Imprinting

For a small number of disorders, phenotypic expression of a trait depends upon whether inheritance was from father or mother. This results from differences in activation of genes. Methylation of DNA prevents its transcription. A gene may be active only on a chromosome from the father, or vice versa. A mutation will only produce an effect if it involves the active gene. The best example of this is Prader-Willi syndrome (mutation inherited from father) versus Angelman syndrome (mutation inherited from mother). In Beckwith-Wiedemann syndrome, uniparental disomy can give a child two paternal active abnormal alleles, where normally at least one inactive maternal copy is present.