Question about the X and Y chromosomes

[JAR]

I have a question about the X and Y chromosomes.

Are the X and Y chromosomes both descended from a chromosome that was neither X nor Y?

[Dr. Imago]

Originally posted by JAR
I have a question about the X and Y chromosomes.

Are the X and Y chromosomes both descended from a chromosome that was neither X nor Y?

The interesting thing is that, in the normal karyotype for women (46, XX), one of the X chromosomes in each cells is inactivated by a process of "lyonization" and subsequently rendered unusuable. This lyonized X can be observed as a Barr body on the nucleus of a polymorphonuclear WBC in a blood smear. What's interesting is that this happens during division of the embryo, and that the lyonization is balanced - some cells get one X (from the father) and other cells get the other X (from the mother).

This does not happen to the normal karyotype for men (46, XY), where the Y chromosome contains the genes that determine the sex. One of the two X's, as I'm sure you know, is always from the mom. The other X or Y is from the father, who determines the sex of the offspring.

Since the Y chromosome essentially determines the sex (it's absence always produces a female phenotype, even in the genetic disorder Turner's syndrome - 45, XO), it is unlikely that they are a hybrid, different descendants, or otherwise. What it may be is that the Y chromosome is a completely different and special type of "extra" chromosome, with the normal somatic cell phenotype truly only needing a complement of 45 chromosomes for most cells to carry out normal function.

I think you'd have to go back very, very far evolutionary-wise to fully answer this question. And, that's far beyond my personal knowledge database. ;)

[Jon_in_london]

Turners syndrome....

what would happen if you deleted the entire y chromosome from a male zygote?

[Soapy Sam]

JAR- Next time you're in a bookshop, browse the chapters on sex chromosomes in Matt Ridley's excellent book "Genome".

There sre some very counter intuitive things going on in there.

[Dr. Imago]

Originally posted by Jon_in_london
Turners syndrome....

what would happen if you deleted the entire y chromosome from a male zygote?

You'd end up with a female with Turner's syndrome.

[athon]

Although it is far from certain, most molecular evolutionists agree with the theory that the Y chromosome is a weakened sex-chromosome, most likely an early form of the X. The thing is, the Y chromosome is tiny in comparison, only holding a few genes. None of these genes are essentialy for life (obviously).

There are instances of small parts of chromosomes splitting away and forming 'min-somes' on their own. Molecular biologists can use this and make artificial chromosomes. It has been suggested that Y was a truncated version of an X.

Also an interesting side note, the slight bias towards females in the world population has been attributed to the delicate nature of the Y chromosome. Anecdotally, the father of an ex of mine worked in radiography, where there were a large number of fathers who had no sons.

Athon

[Jon_in_london]

Originally posted by ThirdTwin


You'd end up with a female with Turner's syndrome.

Have you got a link for Turner's? ta.

[ceptimus]

I believe in birds, and possibly other groups, it is the male that has the XX chromosome pair and females are XY.

Also I find it interesting that the Y chromosome is the smallest of all - it has a lot less 'junk' DNA in it than any other chromosome. This must be something to do with the Y chromosome (in mamals) only travelling down the male line, from father to son. There must be some evolutionary benefit in ejecting the junk from this particular chromosome, but I don't know what that benefit is.

ceptimus.

[Jon_in_london]

Originally posted by ceptimus
There must be some evolutionary benefit in ejecting the junk from this particular chromosome, but I don't know what that benefit is.

ceptimus.

Well, an obvious is benefit of having less DNA is that you have less DNA to worry about.

Mayhap the Y chromosome is degenrate, containing only what is needed. Mayhap we will someday lose all our microsats and introns?

[Shane Costello]

Originally posted by Third Twin:
The interesting thing is that, in the normal karyotype for women (46, XX), one of the X chromosomes in each cells is inactivated by a process of "lyonization" and subsequently rendered unusuable. This lyonized X can be observed as a Barr body on the nucleus of a polymorphonuclear WBC in a blood smear. What's interesting is that this happens during division of the embryo, and that the lyonization is balanced - some cells get one X (from the father) and other cells get the other X (from the mother).

IIRC it isn't a given that one whole X c'some is shut down (Disclaimer: I may be mistaken), rather different parts of each homologous X c'some are shut down in each cell. That or else a different X-c'some is shut down in each cell. Since the X c'some contains a lot of useful genes, this has interesting results. Tortoiseshell cats are always female, because their characteristic colour pattern results from variable X-c'some inactivation. CC the cloned kitten had a different colour pattern to it's donor animal, because of differential inactivation of colour alleles on X c'somes. This raises some interesting questions about cloned animals, particularly in terms of gene expression and resulting phenotypes.

[Dr. Imago]

Originally posted by athon
There are instances of small parts of chromosomes splitting away and forming 'min-somes' on their own. Molecular biologists can use this and make artificial chromosomes. It has been suggested that Y was a truncated version of an X.

I'd be interested to see this, because it's kind of counterintuitive. For example, the Y chromosome contains sex-determining factor and Mullerian inhibiting substance genes that are specific to the Y. In the presence of these genes (which are activated during fetal development), the fetus forms testes instead of ovaries and the ducts that become the fallopian tubes degenerate.

I'm not an embyrologist, but find embryology fascinating and would enjoy reading some material (if available) that discussed this.

Originally posted by Jon_in_london
Have you got a link for Turner's? ta.

Not a very good one, but you need to be a member now for emedicine...

http://www.icondata.com/health/pedbase/files/TURNERSY.HTM

... don't worry so much about the 46,XX and mosaics (I can explain what happens there, if necessary), because it's essentially still the same outcome. But, in short, you basically lose a whole chromosome(s) early on in embryonic development for various reasons.

Originally posted by Shane Costello
(snip) That or else a different X-c'some is shut down in each cell. (snip)

This is what happens. Sorry if that wasn't clear in my original post. It's fairly balanced, most of the time, and is the reason why women are not proned to X-linked diseases, but are by definition the carriers. And, what happens in Turners is you actually don't have ENOUGH genes for transcription and translation to make enough protein product to develop normal secondary sex characteristics. Women with this syndrome tend to infertile and have "streak" ovaries in lieu of fully developed ones.

-TT

(EDIT: I'm AWOL this weekend for a wedding - leaving tomorrow AM - so, don't think you're additional responses are being ignored. Will get to when I get back.)

[Dr. Imago]

Here's a better link...

http://www.turner-syndrome.com/

-TT

[athon]

Originally posted by ThirdTwin


I'd be interested to see this, because it's kind of counterintuitive. For example, the Y chromosome contains sex-determining factor and Mullerian inhibiting substance genes that are specific to the Y. In the presence of these genes (which are activated during fetal development), the fetus forms testes instead of ovaries and the ducts that become the fallopian tubes degenerate.

I've upended all my old boxes of lecture-pads and stuff, and while I've found the hand-written lecture notes, the references I've got to this I can't figure. Hmm, maybe I had been to the uni-club that day...

It's argued that both X and Y are descended from an identical chromosome, and that sex-determination revolved around environmental factors such as temperature and nutrient availability, ala modern reptiles.

The best link I can find to validate this claim is this:

http://www.cnn.com/2000/HEALTH/men/10/30/guy.chromosome.ap/

The ancestors of the human X and the Y were a pair of identical chromosomes. They were found some 300 million years ago in reptiles, long before mammals arose. Genes didn't decide sex on their own in these creatures. They responded to some environmental cue like temperature. That still goes on today in turtles and crocodiles.

Anyway, to refute what many are saying here, the Y chromosome is actually a junkyard of DNA in istelf. Much of it (about 80%) doesn't code for anything obvious.

http://www.nature.com/nsu/010215/010215-5.html

Athon

[zakur]

This just in:Rumors Of Male Chromosome’s Demise Greatly Exaggerated, Study Finds (http://www.sciencedaily.com/releases/2003/06/030619081229.htm)

WASHINGTON, D.C. (June 18, 2003) – In the biological battle between the sexes, the Y chromosome has suffered defeat after defeat. The male-determining chromosome has seen its gene supply shrink from more than 1,000 genes when sex chromosomes first evolved, to what scientists once thought was only a handful of genes, a downward trend predicted to continue until the Y disappeared altogether.

But two studies published in this week's issue of the journal Nature suggest that the rumors of the Y's demise have been greatly exaggerated. Researchers from Whitehead Institute for Biomedical Research in Cambridge, Mass., and Washington University School of Medicine in St. Louis found that not only does the Y contain far more genes than scientists thought – the team found about 78 genes – it also includes a large number of genes arranged in pairs along this single chromosome in ways that may allow the Y to mimic the paired chromosome structure of the rest of the genome.

The findings, involving observations in both human and chimp male chromosomes, could explain how the Y repairs injured genes without the benefit of sexual recombination – the method of gene repair used by all other chromosomes. It's an elegant system that would debunk the theory of a "rotting Y" – the widely held notion that the male chromosome and its dead or dying genes will continue to rot away over the next 5 million years until there's nothing left.

[...]

"Genes constantly are being bombarded with little injuries – mutations. Mutations can either be beneficial or detrimental, but they are far more often detrimental," said Page. "On the Y, detrimental mutations cannot be discarded."

There's no question that this inability to discard has cost the Y hundreds of genes over time. Many of the chromosome's genes either have weakened or died out altogether. Sexual recombination is a card game the Y just can't win. But this new research suggests it doesn't always need to. For critical genes, it swaps with itself.

"This study shows that the Y chromosome has become very efficient at preserving its important genes," said co-lead investigator Richard K. Wilson, director of the Genome Sequencing Center at Washington University School of Medicine in St. Louis. "It's found different ways to do the things chromosomes must do to evolve, survive and thrive."More news stories on the recent Y chromosome findings can be read here (http://news.google.com/news?hl=en&edition=&q=%22y+chromosome%22).

[Dr. Imago]

Yeah, saw that too. Interesting stuff. And, Athon, good posts. I'll defer to the experts (and, in my own defense, I will claim partial ignorance because we didn't actually study the evolution of the Y in genetics).

Interesting thread Sou has started as well.

-TT