Posted 1:17 PM by Robert

Hyperdowdified III: It Ain't Necessarily So

One of the more common - in both senses of that word - errors of the Maureen Dowd excursion into sexual genetics discussed in posts below is her naive conceptualization of cytogenetics, the science which attempts to correlate cellular events with genetic phenomena. Some might start with Big Mo's obvious obsession with the significance of size in a sexual context. But the less of that kind of thing the better.

Big Mo commits many peculiar cytogenetic errors. But her personal confusions seem to reflect the confusions in her personal life, and therefore have little general significance, so they're not worth commenting on in specifics.

What I would like to do here is note a fact that the reader may want to keep in mind the next time he or she comes across the kind of crass analogizing and bull-slinging about inter-species sexual characteristics and genetics that Big Mo's column in some ways typifies and which increasingly plagues popular science writing. Specifically, this:

There are two categories of chromosomes, autosomes and sex chromosomes. With cockatiels [and all birds], sex is determined by a heteromorphic (i.e. morphologically dissimilar) pair of chromosomes called sex chromosomes. In humans and some other species, these chromosomes are labeled X and Y. A male human has the XY complement (the heterogametic sex) and a female human has the XX complement (the homogametic sex). In some species, such as birds, the complement of sex chromosomes is the opposite of the above. Geneticists have assigned a different lettering scheme to make this distinction. The letters used are Z and W. A male cockatiel [and every male bird] has the complement ZZ (the homogametic sex) while the female has ZW (the heterogametic sex).

That is:

Human females are the ones that have two of the same sex chromosomes and human males are the ones that have two different sex chromosomes

- but

bird females are the ones that have two different sex chromosomes and bird males have are the ones that have two of the same sex chromosomes .

The significance of this inter-species genetic structural sexual difference is not understood. But it should serve as a warning against facile cross-species thinking. And one might also want to keep in mind some other molecular genetic history:

Already in 1949 Murray Barr and E.G.Bertram found in the nervecells of a female cat a small dark body as well as in most of the body cells. They identified this later on as sex-chromatin (a sex-chromosome in shrinked condition). This sex-chromatin is also called the Barr-body after its discoverer. Later on it was also found in the cells of other mammals but only in females.

Eventually it was Ohno who proved in 1959 that strictly speaking the Barr-body is one of the two sex-chromosomes in a female. The observations of Ohno soon were confirmed for other female mammals including humans. Although one expects to find two sex-chromosomes in all body cells of a female mammal, only one is to be found the other is the so called Barr-body.

During evolution in placental mammals the Y-chromosome has lost all genes which were allelic to the genes on the X-chromosome. The result is that most and maybe all X-linked genes in hemizygous state (XY) occur in males. Each X-linked gene must have been adapted to the hemizygous state by doubling the amount of "product output".

When this redoubling was achieved in an efficient way (during evolution), the genetic difference between the male having only one X and the female having two X-chromosomes became to large. A certain need to compensate the dosage effect for X-linked genes between the two sexes arose. In mammals this is achieved by inactivation of one of the two X-chromosomes in individual somatic cells in the female. In consequence this means that the expression of X-linked genes in individual somatic cells of both sexes is hemizygous making the female mammal a genetical mosaic dependent on the sex-linked genes.

A good example are cats with orange and black marks on their fur. The orange-black fur is caused by two alleles of one gene on the X-chromosome, one for orange and one for black. Dependent on which X-chromosome is deactivated, cellclones appear with either the active gene for orange or the active gene for black. The mechanism of inactivation is rather involuntary.

In birds the X-linked genes occur in hemizygous condition in hens. Yet there is no indication at all for dosage compensation of these genes. On the contrary, the X-chromosome of birds even shows a certain dosage effect. The full expression of an X-linked mutant phenotype requires even the presence of two dosages of a mutant gene in the heterozygous cock.