heterodisomia

Heterodisomy is a genetic condition characterized by the inheritance of two copies of a specific chromosome from one parent, with the other parent contributing none. This results from a form of uniparental disomy (UPD), where both homologous chromosomes from one parent are inherited, rather than one chromosome from each parent. Unlike isodisomy, which involves two identical copies of a chromosome, heterodisomy involves two different homologs.

Heterodisomy typically occurs due to errors in meiosis, the process that produces reproductive cells. For example, nondisjunction during meiosis I can lead to a gamete with two different homologous chromosomes, which, when fertilized, results in uniparental disomy. If the disomic chromosome is then duplicated or involved in chromosomal rescue mechanisms, heterodisomy can ensue.

Clinically, heterodisomy may be associated with various genetic disorders, especially if the affected chromosome contains imprinted genes—genes whose expression depends on parental origin. The presence of heterodisomy can disrupt normal gene expression and lead to conditions such as Prader-Willi syndrome or Angelman syndrome, depending on the specific chromosome involved. However, heterodisomy may also be asymptomatic if the genes involved do not exhibit imprinting effects.

Diagnosis typically involves molecular genetic testing, such as parentage analysis or methylation studies, to determine the parental origin and whether the inheritance pattern deviates from Mendelian expectations. Understanding heterodisomy is crucial for genetic counseling and diagnosis of uniparental disomy-related disorders. Overall, it reflects a rare but significant mechanism in human genetic variation and disease.