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Dosage compensation is used to make up for different copy numbers of the major sex chromosomes in males and females (One X or Z chromosome in one and two in the other). Two main mechanisms of dosage compensation exist, inactivation and upregulation.

But dosage compensation is not entirely perfect, the amount of gene product from an upregulated X/Z-linked genes in the heterogametic sex is not exactly that of the two in the homogametic sex.

"In Drosophila, the male-specific lethal (MSL) ribonucleoprotein complex mediates dosage compensation by upregulating transcription from the single male X chromosome approximately twofold."

On average how well is dosage compensation performed? What is the variance like:

  • within a single chromosome (between genes)?

  • within species?

  • between species?

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  • $\begingroup$ You mean Y chromosome? $\endgroup$
    – AliceD ♦
    Commented Nov 6, 2014 at 13:36
  • $\begingroup$ @ChrisStronks no. there is one X in males, two in females. To compensate for this males either upregulate their X or females down regulate (or silence) one of theirs. $\endgroup$
    – rg255
    Commented Nov 6, 2014 at 14:01
  • $\begingroup$ Note that there are several sources of variation. There may be stochastic, environmental and genetic among individuals, within individual and within chromosomes variations. For example, in the pseudo-autosomal region (PAR) were the genes are present in two copies in both males and females, you may have much lower change in expression as there is nothing to compensate for. It may (or may not) depend on the protein beeing made and how a shift in the concentration of this protein influence the rate of chemical reaction according to a Michealis-Menten dynamic. $\endgroup$
    – Remi.b
    Commented Nov 6, 2014 at 14:44
  • $\begingroup$ I may have too a selectionist view though and I may overlook how complicated is the question. It is a very interesting question +1 $\endgroup$
    – Remi.b
    Commented Nov 6, 2014 at 14:44
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    $\begingroup$ @ChrisStronks ah now I see what you mean. It's in reference to zz/zw systems (and should be a z not a w) $\endgroup$
    – rg255
    Commented Nov 6, 2014 at 22:14

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