Dosage compensation

Dosage compensation, also known as Ohno's Hypothesis, is a hypothetical genetic regulatory mechanism which operates to equalize the phenotypic expression of characteristics determined by genes on the X chromosome so that they are equally expressed in the human XY male and the XX female. In 2006, literature was in support of this hypothesis. More recently (2008, 2011), the general consensus in literature refutes this hypothesis.

Older Research (May no longer be valid)
Dosage compensation may occurs in other organisms like the fruit fly Drosophila melanogaster and the round worm Caenorhabditis elegans.

Species can have different mechanisms of dosage compensation. In human females (XX), one chromosome is inactivated (see X-inactivation), resulting in a heterochromatic and largely genetically inactive Barr body. Drosophila males (XY) double the expression of genes along the X chromosome. In C. elegans hermaphrodites (XX), both X chromosomes are partially repressed. Any of these mechanisms results in balancing the relative gene expression between males and females (or, in the case of C. elegans, hermaphrodites and males).

In plants (which lack dimorphic sex chromosomes), dosage compensation can occur when aberrant meiotic events or mutations result in either aneuploidy or polyploidy. Genes on the affected chromosome may be upregulated or down-regulated to compensate for the change in the normal number of chromosomes present.

In Humans*: Women with two normal X chromosomes have the same blood levels of X-chromosome protein products, such as factor VIII, as normal men, who of course have only one X chromosome. An exception to this phenomenon of dosage compensation is the level of steroid sulfatase in blood, which is increased in women compared with men. Not surprisingly it has been shown that the locus for steroid sulfatase (deficiency of which causes a skin disorder known as ichthyosis) is in the pseudoautosomal region.