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Unformatted text preview: Lecture 6 24/1/08 Dosage compensation; extensions and modifcations oF Mendelian principles Dosage compensation Human females have 2x dosage of X-linked genes compared to males. How do cells cope with 2-fold difference in chromosome dosage? Murray Barr, Susumu Ohno, and Mary Lyon: Barr bodies and X-inactivation. Inactivation occurs randomly in embryonic cells and is stably propagated in cell division: an epigenetic change. Chromatin changes from decondensed (active ) euchromatin to condensed (inactive) heterochromatin . Mechanism is not understood but requires X-linked RNA, Xist. Random inactivation so females are mosaics of tissue in which one or the other X is active (cf tortoiseshell cats). Drosophila: males hyperactivate X-linked genes. Revisiting dominance and recessivity Incomplete dominance (partial dominance, semidominance): heterozygous Aa is phenotypically distinguishable from homozygous AA. One mechanism for semidominance is haploinsufFciency....
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- Spring '08