Every credit to Steve from Kernow- will someone summarise what he is saying- are men or women the superior X

The Joy Of X.

The defining difference between women and men lies within our chromosomes. To be precise, women have two copies of the "X" chromosome, while men have one X and one Y chromosome. Reports in the latest issue of the journal Nature describe the sequencing and analysis of the X chromosome. The results shed more light on the secrets of the sexiest and most mysterious of all the chromosomes.

Our DNA is packaged into compact bunches, chromosomes, enabling the precise segregation of genetic information when cells divide to give rise to two daughter cells and allows the successful passage of genetic information to daughter or son. Our chromosomes, one from our mother and one from our father, are orchestrated into pairs during meiosis, cell division in preparation for sexual reproduction. We have 23 pairs of chromosomes and one of each pair will be inherited by our children. 22 of the pairs are virtually identical and are called autosomes. One pair differs quite dramatically. Women have a pair of almost identical "X" chromosomes, while men have one X and one Y.



Above is what's known as a karyotype, a collection of one person's (in this case a man's) chromosomes arranged on a grid. The chromosomes are "painted" with fluorescent probes which recognize and stick to particular DNA sequences. The fluorescence pattern shows the near identical nature of each chromosome, one from each parent, in the autosomal pairs ( 1 through 22). The near identity reflects the presence of almost identical genes in the same position on each chromosome; the two corresponding genes on the chromosome pair are called alleles. During meiosis, an event called recombination takes place in which adjacent autosomes exchange pieces of chromosome between one another. Since the pairs are so similar, and since recombination occurs at the same positions on the chromosomes, there is usually little change in the genetic composition of the chromosomes. However, the little change that does occur contributes to genetic diversity, which is essential for the survival and evolution of species.
We can clearly see that X and Y are the odd couple of the karyotype. A woman's karyotype would show two almost identical X chromosomes, however, the male karyotype above reveals an X that is painted differently and is much larger than the Y, which looks like it could use some help from the drug discussed in a previous post. Why are the X and the Y so different?

The Evolution of X.
About 300 million years ago, the ancestors of the X and the Y were probably another autosomal pair. As mammals started to evolve, the X and the Y started to develop their own identities, forming the basis of gender determination; they became the sex chromosomes. Over the course of the next 300 million years, the Y gradually shed many of the genes that were also present on the X, since both females and males had at least one copy of the X. On the other hand, the X kept most of its genes and probably added some, the result being that it now has about 1,100 genes along its length, while the Y has less than 200. As the X and the Y diverged, they began to share their genetic information through recombination less and less, the result being that they now have very few genes in common. There is a small region of commonality, called the pseudoautosomal region, where they can recombine, Since this region is probably essential for pairing during meiosis, it is likely that the divergence between the X and the Y has gone as far as it will go.
One consequence of the X and the Y not switching genetic information through evolution has been that the X has probably been subject to different evolutionary selection pressures than all the other chromosomes. Genetic diversity, as mentioned above, is fostered by recombination between parental chromosomes during meiosis. The X can not do this and so is, in a sense, stuck with what its got. However, because males only possess one copy of the X, any change in the X, by rare recombination or mutation events, will be completely exposed and not masked by a normal allele on the other X. Because males had the ability to foster multiple offspring with multiple partners, any change in the X "for the better" would have prospered over those changes "for the worse". It is quite likely, therefore that a certain category of beneficial genes may have been evolutionarily selected for on the X in a way that they might not have been on other chromosomes. I'll discuss what this category might be at the end.

The Consequences of X.
More than a hundred years ago it was recognized that certain hereditary diseases that were expressed early in life were far more prevalent in boys than girls. Since then a large number of hereditary diseases have been found to be "X-linked", including certain forms of hemophilia and muscular dystrophy, and a wide range of mental impairment disorders, the most common of which is Fragile X Syndrome. The X-linked disorders arise from inherited mutations in genes on the X that result in the production of defective protein products from these genes. These mutations are recessive, that is, in the presence of a normal allele, the normal protein produced from this allele overcomes the effect of the mutation. Therefore girls with one recessive X allele are disease-free, since there is always a normal allele on the other X (it's actually a bit more complicated, as we'll see). Of course boys don't have the protection of the normal X and will develop the disease; girls generally inherit an X-linked disease in the rare event of a "carrier" mother having sex with a father with the disease.

The Silencing of X.
You may have been asking yourself (if you weren't, pay attention, dammit!); "If women have two copies of the X and men have just one, does that mean that women produce twice as much of X-coded proteins as men? Can this account for the obvious differences between the sexes?" Well, if both X chromosomes were active, the answer to the first question would be yes. Gene dosage, in other words the number of active alleles, generally correlates with gene product (protein) levels. However, early in embryonic development females randomly inactivate one of their X chromosomes, leaving only one active. X-inactivation is initiated by one of the genes on the X itself, which produces an RNA molecule called XIST. The XIST molecule randomly sticks to one, and only one, of the X chromosomes and this triggers a chemical modification of the DNA in that particular X which prevents it from coding for protein production. The modification is permanent and is passed on to all successive generations of cells. The result is that women are a mixture of cells in which some cells have one X, and other cells the other X, inactivated. This mosaic explains why girls usually escape X-linked disease, even in the face of X-inactivation; while some cells have an active recessive, "bad" gene, others have an active normal gene. This is apparently sufficient to protect against the disease phenotype.

The Sequencing of X.
A large consortium of scientists reported the complete sequencing of the X chromosome in the March 18th issue of Nature. Although there were no major surprises in the sequence, they were able to gather some important information. It has been thought that genes on the X play little role in the control of cell growth, and therefore in proliferative diseases like cancer. However, the consortium found that a surprisingly high number of X genes code for so-called cancer related antigens. These are proteins normally expressed only in the testis in healthy individuals (whether this is an ironic coincidence or has some meaning is not known) but are also expressed in tumors. Although they probably don't play a role in cancer progression, they have been regarded as good targets for anti-cancer vaccines. It will be important to find out why they are clustered on the X.
In another paper in the same issue of Nature, researchers from Penn State and Duke examined more closely the efficiency of X-inactivation. When an inactivated X chromosome is transferred from a human cell to a mouse cell in the laboratory it remains inactivated. Using this knowledge, Carrel and Willard devised an experiment to find out whether any X-linked genes might escape inactivation. They found that a surprising 15% of all genes repeatedly remain active on a silenced X, and that another 10% sometimes escape inactivation to variable levels. This means that 15-25% of X-linked genes are more active in women than in men, a finding that may have profound implications in terms of our understanding of inherent differences between the sexes. This is a subject that has always been hotly debated, even more so in recent weeks, following controversial comments from the president of Harvard University, suggesting that women are innately less equipped, on average, to do well in the fields of science and mathematics*. Moreover, the 10% of variably inactivated X genes suggests an unexpected diversity in the female population that can not be present in the male population. (Remember, in an evolutionary sense, genetic diversity is a good thing .....).

The Secret of X.
It has become clear in recent years that the X, once regarded as somewhat dull, is the most unique and intriguing of the chromosomes. It, along with the Y, determines an individuals sex, it is the only chromosome to inactivate one of its copies, variants among its genes are responsible for a significant number of hereditary diseases, and now it seems it may play unexpected roles in other diseases and in genetic diversity between and within the sexes. All of these features notwithstanding, however, the X may hide an even greater secret.
The first clue that the X may harbor more than its share of genes involved in higher brain function came from the realization that a large number of X-linked diseases cause mental impairment, far more than those associated with other chromosomes. More circumstantial evidence has come from studies of identical twins. Identical twin girls inherit one X from their mother and one from their father, however, one of these is randomly inactivated. Moreover, as mentioned above, there seems to be variable expression of ~10% of genes from the inactivated X. Identical twin boys, on the other hand, have just the one X from their mother. Therefore, variations in behavior that are observed in twin girls, but not twin boys would be expected to involve X-linked genes. It was found in a large survey of identical twins that attributes linked to intelligence, such as verbal skills and social behavior, were more variable between identical twin girls than twin boys, pointing to a link between the X and intelligence traits.
As discussed above, the X has evolved in a unique manner. At some point in evolution, increased intelligence probably became the primary motivator for survival and it has been suggested that early in evolution females became more attracted to "more intelligent" males. As a result, it is likely that genes associated with intelligence and cognition co-evolved and became selectively clustered on the X.
So, it may well be that the X harbors many of the genes that separate us from our closest animal relatives. The genes that enable us to appreciate the genius of Mozart or Bowie, or to wonder at the beauty of a sunset over the Pacific or a meadow of desert flowers in the morning light, or to carry out the mass deception and destruction of our own kind.

Is it possible that the X acts as the genomic guardian of humanity? If so, how ironic would it be that women, who in many of the world's societies are denied the same privileges and freedoms enjoyed by men, have two copies of the X to man's one?

Gordon Confusious