What percentage of a man's genes comes from each grandparent?

Especially the ones which could influence the mind, personality.

Do I understand correctly that a man would have no X chromosome elements from his paternal grandmother?

I think you meant this for GQ? :slight_smile:

Moved to GQ.

No, actually there is some crossing over in parts of the X and Y chromosomes.

Your’e correct that a human genetic male gets no X chromosome from their father, so no X-only genes from their paternal grandmother.

That said, saying ‘this percent of genes comes from that ancestor’ is only a rough estimate of probability. The egg or sperm isn’t going to have a perfect 50/50 mix of genes from each grandparent, many genes are going to be identical anyway, there’s going to be some mutations, mitochondrial genes usually come from the mother but very occasionally from the father (and they’re probably virtually identical from both parents anyway) etc. etc.
And then the X is only one of 23 chromosomes and the Y still has a few genes, etc. I saw an estimate of 51% mother/49% father, but that’s maybe putting too much precision on what’s only a rough estimate anyway.

You know how, when you’re a kid, you think some adults are quirky then, as an adult, you think back and realize they probably had issues? I noticed that about my paternal grandmother. Some ways of speaking, acting that were reminiscent of young children, like when 5-year-olds are gesticulating semi-randomly and going “la la la”. I noticed that about my father and paternal uncles and aunts. I don’t think I behave like that and it often felt like I didn’t fit in that family, hence my question.

Granted that this is only probabilistic you should expect about 25% of your genes from each grandparent. Yes, it will depend a bit on your sex, but not that much. And you almost always get 100% of your mitochondrial DNA from your maternal grandmother.

Believe it or not, we still don’t know exactly how many genes people actually have, which includes how many on each chromosome. But calculating from the number of base pairs from this table, females contribute around 3 billion base pairs and males contribute around 2.9 billion base pairs, so there is around a 3 to 4 percent difference in number of base pairs. But thanks to crossing over, the exact percentage that comes from each grandparent is unpredictable. Statistically taken over large populations it is probably pretty close to 25 percent from each one, but if the genes are reshuffled right in theory you could get pretty close to 50 percent from one and close to zero percent from another.

Well…perhaps we might reconsider.


And for the other part of your question, we have almost no idea of what genes influence the mind or personality. Presumably, though, there are a lot of them, and they’ll be distributed the same way as any other gene, mostly not on the X or Y and hence mostly equal from each grandparent.

Of course, further complicating matters is that you don’t know his grandfather’s genotype, only his phenotype (that is, what traits he actually displayed). So to estimate the chances that he got the relevant gene, you’d need to know whether that trait was dominant or recessive, and how common it is in the population as a whole. Or more precisely, you’d need to know that for each of the genes involved, because for something as complicated as personality, there’s probably an interaction between many genes.

If I understand from prior threads (and I may not have) every person gets exactly 50% of his DNA from his mother and 50% of his DNA from his father. However, and importantly, he does not get the same proportion of DNA that his parents have.

So, for example, if we have a mother and father who both have 80% sub-saharan Africa DNA, and 20% Irish DNA, and both have dark skin, identify as black, and are perceived to be black, it is unlikely, yet possible that they have a child that is born with white skin, red hair, holding a Guiness in each hand and wearing a leprechaun suit, IOW, 100% Irish.

Is that correct?

A child gets (roughly) 50 percent from each parent (neglecting the small percentage difference from XY chromosomes mentioned above) but they don’t get the same chromosomes that the parents have–they get an entire set of brand-new chromosomes. In the “crossing over” mentioned above, during the process of creating sex cells, the 23 pairs of chromosomes line up beside each other and are cut apart at various places and then reattached to opposite chromosomes.

So, for example, in one particular precursor cell for sperm, on chromosome 12 after this swapping takes place one sperm cell could end up with 60 percent of the guy’s father’s chromosome 12 and 40 percent of his mother’s, while the other sperm cell from that division would have 40 percent of Dad’s and 60 percent of Mom’s. And even with X and Y chromosomes while most of the length doesn’t match up well enough for the swapping to take place, small parts of each end do, so men don’t have “their father’s Y chromosome”, they have most of their father’s Y chromosome and small parts of one of their mother’s X chromosomes.

If both parents have 80% of their genetic material coming recently from sub-Saharan Africa, then the kid is going to have, at minimum, 30% of his genetic material coming from there (and probably more). Though depending on which 80% (and which 30%) that is, this may or may not be perceptible, or even measurable (an Irishman and a Zulu are still very similar genetically, after all: Both are the same species, and even as species go, humans have relatively low biodiversity).

Anecdotally, in college I used to have a friend who was a woman of Mexican ancestry whose parents looked straight out of a stereotypical Mexican peasant village. Black hair, looked like they have lots of native ancestry, on the darker side for Hispanics and definitely brown eyed.

What did my friend look like? Auburn haired, green eyed and extremely fair. Fairer than me, a garden variety, off-the-shelf white man. If I hadn’t known she was Hispanic, I absolutely never would have guessed it by looking at her.

So genes are funny things- there’s a lot of random chance involved, and you can end up with strange stuff from your ancestors that your parents don’t exhibit.

The shared centimorgan project collects data from people doing genetic genealogy. Their histogram results in a 99 percentile range from 14-35% when comparing grandparents and grandchildren.

I share around 19% with my paternal grandmother, so I would have 31% in common with my grandfather.

(Of course I actually share 99% with either of them, since we’re all humans. The sharing is about specific patterns indicating segments of DNA have been inherited from the same source.)

Just a slight clarification, since it’s confusing whose mother/father you’re talking about here. Crossing over occurs in the father’s sperm cell precursors and the mother’s egg cell precursors between their two copies of each chromosome, which come from the grandparents. So a child gets one copy of each chromosome entirely from their father, which is a mosaic of the two paternal grandparents; and one copy entirely from their mother, a mosaic of the two maternal grandparents.

So a boy’s Y does come entirely from his father, and his X comes entirely from his mother, but in each case both grandparents contribute genetic material. So although a boy gets his Y entirely from his father, it will not be exactly the same Y that his father has - it will contain some genetic material that came to the father from the grandmother on the father’s X, and was transferred to the Y that the father passed on to the boy through XY recombination in the father’s sperm. Likewise, a girl can inherit genetic material on the copy of X that she gets from her father that was originally present on the Y that her grandfather passed to her father.

You live and learn! I read the cited article as much as I could. It seems it is no so simple as I was taught. Still it seems like a mitochondirian is at least 10000 times as likely to come from your mother as from your father. Them’s pretty good odds.

Nothing in genetics turns out to be as simple as it seemed at first. Remember “junk” DNA. It isn’t junk at all. And the idea that two blue-eyed parents couldn’t have a brown-eyed child. Or that two type O parents couldn’t have a type A child. All false.

Only 4.9% of the Y-chromosome can recombine with the X, so very little of a boy’s Y-chromosome can come from grandmother.

Moreover, many discussions of the Y-chromosome deal specifically with the other 95.1% — the NRY (non-recombining region of the Y chromosome), ALL of which is passed specifically father-to-son.

Yup, and the fact that it’s non-recombining and passed down the male lineage gives us some really interesting information. About 1 in 200 men alive today have Genghis Khan’s Y chromosome.


SDMB’s own Tamerlane (Emperor of Tartars) is allegedly an agnatic cousin of Genghis Khan, so shares that same chromosome (or a close variant).

But it’s a matter of historical record that one of the Great Khans had a giant encampment of thousands of people — all of whom were his wives, concubines, children (or grandchildren?). I’m more fascinated by Y-haplogroups which tell us new things about prehistory.

For example, a majority of ALL Western Europeans are agnatic descendants of a single man who probably lived near Bavaria about 2500 BC. That clade magnified at mind-boggling speed after 2500 BC, an expansion that was coincident with the “Bell Beaker expansion.”