Bruce writes:

> Wirt Atmar writes:
>
>  >The amount of information that's required to encode any single individual
>  >human being is only 1GB (4 billion base pairs * 2 bits (4 choices) / 8
>  >bits/byte). Twenty, thirty or forty years ago, when computers were still
>  >young, that number would have seemed quite large, but now it seems
>  >surprisingly small.
>
>  it's even smaller than that, actually, since very few (< 5%) of those
>  base pairs are actually translated. The rest are just hanging on for the
>  ride. It takes more than the notional 50MB of data, however, because some
>  information is encoded in what might be called "tags" on those
>  "bitfields." One important function for those tags is in making sure that
>  males and females make the right amounts of certain gene products even
>  though they have different numbers of the genes that code for them.
>
>  I haven't seen an estimate of the amount of information encoded by these
>  "tags", but it could theoretically double the total information content.
>  (It's known to be smaller than that.)

One of the problems with analogization is that I'm unsure about what Bruce is
calling "tags" in this context. There are two characteristics that separate
male and female mammalian genomes. One is called "imprinting". The other is
called "dosage compensation."

In imprinting, certain genes are turned off (rendered quiescent) in each of
the various 22 somatic chromosomes that constitute a male spermatazoon (22
"somatic" chromosomes and 1 sex chromosome, the Y). Exactly the same effect
occurs in females, but different genes are suppressed during its
gametogenesis. The explanation that I've long advocated for this phenomenon
is that imprinting acts as an evolutionary brake, preventing mammals from
backsliding into parthenogenesis (literally "virgin birth"). So long as
critical, but different, genes are suppressed during spermatogenesis and
oogenesis, one gender by itself cannot successfully build a new, functional
diploid individual by simply autonomously doubling the chromosomes of the
gamete sometime gametogenesis.

In order to keep the process of sex alive in the phyletic lineage, no
mechanism could be simpler. Having different genes on the same chromosome
disabled in the two genders critically requires the union of
paternally-derived and maternally-derived genomes before an informationally
complete individual can develop. But this process doesn't add any information
to the genome as a whole; rather it subtracts it.

The other effect is dosage compensation. In mammals, as in flies, lizards,
and a great many other animals, males have a specific sex chromosome, the Y.
The Y chromosome is a much reduced version of its original pair, the X. In
mammals, individuals bearing an XX combination are female. Males are XY.

Out of the 23 pairs of chromosomes in humans, the X is the only chromosome
that doesn't have a backup copy in males, thus males come to the table with
only half the  capacity to generate the gene products that reside on the X
chromosome as females do. Given that males are put at such a distinct
disadvantage, the question is how do males get alone as well as they do?

The answer was first determined by Mary Lyon in 1961. In ends up that the
problem isn't solved in mammalian males but rather in females. During female
embryongenesis (development), in large swatches of cells, one or the other
inherited X genomes is turned off. For a relatively large segment of cells,
only the paternally-derived X might be translated. In other segments, only
the maternally-derived X will be expressed. Calico cats are the most familiar
result of this "mosaicism" (and thus only female cats can be calico).
Nonetheless, regardless of which X is quiesced, the result is the same:
females express the same dosage level of gene products on their X chromosomes
as males do.

Flies, btw, solved the problem differently. Although male flies are XY too,
in the males, the X chromosome is doubly translated into gene product, thus
matching the female fly's natural gene product dosage level.

But once again, in either the case of mammals or flies, the process adds no
new information to the total genome.

In all animals, maleness is a quality built on a female platform. Neither
males nor sex are necessary in a mathematical sense to life and both would
seem to be highly dispensible. A mother-daughter parthenogenetic line of
inheritance could theoretically suffice. Indeed, there are a lot of
theoretical advantages to an all-female population. These advantages are
generally called the "cost of males", the "cost of sex" problems in biology.
The reason that such lineages don't exist undoubtedly lies in the rate of
accumulation of errors in germline DNA that such an informational
architecture implies.

Otherwise, males and females in every species are virtually identical
genetically. Only a very little tissue is modified in order to create a male
or female phenotype, and in mammals that little difference is derived from a
cascade of signals and triggers that appear to be derived from the expression
of just one gene on the Y chromosome, the SRY (which is short for "sex
deteriming region on the Y chromosome"). Except for this (and perhaps a few
other genes on the Y), all of the information to build a male phenotype is
present in every female, and vice versa.

Wirt Atmar

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