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Mammals also compute X/A ratio: Hodgkin

Fri Mar 5 08:58:03 EST 1993


  The ratio of X chromosomes to autosomes (X/A ratio) is critical in
establishing sex and dosage compensation in the fruit fly (Drosophila) and
in the nematode worm (C. elegans). In a 1990 review Hodgkin pointed out
that "In each organism therefore, a sharp threshold is in operation. This is
one of the best examples of a threshold response in developmental
biology, and raises the question of how the embryo is able to compute
this ratio so accurately and reliably"(1).
  Mammals, however, seem to be left out of the picture. Hodgkin stated
that in mammals sex is "determined by the presence or absence or
absence of a Y chromosome, and dosage compensation by the number of
X chromosomes"(1). In a recent review he repeats that "For mammalian
dosage compensation, the Y chromosome is irrelevent and the determinant
is the number of X chromosomes."(2) (my italics).
  Hodgkin's otherwise excellent reviews, will have left many with the
impression that mammals do not compute the X/A ratio. In the case of
dosage compensation this would not seem to be correct. In an early
literature which has been reviewed by Mittwoch(3), it has been shown that
in mammalian polyploid states, as the number of sets of autosomes
increases the number of active X chromosomes increases(4,5,6). This
implies an ability to compute the X/A ratio.
                      Yours sincerely,
                      Donald R. Forsdyke,
                      Department of Biochemistry,
                      Queen's University, Kingston,
                      Ontario, Canada K7L3N6

1 Hodgkin, J. (1990) Nature 344, 721-728
2 Hodgkin, J. (1992) BioEssays 14, 253-261
3 Mittwoch, U. (1973) Genetics of Sex Differentiation. Academic Press,
  New York
4 Book, J. A. and Santesson, B. (1961) Lancet 2, 318
5 Harnden, D. G. (1961) Lancet 2, 488
6 Jacobs, P. A. and Midgeon, B. R. (1989) Cytogenet. Cell Genet. 50, 75-

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