<s>[INST] context: Single nucleotide polymorphisms are the most common DNA variations found in the human genome. SNPs influence disease susceptibility and some are usually 'neutral'. The effect of most SNPs on disease susceptibility is weak. Genomic imprinting Differences in gene expression result from an epigenetic process called imprinting. In most cases, imprinting selectively inactivates either the maternal or the paternal allele. Thus, maternal imprinting refers to transcriptional silencing of the maternal allele, whereas paternal imprinting implies that the paternal allele is inactivated. The molecular basis of Prader-Willi/Angelman syndromes lies in the process of genomic imprinting. Three mechanisms are involved: Deletions Prader-Willi syndrome - genes on maternal chromosome are imprinted, only paternal allele is functional. Thus deletion of paternal functional allele leads to Prader-Willi syndrome (pic) Angelman syndrome - genes on paternal chromosome are imprinted, only maternal allele is functional. Thus deletion of maternal functional allele leads to Prader-Willi syndrome (pic) Uniparental disomy Inheritance of both chromosomes of a pair from one parent Prader-Willi syndrome - both maternal alleles inherited which are imprinted and no functional set of genes (paternal allele) Angelman syndrome - both paternal alleles inherited which are imprinted and no functional set of genes (maternal allele) Defective imprinting In a small minority of patients (1% to 4%), there is an imprinting defect. In some patients with Prader-Willi syndrome, the paternal chromosome carries the maternal imprint In Angelman syndrome, the maternal chromosome carries the paternal imprint (hence there are no functional alleles) [/INST] question: Which of the following genetic aberrations is not a causative mechanism in cases of Prader-Willi syndrome? | options: A: Gene deletions B: Defective genomic imprinting C: Single nucleotide polymorphisms (SNPs) D: Uniparental disomy | answer: C </s>