Overview Changes in gene expression caused by mechanisms other than changes in actual DNA sequence Examples X-inactivation imprinting histone modification login to view 1 more bullet X-inactivation Overview normalizes the genetic amount of males and females (lyonization) inactivates # of X chromosomes - 1 in a Barr body login to view 1 more bullet Mechanism mediated by XIST gene inactivation through methylation occurs at blastocyst stage in female embryos X copy chosen for inactivation is random login to view 1 more bullet Clinical relevance mosaicism login to view 2 more bullets Imprinting Overview describes differences in transcriptional activity based on whether the chromosome is of maternal or paternal origin at a single locus login to view 2 more bullets creates a hemizygous state Mechanism inactive ("imprinted") allele is methylated during gametogenesis login to view 1 more bullet is maternal/paternal specific login to view 1 more bullet all cells of an individual have same imprinting level during gametogenesis of the individual the methylation state is erased login to view 1 more bullet Clinical importance Prader-Willi syndrome login to view 10 more bullets Angelman's syndrome login to view 13 more bullets