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From Chapter 20 re: Methylation

Imprinted Genes
The genetic disease that a child with a 15q11 defect will get depends on which parent supplied the mutated chromosome. If it came back from the mother, it will be Angelman syndrome, and if it came from the father, it will be Prader-Willi. The gene that underlies these disorders is imprinted -- that is, carries a biological marker that tells cells to express only the maternal or paternal copy. Imprinted genes can "remember" their parental history, through a process known as methylation that leaves some switched on and others switched off.

Methylation

Imprinting works because of a process called DNA methylation by which the function of geners is altered by chemical modifications. It involves the addition of a chemical tag, know as a "methyl group", to the DNA base cytosine. This can turn down a gene's activity, or switch it off entirely. It is critical to ensuring that genes are expressed only at the right times in an organism's life cycle, and in the right kinds of tissue.

Most of these methyl tags are wiped away during the early stages of embryonic development. The main exceptions are the imprinted genes, which retain these marks to flag up their maternal or paternal origin.