Only one X chromosome active in a woman? But, but ...

Factual Questions
1

I’ve been reading “Adam’s Curse: A Future Without Men” by Bryan Sykes, right?

In it, Sykes says that gender is determined not by the number of X chromosomes but whether or not we have a Y chromosome. So far, so good. People who have Klinefelter’s syndrome (XXY) are boys, then, and women with Turner syndrome, who have only one X chromosome, are still girls. It’s all about the Y.

So when a boy is conceived, he gets an X from Mom and a Y from Dad.
When a girl is conceived, she get an X from both parents.

And then one of the X’s in the girl shuts down forever. The book says it’s random which one it is, and once the decision has been made it’s permanent. Every woman has just one functioning X chromosome.

I can deal with the “whys” of this. Boys get along just fine with just one functioning X, and if both X’s in a girl worked we’d have a train wreck on our hands, with genes being duplicated and all.

A badly written sentence in the book is tripping me up, though:

“If you are a woman you are literally a mosaic made up of patches of cells in which one of your X chromosomes is working while the other one is doing nothing, condensed down as a Barr body, and patches in which the other one remains active while the first one is closed down.”

What I’m getting out of this sentence is that some of my cells have my mother’s X chromosome active, and some of my cells have my father’s X as the active one.

I don’t get it. He just said a couple of sentences before that that the cells pick an X and shut down the other one, and in every cell that comes after it the SAME X chromosome is shut down!

So I’m seeing two scenarios:

  1. At conception, my cells decided to, oh, keep my mom’s X functioning and shut down my dad’s. From that point on, in every one of my cells only my mother’s X is working.

OR

  1. At conception, my cells picked a parent and decided that for that cell only, mom’s X would be functioning. Every other cell that came after that made the same decision on its own.

#1 makes more sense, in which case the sentence I quoted is really badly worded and the editors didn’t catch it. But #2 also makes sense to me, because you’d think that my cells would want a 50/50 split overall between Mom and Dad’s X chromosome, leading to more variety in my genes.

So which is it?

If the answer is #1, here’s what I also want to know: how does the body decide which parent’s X to use? Does it pick the “best” one? Or is it just totally random, a snap decision?

Also, again provided that #1 is the case, is there any way to tell which parent’s X is doing all the work? Like, if you see someone who looks just like their father, could you assume that their father’s X chromosome must be the active one in their cells?

2

According to this site, the decision is made by each cell about the 100-cell stage of development.

So it’s neither #1 nor #2. At the 100-cell stage, each of the cells makes a decision of which X to shut down at random, so half will have the father’s X active and half the mother’s. All of the cells descended from that cell will have the same X shut down, so your body will be a mosaic, with roughly half each.

3

Maybe if you showed where he says that the same one shuts down after mitosis? IIRC from my biology (almost a decade ago now), the choice is made on a cell-by-cell level, and any passing-down that gets done is only pretty far into the game. The fertilized ovum doesn’t force the choice to its immediate decendants, for instance.

As a side note, I’ve seen a lot of debate against this “Adam’s Curse” idea – references escape me at the moment, but I’m sure someone will fill them in. Anyhow, one of the suppositions for the decay of the Y chromosome is that there’s no cross-checking method to repair it like there is for the X and all other chromosomes which have a matched pair (at least half the time). To the contrary, though, researchers are finding long “palindromic” strings of bases – matched patches running in opposite directions along the Y – which allow the Y chromosome to check for consistency against itself. This was the biggest counter-argument to stick with me, but there are others out there.

4

oop, here’s one I just recalled: [url=http://www.sciamdigital.com/browse.cfm?sequencenameCHAR=item2&methodnameCHAR=resource_getitembrowse&interfacenameCHAR=browse.cfm&ISSUEID_CHAR=2DCBE7F7-2B35-221B-6DF929E0602B991E&ARTICLEID_CHAR=2DDBE72E-2B35-221B-6CE7BE0B60674D85&sc=I100322]an interview in Scientific American* from a few months ago. Unfortunately it seems you’d have to buy access to the article if you don’t have a digital subscription already. Still, it’s a reference you can chase down in any decent library.

5

GAH: trying again

6

Ooohhhhhh! That makes a lot more sense, Colibri. I wonder why he didn’t add in the part about it happening at about the 100 cell stage, that would have explained everything.

Ok, then, what happens with egg cells? Does that mean half the eggs a woman’s ovaries pops out contain the X she got from her father, and the other half have the X she got from her mother?

If that’s the case, let’s say, for example, that I ovulate and the egg produced that month has an egg with my father’s X chromosome. I get pregnant and conceive a boy. Nine months later the boy is born and looks a whole lot like my dad. Would that be because he came from an egg containing my father’s X, or is it just concidence?

Mathochist, I’m only halfway through the book but I take what Sykes says with a grain of salt anyway. The guy’s obviously brilliant but I just can’t get behind his evolution stuff. I think he’s right on the money as far as clan mothers and mitochondrial DNA and all, but I think he’s got his dates a bit off.

I don’t look for men to become extinct anytime soon, either. He says it’ll be another 125k years; I doubt we’ll BE here 125k years from now. We’ll have found a way to kill each other off by then, unfortunately.

7

Yep. That’s exactly what it means.

Not entirely coincidence, but the X chromosome will be just one of 23 chromosomes that you gave him, and half of those, on average, will have come from your father. They’ll all contribute to his looks.

As to the mosaicism that’s been explained already, keep in mind the classic example that’s in all the textbooks: calico cats. The fur color gene is on the X chromosome in cats. So a female with an orange gene and a black gene will become calico, because patches of skin will have either the orange or black gene active. Each patch will have come from one cell at around the 100 cell stage that randomly deactivated one or the other.

8

From the same site I linked to:

That is, in ovarian cells both Xs remain active. However, in the course of cell divison, as in the case of other chromosomes I believe “crossing over” will occur between the two active Xs, and they will exchange genetic material. So that although each haploid egg cell contains only one X chromosome, it will contain a mixture of genes from your father and your mother (though of course, only one copy of each gene.)

9

So why don’t human women have similar patterns in their hair color?

10

Abbie, you’re generally on the mark about things, but there are a lot of other people who will read this thread. To have the chance to stamp out this bit of shaky science is welcome. Sykes is far from the first or only person to advance these ideas, but he’s among the closest to doing well-supported science. This idea – that the Y chromosome is all “junk” and/or on its way out – has gained a lot of credence in a lot of circles. In particular, it gets blown up (via “as above, so below” arguments) to make the same assertion about maleness in general.

11

Like I’ve said, I’ve only read half the book. I haven’t even gotten to the part about why the Y is doomed, etc. I’m having more fun with the X chromosome thing and how Iceland was settled. And I’m the last person who would even try to refute Sykes’ science; I only argue stuff I know :wink:

But if you wanna take a whack at it, by all means, go for it.

12

AFAIK, the hair color gene in humans isn’t on the x chromosome. It’s on one of the chromosomes where we all have two active copies, so it’s completely unaffected by all this.

13

I imagine the hair color gene is somewhere else, but I would point out that everyone has many shades on their head. In a way, we’re kind of calico too.

14

Different hair colors on human heads come from completely different processes, because the genes aren’t on the X chromosome.

15

Given that the mammalian Y chromosome has probably been around for more than 80 million years, rumors of its demise in the next 125,000 are likely to be exaggerated.

16

Exactly. What makes human beings unique among mammals that we will be the only species to stop producing males?

There’s nothing particularly different about human chromosomal arrangements compared to other mammals. And while there are some reptiles that don’t have genetically controlled genders, all mammals have both males and females. If we were on our way to losing the male gender, why haven’t other species done it? There are thousands of mammal species, why aren’t there any that have only females?

The argument makes no sense.

17

I’ve read more of the book. I skipped ahead and read his chapter on the gay gene; I think it’s a crock. He said the gay gene might be there as an “altruistic” gene.

Warning: this is a very bad job of explaining what the book says. If any of you have read it or are familiar with this theory, feel free to fill in the gaps:

For example, biologically if you have 3 or more siblings, he says it makes sense that your genes would “want” you to die for them if you all were threatened. Your genes apparently don’t care WHO passes them on, so they’re happy to “know” that even if you die, there are 3 or more other people who will pass on copies of themselves. (I should add that the book’s whole premise is that we are controlled by our genes, which I also think is BS.)

So the idea of there being a gay gene is that the mitochondrial DNA sees that there’s been a boy conceived, and so it’s like “dammit. Can’t be passed on. Ok, let’s make him gay so he won’t reproduce and give the girls a better chance of passing on the genes.” (Presuming that a gay man won’t make babies. Yes, I know they do, and so does the author. In general, though, they don’t make as many babies as straight men.) Basically, the mDNA “attacks” the baby boy somehow and makes him gay because, biologically speaking, he’s useless for the purposes of passing on his mDNA to any future offspring. This makes no sense to me, though. Why isn’t the boy’s Y chromosome fighting to keep him straight. Doesn’t the Y want to go forth and have this boy make more boy babies?

Plus I don’t see how having a gay brother makes any of his sisters MORE likely to reproduce. Who says they’re gonna have babies to begin with? How exactly does having a gay brother make a woman more likely to mate?

Plus why does the mDNA care if there’s a son? Sure, it can’t be passed on, but he would still grow up to pass on his mother’s genes, even if it’s not her mDNA. I can see how mDNA is very important (and truly amazing), but I can’t see the rest of the DNA allowing it to sabotage any baby’s potential to reproduce.

Plus it makes no sense for our mDNA to want to stop us totally from having straight boys. Sure, it would prefer a girl so it can be passed down to another generation. Ultimately, though, if there’s no boys around, we would die out because we must have men to make babies.

I still haven’t gotten to the part about why the Y itself is supposedly weak. Maybe there is a gay gene: I doubt it, but maybe there is. If there is one, though, I can’t accept any of the reasons the author gives for why, biologically speaking, it would be a helpful gene.

I’ll read on.

18

Not quite. There are some children born with a form of Turner’s mosaicism (aka mixed gonadal dysgenesis) whose sex chromosomes are mosaics of xo/xy, xo/xy/xyy, etc. Some of these kids are girls.

FYI

19

Since mitochondria are passed on only from the mother, except in very rare circumstances, a son would never pass on his mitochondrial DNA anyway.

Another theory about the “gay” gene is that it makes (straight) females carrying it more fertile, while having the (unintentional) side effect of making men who inherit it more likely to be gay. Thus, the “gay” gene (for male homosexuals only) will persist long-term, just like other genes (i.e. for sickle cell anemia) that confer both advantages in many individuals and disadvantages in others.

I’d recommend reading “Living with Our Genes: Why They Matter More
Than You Think” by Dean Hamer, for a thorough discussion of these theories, and other behavioral-genetic topics.

20

If one of the X chromosome turns off in the 100 cell stage why is it that color blindness and hemophilia are much more prevalent in men than women? One would think out of 100 cells only one of them would be destined to become eyes so if oyu were unlucky enough that the X with color blindness stayed active you’d be sunk.

Now I’m not at all sure where hemophilia becomes “active”. I’d guess the marrow, and it might well be that more than one of those 100 cells is destined to do whatever is needed to prevent hemophilia.

But in any case this is very different from the story I heard as a child as to why some diseases are sex linked.