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Which X Chromosome did I inherit from my mother?
August 27, 2011 10:06 PM   Subscribe

Which X Chromosome did I inherit from my mother?

I just got into genetic genealogy, and I am curious (can't find it on Google): which of my mothers 2 X chromosomes did I inherit?
posted by brownrd to Science & Nature (12 answers total) 15 users marked this as a favorite
 
Can you tell us how you are distinguishing between them? Thanks.
posted by Knowyournuts at 10:12 PM on August 27, 2011


She inherited one from her father and one from her mother. Is the paternally derived one passed on by her in procreation, is the maternally derived one?
posted by brownrd at 10:14 PM on August 27, 2011


Both of them.

When chromosomes are duplicated, they get pulled apart into pieces. The pieces are then doubled, and afterwards the pieces get reassembled into complete chromosomes. The interesting thing is that parts of both original chromosomes got into each of the four child chromosomes.

That fact is how they originally started mapping genes. Genes on the same chromosome which are very near one another will tend to inherit together. Genes on the same chromosome which are further apart have a greater chance of not inheriting together. By doing statistical analysis of character inheritance over large populations, it was possible to identify which genes were on the same chromosomes and get some idea of how far apart they were.

(Later, of course, we started sequencing them directly.)
posted by Chocolate Pickle at 10:15 PM on August 27, 2011 [8 favorites]


By the way, this is an evolutionary advantage. It makes it possible to ditch bad genes without losing good genes on the same chromosome.

One of the coolest findings recently answered the question: why doesn't genetic garbage accumulate on the Y chromosome? The answer turns out to be that the Y chromosome contains two copies of all the unique genes it carries. It is its own duplicate. And during duplication of the Y chromosome, some get a copy of the A side and a copy of the B side, somee get two copies of the A side, and some get two copies of the B side. So if one of them was a garbage gene, there's a chance that it got dropped in the duplication step.

Among other things, what this means is that you don't have an exact duplicate of any chromosome from any of your grandparents. It also means that no two chromosomes of the same kind anywhere in your body are exactly the same. This mixing process happens during every cell division, and it happens differently every single time.
posted by Chocolate Pickle at 10:21 PM on August 27, 2011 [7 favorites]


"you don't have an exact duplicate of any chromosome from any of your grandparents."

Correct, but:

"This mixing process happens during every cell division"

Recombination happens during meiosis (gamete production), but generally not during mitosis (ordinary cell division), so it doesn't make your somatic (non germ-line) cells differ from each other.
posted by Canard de Vasco at 10:47 PM on August 27, 2011 [4 favorites]


Yeah, parts of both of them, due to crossover.

But you know what's even better? X-inactivation. If you're female, it is not unlikely that some of your cells have effectively a different genome than some others of your cells, which makes you a mosaic. Like a Calico cat! Or (albeit maybe not due to the X-chromosome) my mom! These are all examples of epigenetics in action, which metabugs has explained beautifully.

I am a phentotypically ordinary human person, and so is my sister, but both my kids have heterochromia iridum too. Autosomal dominant, right. Hmph.
posted by pH Indicating Socks at 11:00 PM on August 27, 2011 [3 favorites]


Maybe I geeked out a little too hard there.

Mitosis -- what your cells are doing right now, where you start with a cell that has pairs of chromosomes, and you end up with two cells that have pairs of chromosomes. this is the ordinary somatic (body) cell division. When this is not happening as it should, that's what cancer is.

Meiosis -- a two part process, where you start with a cell with paired chromosomes, and you end up with four cells with single copy chromosomes. This only happens in cells that lead to cells specialized for sexual reproduction, like eggs, or sperm, or pollen -- gametes. The first part is like mitosis, and that's where crossover happened, and all the eggs you are ever going to have did this before you were born. The next part, where you end up with one egg cell (single copy chromosomes) and three things that are not egg cells (polar bodies, single copy chromosomes) you start to do every month when you ovulate. You only finish meiosis with a particular egg cell, if the egg-to-be meets a sperm. Which is odd, and I don't know why your body would want to do it that way, but so it is.

So crossover. You put two masses of spaghetti -- and let's say your masses of spaghetti are each a whole pound of spahetti, and each one single super-long strand. This is overcooked spaghetti, so it's sticky. With me? So you put two masses of spaghetti and mush them together, then pull them apart. Your spaghetti strands broke in many places, and each mass now contains some from the other mass. Well, that's your chromosomes during crossover. Your cells have ways of fixing breaks, and of making sure that the splices happen in the right places, so you end up with two single strands of DNA, that each include many, probably hundreds, of splices between the chromosome from your mom, and the one from your dad.

Now X-chromosomes are a special case, because they are huge and contain many genes (for example, genes related to color vision, and blood-clotting) that you need one of but not *two* of. Right? Because men have livers and skin and blood cells and things, and they only get one X-chromosome. But you have *two* X-chromosomes. So what does your body do? Epigenetics, specifically, X-inactivation! It adds little molecules to random (or maybe not so random, we're still working on that) parts of each of your X-chromosomes, which makes that part of the DNA unreadable by your cells. So you still have two X-chromsomes, but enough parts of them are inactivated, that you have one X-chromosome's worth of genes your cells can use.

Y-chromosomes, in case you're wondering, are very, very small and contain hardly anything except genes specific to maleness. Nobody needs those except men.

I wish I were you, just starting on learning about genetics. It is the single most interesting thing in the world.
posted by pH Indicating Socks at 11:49 PM on August 27, 2011 [3 favorites]


Oh yeah, "pheotypically" -- how having a particular gene plays out, in the living thing. You might have a gene for Disease X -- your genotype includes gene "disx" -- but you may or may not have the disease, that is, your phenotype could be normal or diseased.
posted by pH Indicating Socks at 11:53 PM on August 27, 2011


If you're female, it is not unlikely that some of your cells have effectively a different genome than some others of your cells, which makes you a mosaic. Like a Calico cat!

One of the neatest things about this: if you clone a calico cat you get a kitten which doesn't have a similar coat. X-inactivation means that even cloned females aren't phenotypically identical!
posted by vorfeed at 12:02 AM on August 28, 2011 [6 favorites]


If you're tracing matrilinear descent, mitochondrial DNA is a fairly sound way to go about it.
posted by Jilder at 1:05 AM on August 28, 2011


I can't believe no one has mentioned Richard Dawkins The Selfish Gene as an excellent introduction to genetics. Even reading the first three or four chapters will be enough to give you a great overview.
posted by pandabearjohnson at 8:08 AM on August 28, 2011


If you're looking for something to read about this, IMHO you'd do much better with The Cartoon Guide to Genetics. (And no, I'm not kidding. It's excellent.)
posted by Chocolate Pickle at 9:46 AM on August 28, 2011 [2 favorites]


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