Lecture 7 (deleted 4f50fb8f-1dc8ae-5a2a49fd)

39 barr body male cells female cells in 1949 murray

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Unformatted text preview: ge adult sex) Environmental variables determine sex (during development or in adults) 32 16 2/14/12 Ques1on: But how did sex chromosomes evolve? (example from mammals) 33 Sex Chromosome Evolu1on (mammals) Sex chromosomes evolved (gradually) from autosomes 34 17 2/14/12 Problem: If the male Y is unable to recombine, then will it degrade and eventually become ex1nct given enough 1me? 35 David Page Whitehead Ins1tute MIT Sequenced the human Y chromosome and found…… 36 18 2/14/12 Palindromes: the Y chromosome savior Sequences that read the same forward & backward ‘Recombina1on’ between two arms of the Y can serve for self ­repair of muta1ons 37 Problem: If females have 2 X chromosomes and males only one, then females will produce twice as much protein from X ­linked genes! 38 19 2/14/12 Dosage Compensa1on Equalizes the amount of protein produced by X ­linked genes in males and females Flies: Double the ac1vity of the genes on the X chromosome in the male. Worms: Halve the ac1vity of genes on both X chromosomes in the female. Mammals: Genes on one X chromosomes in the female are inac1vated. 39 Barr body Male cells Female cells In 1949, Murray Barr observed condensed, darkly staining bodies in the nuclei of female cells. 40 20 2/14/12 Lyon Hypothesis In 1961, Mary Lyon hypothesized that the Barr body was an inac1vated X chromosome. 41 Lyon Hypothesis In 1961, Mary Lyon hypothesized that the Barr body was an inac1vated X chromosome. Within each female cell: One of the two X ­chromosomes is inac1vated. Which X ­chromosome is inac1vated is random. All but 1 X ­chromosome is inac1vated. *Females are func)onally hemizygous at the cellular level for X ­linked genes. *Females are mosaics for the expression of X ­linked genes. 42 21 2/14/12 Random X ­inac1va1on Within each female cell: X ­inac1va1on occurs early in development. That X remains inac1ve in all soma1c descendant cells (e.g., ‘patches’ of cells). Tortoiseshell cats: X ­linked color locus with 2 alleles X+ = non ­orange (usually black) X0 = orange Females: X+X+, X0X0, X+X0 Tortoiseshell cat...
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