits two copies of a chromosome from the same parent and no copies from the other. Disomy may be the consequence of meiotic failures such as non-disjunction, or it may occur early in foetal development. It may also be a consequence of trisomic rescue.

Uniparental disomy comes in two flavours:

  • Heterodisomy where two homologous chromosomes (one from each grandparent) are inherited from the same parent. This implies that non-disjunction occurred during meiosis I.
  • Isodisomy where two identical chromosomes are inherited from the same parent. This implies that non-disjunction occurred during meiosis II, or perhaps a postzygotic chromosomal duplication.

There are usually no phenotypic manifestations of uniparental disomy, although isodisomic inheritance (non-disjunction in meiosis II) can lead to rare recessive disorders, such as Prader-Willi and Angelman syndromes, both potentially caused by UPD of chromosome 15. Recessive disorders in children where only parent is a carrier of the recessive allele imply that an isodisomic inheritance of the recessive allele is likely to have occurred.

UPD inheritance of genomically imprinted genes can lead to the null expression of a gene and this can have various phenotypic manifestations including delayed physical development and mental retardation.