Re: XY and XX chromosomes

On Apr 24, 3:23 pm, John Harshman <jharshman.diespam...@xxxxxxxxxxx>
wrote:
usenetposts wrote:
On Apr 17, 3:41 pm, John Harshman <jharshman.diespam...@xxxxxxxxxxx>
wrote:
Perplexed in Peoria wrote:
"Viktor D. Huliganov" <jerzy.jakubow...@xxxxxxxxx> wrote in message
news:967c43b8-8735-4ab3-b84e-4a26eeb80c87@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
I was wondering whether anyone could explain in layman's terms where
it is thought to have originated that the male has a Y chromosome that
comes from the father only, whereas there are two X chromosomes in the
female. The points that interest me are as follows:
1. Is the presence of a Y chromosome the main defining feature of
"maleness" in biology?
No. The *defining* feature is that males produce smaller gametes than
do females. Sperm vs egg. Pollen vs Ovule.
Y vs X chromosome is just one way of *causing* the difference between
male and female.
See:
http://en.wikipedia.org/wiki/Sex_determination
It is a complicated subject, and far from completely understood for many of the
more obscure branches of the tree of life. My reply here is only going to scratch
the surface.
2. Does plant-life also have the XX/XY feature?
See the wiki article.
3. At what point in the evolutionary tree is this chromosome thought
to have emerged?
It emerged many times. The XY system is used in placental mammals. WZ is
used in birds. The monotremes (platypus, etc.) apparently use a very complex
system. So, based on that evidence, the placental mammal version of the XY
system probably arose around 100 million years ago, though maybe as early
as 300 million years ago.
4. By what mechanism is it thought to have emerged?
See the wiki article. And then go on from there. It is very, very complicated,
since aspects of it have emerged so many times at so many places in the tree.
5. Have there been any alternatives to the XX/XY chromosome mechanism
for biological gender?
Many. And quite a few alternatives to the notion of only having two genders.
There are at least three very popular chromosome mechanisms - XY, WZ,
and haplodiploid - each of which probably arose multiple times in evolution.
Plus dozens of more complicated systems.
As an alternative to the two-gender approach, hermaphroditism is fairly common
(both reproductive genders in a single individual) and serial hermaphroditism
(one gender when young, another when more mature) is a fairly common variant.
Many microorganisms don't have a size difference between male and female
gametes. In these, you have 'mating types' rather than 'sexes', with the rule
being that you can't mate with your own mating type, but you can mated with
anyone else. And some species have as many as a dozen mating types.
Time to cite one of my favorite articles: Ogata, M., H. Ohtani, T.
Igarashi, Y. Hasegawa, Y. Ichikawa, and I. Miura. 2003. Change of the
heterogametic sex from male to female in the frog. Genetics 164:613-620.

A species caught in the act of a major divergence (at least at the
chromosomal level) in sex determination. At least from the "evolutionist
perspective". I have no idea what a creationist would make of it.
For us it is never problematic to find consistent explanations for
these things, as we will simply invoke the higher power.

That's not exactly an explanation, is it? That's declaring that you
don't got to show me no stinking explanations. God can do anything,
therefore anything can be "explained". There is no conceivable evidence
that could be inconsistent with your "explanation", so you have
explained nothing.

Well, that comes down to a philosophical view of the world. One view
says there should be explanations, the other says there are not
necessarily explanations. I say this, I'll be happy to hear your
explanations, but I'll be surprised if they ever cover enough open
questions as to make the case for creator/impersonal algorithm cut and
dried.




The more
interesting thing is to see how evolutionists account for these open
questions. In particular what emerges from this thread, the fact,
(which I did not know) that XY is relatively limited to mammals

No, that's not true. You have misunderstood. XY is quite common in life,
in fact extending to most insects, many frogs (as in the paper I cited),
and lots of other groups. It's just that these systems change through
time; XY systems have been gained and lost many times.

Is the new XY different to the old one? What is the difference between
a ZZ/ZW system and a XX/XY system? Why have none of the mammalia gone
back to a ZZ/ZW system?


and
does not even strictly apply to the monotremata, makes me really
wonder how on earth you can ever switch from one sexual system to
another when the risk must be so high that as a result you will lose
the ability to reproduce sexually and there is, after all, no asexual
fall back method in most land vertebrates.

That's why the frog paper is so interesting, as it catches a species in
the act of doing just that.


Reading the paper of 緒方 et al, the explanation of two competing gene
systems, the ZZ/ZW and the XX/XY at once explains how the risk is
taken away that a sexual animal won't be able to reproduce at all.
However, at the same time it makes new questions arise in my head,
such as how is there space for two competing systems in the codes and
if there is space for such, why don't more animals have more than one
system? And if there are two, the ZZ/ZW and the XX/XY, then why only
two? Why not a whole series in post amphibia animals? The monotremata
show that even quite zany variations on the theme will work, and they
also have interesting sexual dimorphisms, even down to the male
possessing poisons that the female doesn't have. (Unlike humans, where
it's the other way round - joke)

I need to get some kind of grasp on what makes ZW/ZZ and XX/XY
inherently different. Otherwise I can't get my head around this
conundrum.


And if we say that sexual dimorphism in reptiles is environmentally
determined, then isn't that a stronger survival feature that stripping
out the ability to react to circumstances with a different proportion
of males and females? How could the early mammalia have "gotten away"
with abandoning that competitive edge?

You assume it's a competitive edge. I don't think it is.

Only if the XX/XY system also contains some elements that can react to
environment. Does it, though?

Best,

Davey


.

Relevant Pages

  • Re: XY and XX chromosomes
    ... male and female. ... There are at least three very popular chromosome mechanisms - XY, WZ, ...  And some species have as many as a dozen mating types. ...
    (talk.origins)
  • Re: XY and XX chromosomes
    ... Is the presence of a Y chromosome the main defining feature of ... male and female. ... (one gender when young, another when more mature) ...
    (talk.origins)
  • Re: XY and XX chromosomes
    ... Y vs X chromosome is just one way of *causing* the difference between ... male and female. ... (one gender when young, another when more mature) ...
    (talk.origins)
  • Re: XY and XX chromosomes
    ... male and female. ... There are at least three very popular chromosome mechanisms - XY, WZ, ... heterogametic sex from male to female in the frog. ...
    (talk.origins)
  • What was a king anyhow? (was Re: Granada - king or Emir ?)
    ... as SRY and TDFY that are required to be male, ... differences in the Y chromosome. ... And tried to find some reasons in genetics for such a ... has had no basis in genetical heritage, ...
    (soc.genealogy.medieval)