What is the implication of having both copies of a gene on one chromosome with no copies on the other?

The situation described, where both copies of a gene are on one chromosome with no copies on the other chromosome, typically indicates a form of chromosomal alteration, such as uniparental disomy or deletion on the homologous chromosome. This arrangement can lead to misleading interpretations regarding carrier status.

When a gene is duplicated on one chromosome but absent on its homolog, it can create the false impression that an individual is not a carrier of a recessive trait. If a person carries a recessive mutation on one chromosome, and the second chromosome lacks that gene entirely, it can prevent detection of the carrier status during standard genetic testing. This phenomenon may lead to the conclusion that the individual does not carry a genetic predisposition to certain conditions, when in fact they may carry a pathogenic variant but are simply presenting with the duplication rather than the typical heterozygous state.

This understanding is crucial in genetic counseling and testing since identifying carriers accurately is important for assessing risks for offspring and providing appropriate counseling to affected families.