Does this mean that when a mother has a son, she passes to him a specific 1 of her 2 X’s, meaning that the son’s X is either totally his maternal grandfather’s or totally NOT his maternal grandfather’s?
How is that possible – that would mean that the son is totally unrelated to 50% of his maternal tree?
I was thinking there was a recombination of the mother’s two X’s into the son’s X. That way, the son has genes from both sources of his maternal X. But according to the Wikipedia article it’s either totally 1 X or the other X, which is surprising.
(The son gets the Y from his father, so here we are only concerned with X’s, which the mother gets 1 from her father and 1 from her mother.)
I believe this has been beaten to death in assorted biological and evolutionary threads - that during the process where sperm and egg cells are created, some genetic material could sometimes randomly shuffle between corresponding chromosomes - i.e. the one X in the egg may become a mix of material from both of the mother’s parents. The only time this does not happen (usually) is when a male creates sperm, the X and Y will not exchange material as they do not correspond.
But then, there are 22 other chromosomes the mother can pass on from either side of the family - not all critical information is on the X. Plus, the egg contains the rings of DNA, the mitochondria.
In oogenesis, normal meiotic recombination occurs between the two grandparental X chromosomes. So the X chromosome that is inherited from your mother is a recombined mosaic of the two maternal grandparents’ X chromosomes, just like any autosome.
However, in spermatogenesis, recombination between X and Y occurs only in the short pseudoautosomal region. So sons get their paternal grandfather’s Y almost intact, and daughters get their paternal grandmother’s X almost intact.
In addition to all of that, there are 22 other chromosome pairs that get mixed up pretty freely. So you will still have roughly 1/4 the genetic information from each grandparent, regardless of your sex chromosomes.
During production of an egg a woman’s two X chromosomes pair up. During prophase 1 ( a part of meiosis I) the homologous pairs of chromosomes undergo a somewhat random* breaking at a few points and recombination occurs when the parts are in essence exchanged between the maternally and paternally derived chromosomes. This is known as chromosomal crossover.
So The X chromosome that a son inherits from his mother will be a combination of parts of the X chromosome from his maternal grandmother and maternal grandfather. But it may not be a 50/50 split since there are a relatively small number of crossovers per chromosome, perhaps just one or two in some pairings.
To nitpick md2000’s reply, even though the X and Y chromosomes are very different they do pair up and sometimes have a crossover in a small section know as the pseudoautosomal region. So the Y chromosome a son inherits from his father *might *contain a very small bit of the X chromosome from his paternal grandmother that crossed over.
The site of a crossover is not truly random. There are some areas of the chromosome where crossovers are much more likely to occur and some areas where crossovers are quite rare.
In terms of the Androgen Receptor (AR) gene which sits on the maternally passed X chromosome, there’s a 50% chance it came from her father’s X and 50% it came from her mother’s X, right?
Historically there’s been a belief that men inherit Male Pattern Baldness or lack of it from the maternal grandfather. It’s been debunked in the sense that men can get baldness from their fathers too, but let’s set that aside for now, and focus on the X AR gene only which is always from the mothers side. My question is, why would the maternal grandfather be singled out in this historic belief, if there’s an equal 50% chance that he didn’t pass his AR to the grandson but the mother’s other X did?
You mentioned something about slightly non-random crossovers, is there some slight bias toward the mother’s father’s X AR passed to the grandson rather than the mother’s mother’s X AR?
People talk about getting male-pattern baldness from their maternal grandfather because even if maternal grandmother had the gene, she wouldn’t have expressed it. If you’re just looking at phenotypes, you’re going to look at males, and you need to look in the maternal line. And grandpap is the most obvious male in the maternal line, though of course you could also look at any of your mother’s brothers.
^ and my issue is with this word “most obvious” - if his chance is 50% then it’s no better or likelier than the other 50% from the mother’s-mother’s side, the other X whose probability is 50%.
No. Recombination hotspots are parts of the genome where meiotic recombination (chromosome crossover) occurs more frequently. The PAR (pseudoautosomal region) of X&Y is one. A recombination hotspot affects linkage (the probability of co-inheritance of genes that are located close together on the same chromosome), but it does not affect the outright probability of which of any two alleles is inherited. It is still 50/50 whether any gene on a boy’s X comes from either maternal grandparent.
Aside from the parts already listed by other people,
You appear to be thinking that there is only one pair of chromosomes. There are multiple pairs: the son could get the X in the XY pair from his mother’s father (the Y has to be from his father and his father and…), but any other of the chromosomes he gets from his mother could be from his mother’s mother, and the mitochondrial DNA will be from his mother and her mother and her mother and…
Note that mitochondrial DNA is a completely separate matter from the nuclear DNA in chromosomes. Mitochondria were originally independent single-celled organisms that, long ago in our evolutionary history, got absorbed into other cells and turned into mere parts of a cell, but they still retain their own DNA. They can’t live outside of their host cells, but within those cells, they reproduce on their own, and express their own traits based on their own DNA. Mitochondrial DNA won’t affect any observable trait of the host organism (or at most, almost no traits: I suppose it’s possible for one person’s mitochondria to function better or worse than another’s, which could have observable effects), and are interesting mostly for purposes of tracing ancestry.
Reading some Wikipedia articles on mtDNA leads to some interesting (to me) stuff:
Cells can contain quite a few copies of mitochondria. Including ova. Sperm not so much and mitochondria either doesn’t enter the ovum or gets quickly destroyed although there is one documented case of a male with mtDNA from his father. Given not a lot of people have been tested for paternal mtDNA inheritance it presumably has happened often enough to affect Mitochondrial Eve type studies.
However, various modern test-tube conception methods mess with this. E.g., In vitro fertilization and 3-parent setups. We’re screwing up the test results for future DNA inheritance researchers!
Anyway: lots of mitochondria in ova. This means that there can be several mtDNA variants in a cell. (There are competing processes to encourage this as well as limit it. Don’t want all your eggs in one basket, as it were, to limit mutation effects. But you want to encourage the better mitochondria to dominate.)
Centenarians tend to have a higher degree of such mitochondrial variants. And since that’s inherited from Mom … hmmm.
By convention the centromere, the compressed point of the X that you see in many karyotypeimages of diplotene chromosomes, is the point that determines the parentage of a chromosome. Even though each chromosome you get is a mixture of DNA from your grandparents’ X chromosomes, we say you received your maternal grandmother’s X if you get an X chromosome with a centromere derived from your maternal grandmother. This is so even though, due to crossovers, much of the chromosome you get may be copies of your maternal grandfather’s DNA.
The AR gene sits very close to the centromere of the X chromosome at Xq11-12. A crossover between the centromere and the site of the AR gene is highly unlikely simply due to their close proximity. So if you receive an X chromosome derived from your maternal grandfather (same centromeres) then it is highly likely you also got his allele at the AR locus too. These factors are said to be linked.
There is no particular favoritism, so a man is just as likely to have received an X chromosome derived from his maternal grandfather as on derived from his maternal grandmother. But whichever he gets he is likely to also get that grandparent’s AR allele as well.
As male pattern baldness phenotype is most easily visible only in males, I suspect the assumption that this trait is inherited via a man’s maternal grandfather is a matter of confirmation bias. A man develops male pattern baldness and then looks to see what trait his grandfather’s had.
And as to hot spots and non-random crossovers… There does seem to be a suppression of recombination near centromeres on many chromosomes, but not on X chromosomes in meiosis in females.
Since my favorite parts of these threads, and biology in general, are the tangents and exceptions:
I’ve heard about a number of examples where quirks in genetics and evolution lead to distortion of perfect 50/50 segregation. The textbook example is “meiotic drive”, where a selfish genetic element distorts the meiotic process in a way that gives it a better than 50% chance of overall inheritance.
In the process of meiosis, there are plenty of opportunities to do so. Consider the fact that centrosomes are the bit of the chromosome that are pulled apart by a huge complex of proteins during meioisis. There are DNA sequences that bind to centromeric proteins with varying efficiency, and vice versa. This sets up a system where every centrosome is under “selfish” selection to bind better to the centrosome proteins than the other homologous chromosome. In the whole genome, you can find examples of “selfish” co-evolution of centrosomes and centrosome-binding proteins that promote their own inheritance.
I’m not exactly sure how this is relevant to human genetics, since most of the detailed examples I’ve heard of are in other organisms. I recall hearing about human relatedness differing by a small but statistically significant bit from the expected levels,
but my google-fu isn’t good enough to come up with a cite right away…
But I’m puzzled as to where centromere origin could possibly be relevant in practice. I’ve never done karyotyping, but I’m skeptical that there’s any distinguishable variation in centromere morphology under a microscope, let alone enough to reliably determine which grandparent the centromere came from. And if you’re using PCR or whatever to genotype people, obviously you’ll look directly at the locus you care about, it’s not relevant which grandparent the centromere came from.