What is the mechanism of genomic imprinting, and name two imprinting disorders?

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Genomic imprinting works through parent-of-origin–specific expression of certain genes, driven by epigenetic marks that silence one allele depending on whether it came from mom or dad. These marks are established during gamete formation and maintained after fertilization, so some genes are expressed only from the paternal copy and others only from the maternal copy. This monoallelic, parent-origin–dependent expression is the hallmark.

Two classic imprinting disorders are Prader-Willi syndrome and Angelman syndrome, both linked to the same region on chromosome 15 (q11–q13). If the paternal copy is lost or silenced (via deletion or maternal uniparental disomy), Prader-Willi occurs; if the maternal copy is lost or silenced (via deletion or paternal uniparental disomy), Angelman occurs. The other options describe mechanisms or conditions that do not fit imprinting: expression that is random, conditions caused by trisomy rather than imprinting, methylation that is exclusively paternal, or allelic expression that is the same regardless of parental origin.