What is genomic imprinting, and how does parental origin influence phenotype?

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Genomic imprinting is an epigenetic process in which the expression of certain genes depends on which parent the allele was inherited from. This is achieved by imprint marks, mainly DNA methylation and related histone changes, that silence one allele in the offspring so only the other is expressed. The result is that the same genetic sequence can have different effects in offspring depending on whether it came from mom or dad. A classic example is a gene pair where the paternal allele is active for one gene while the maternal allele is active for its partner, illustrating how parental origin shapes expression and function. This can influence phenotype because disrupting imprinting or losing the active copy can lead to disorders such as Prader-William syndrome or Angelman syndrome, which depend on whether the paternal or maternal copy is affected. Imprinting is a nuclear phenomenon reset in germ cells and is not about random mutations or the mitochondrial genome.