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

[Mighty_Girl]

Nothing to say, except that this thread is very interesting. I love this board. I would even pay to be here if it went on to be a subscription basis MB.

[MaceMan]

@ggurl

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.

Another common variation is Androgen Insensitivity Syndrome. A female may be born with the XY combination. Because of her inability to respond to androgens, she will not develop as male, but will usually look completely female. Except for the lack of menses and the ability to bear children, many women with this condition wouldn’t be aware of it.

[Colibri]

@OldGuy

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…

Not necessarily. The fate of individual cells is not determined to that level yet. Probably severel dozen cells at least will go on to become ectoderm, from which the skin and nervous system (including retinal cells) arise.

[Mathochist]

@Abbie Carmichael

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.)

How would this apply to lesbians? For that matter, why not just make the kid smarter? Number of children is inversely correllated with academic achievement as well.

[Smeghead]

@OldGuy

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.

There are a couple of factors here. With many genes, even if only half your cells have active functional genes, you’ll make enough functional protein to get along just fine. Additionally, the “inactive” X chromosomes aren’t really completely inactive. There are some genes that are active, even if the rest of the chromosome is condensed and inactive. If this weren’t the case, a Turner syndrome woman (XO) would be completely normal, since their cells would all have one active X chromosome, like normal people, and they’re not.

[irishgirl]

Even with X-linked conditions, if a woman inherits the dodgy gene, and by some fluke it is expressed in the majority of her cells, she will inherit the disease to some extent…usually less severely than a male.

I’ve met one female haemophiliac, the daughter of an affected man and a non-carrier woman, who has mild haemophilia by that mechanism. She is quite different to another haemophiliac patient who is the daughter of an affected man and a carrier woman (who met each other through a support group), this girl is also a haemophilic, but is much more severly affected, since ALL of her cells have the dodgy gene.

Mosaicism can work with other chromasomes too, I met a child with mild learning disabilities who was found to have Down’s syndrome, but a mosaic form, so that only some of his cells contained the trisomy 21.

[Smeghead]

@irishgirl

Mosaicism can work with other chromasomes too, I met a child with mild learning disabilities who was found to have Down’s syndrome, but a mosaic form, so that only some of his cells contained the trisomy 21.

Very true, though that happens through different processes. Instead of one chromosome randomly inactivating, you’d need some sort of nondisjunction event early in development.

[Peter_Morris]

@Abbie Carmichael

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?

@Smeghead

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.

Is that correct?

I’m no expert, but what I seem to remember from biology lessons is somewhat different.

During Meiosis, that is the formation of sperm and egg cells, each pair of chromosomes exchange genes. So any chromosome you pass to your child, half the genes on that chromosome come from your father, and half from your mother.

Am I right?

[Colibri]

@Peter Morris

During Meiosis, that is the formation of sperm and egg cells, each pair of chromosomes exchange genes. So any chromosome you pass to your child, half the genes on that chromosome come from your father, and half from your mother.

Am I right?

It’s not that even or predictable a swap. Crossing over is somewhat random, so that the specific bits and pieces of chromosome exchanged will vary in each instance. But it is true that, although you have one copy of each gene from your father and one from your mother, the chromosomes themselves are not passed on in integral form. Also because of this, probably none of your siblings’ chromosomes are identical to your own.