isodisomy

Isodisomy is a form of uniparental disomy in which both copies of a single chromosome are inherited from one parent and are identical copies of that chromosome. It contrasts with heterodisomy, where the two chromosomes from the same parent are non-identical homologs.

Isodisomy typically arises when a zygote with only one copy of a chromosome (monosomy) duplicates that chromosome to restore disomy, a process called monosomy rescue. It can also arise from post-zygotic duplication of a single chromosome, or from mitotic recombination leading to segmental isodisomy.

Because the two copies are identical, isodisomy increases homozygosity for any variant on that chromosome, which can unmask recessive mutations and lead to recessive diseases if a harmful variant is present. Additionally, some chromosomes carry imprinted genes; uniparental isodisomy can disrupt normal imprinting patterns, potentially causing imprinting disorders depending on the chromosome involved.

Detection relies on genetic testing such as SNP genotyping or microarray analysis, which show long contiguous stretches of homozygosity with absence of heterozygosity for the affected chromosome; parent-of-origin studies or methylation assays help determine parental origin and imprinting involvement.

In most cases UPD is de novo and recurrence risk is low, though it can be increased if a parent carries a balanced translocation or other chromosomal rearrangement. Counseling may be offered.