X inactivation - Mary Lyon hypothesised that the X...

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X inactivation All cases of abnormal karyotypes in which a single autosome is missing (autosomal monosomy) are lethal during embryogenesis even for the smallest autosomes. Yet males with only one X chromosome (a medium sized chromosome) are (comparatively!) normal. How is this accomplished? The answer (as suggested by Mary Lyon in 1961) is by inactivation of one of the two X chromosomes in females so that the normal state for a cell is to have two active sets of autosomes and only one active X chromosome. The other X chromosome is condensed and inactive and is visible as a dark staining "Barr body" pushed against the nuclear membrane. The following image of cells from a female cheek swab was "borrowed" from the US Army . Three nuclei can be seen each with a characteristic dark blob (which is particularly clear in the centrally located nucleus).
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Unformatted text preview: Mary Lyon hypothesised that the X inactivation happened at random early in development so that each female is composed of two populations of cells. In one population one X chromosome is expressed and in the other, the second X chromosome is expressed. Females are thus mosaics , i.e. composed of two genetically distinct cell populations. For genes which are homozygous this will make no difference but for genes for which the female is heterozygous the two populations of cells will be of opposite phenotypes. In this way she explained the patterns of hair colouration in, for instance, the tortoiseshell cat which is always female except for the very rare occurence of an XXY male, which exception proves the rule! (Those of you who are interested might like to try this well written page on cat coat colour genetics .)...
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This note was uploaded on 11/22/2011 for the course CHEMISTRY CHM1025 taught by Professor Laurachoudry during the Fall '10 term at Broward College.

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