Overview Changes in gene expression caused by mechanisms other than changes in actual DNA sequence Examples X-inactivation imprinting histone modification see Chromatin structure topic X-inactivation Overview normalizes the genetic amount of males and females (lyonization) inactivates # of X chromosomes - 1 in a Barr body triploid X will have 2 Barr bodies Mechanism mediated by XIST gene inactivation through methylation occurs at blastocyst stage in female embryos X copy chosen for inactivation is random after choosing every subsequent cell will have the same X copy inactivated Clinical relevance mosaicism non-homogenous X inactivation some cells express paternal X and some cells express maternal X Imprinting Overview describes differences in transcriptional activity based on whether the chromosome is of maternal or paternal origin at a single locus 1 allele is active 1 allele is inactive creates a hemizygous state Mechanism inactive ("imprinted") allele is methylated during gametogenesis creates transcriptional inactivity is maternal/paternal specific gene at one locus always methylated on a specific copy all cells of an individual have same imprinting level during gametogenesis of the individual the methylation state is erased reset to be either maternal or paternal depending on the sex Clinical importance Prader-Willi syndrome cause deletion or mutation of normally active paternal allele on 15q remember: Prader = paternal deletion presentation mental retardation hyperphagia → obesity hypogonadism neonatal hypotonia behavior problems affects both male and females Angelman's syndrome cause disruption of the maternally expressed and paternally imprinted gene UBE3A, which encodes an E3 ubiquitin ligase typically results from deletion of the normally active maternal allele on chromosome 15q same region of the genome as Prader-Willi syndrome deletion but opposite chromosome (not the same gene) presentation severe cognitive disability frequent seizures ataxia speech impairment hyperactivity inappropriate laughter "happy puppet" affects both male and females
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