Edwino & Nebuli, finally you came. So you think that dominance comes first, brain blood supply follows? I just assumed that the right hemisphere receives (slightly) more blood. Perhaps, it’s a wrong conclusion; the left carotid originates directly from the arch and, therefore, carries more blood. But eventually, it evens due to the circle of Willis. And the circulation is anatomically unchanged in lefties, of course. Any comparative PET scan studies done you know about?
Peace,
If your “finally you came” remark was in reference to your earlier request for SDMB M.D.s, I’ve got to be upfront and admit that I’m not a doctor and I don’t play one on TV. My info was just from my reading over the years, including the the info given out in the study I’m participating in. The argument against Coren’s conclusions is my own, but I’m really surprised no one else has come up with it (of course they probably have and I just haven’t read of it).
I don’t know of studies doing PET scans to study Righty vs. Lefty difference, but wouldn’t be shocked to learn that there have been some. I haven’t heard of any proposed explanation of how the conjectured Rightside Dominant gene would actually work. Having it work by creating a difference in blood supply to one cerebral hemisphere seems like one plausible possibility.
nor am I an MD,
but an MD/PhD student (about midway through). I have completed 2/3 of my primary medical education.
I will assume dominance comes first. Blood flow adapts to the areas of usage, just like in other tissues.
The anatomic origin of left versus right carotid IMHO makes little difference, as you mentioned, due to the Circle of Willis and the fractional difference in blood flow between the two. Although it is an interesting consideration – people with unilateral carotid insufficiency often show focal neurologic deficits associated with the cortex supplied by that artery.
The major functional areas – especially the language ones (Broca’s and Wernicke’s) are on the contralateral side of the brain from the hand you use (so in most people, the left cortex is dominant).
This is gonna sound silly coming just after the serious medical posts, but I wanted to thow it in. Is anyone here aware of their nostril dominance? I’m left hand, eye, and foot dominate, and now that I’m just getting over a cold, I think I’m left nostril dominate too. During the cold, most of the itching and the vast majority of the snot came from my left nostril. Has anyone else experienced this?
Attention, mods: this is turning into GD.
Edwino and Nebuli, I called you by your names, not by your titles. I do not care much about titles. What you both said made sense, the rest is immaterial.
Gene hypothesis is interesting. I have a problem with it, though, but this is my problem, due to my limited mind. All genes I know about are not “command type genes”, they are “operative type genes”. i.e. they code for a protein syntesis. In the simplest case, if a gene is present, a protein is synthtized, if it is absent, no protein is syntetized. So, how is “dominance gene” work? Let’s say that Righties have a protein which is absent in Lefties. Now what?
Kferr, I never heard about “nostrillity”. But if you are sure that your nasal septum is absolutely symmetrical, I would not be surprised that there are more blood vessels in your left nostril than the right one. We are all assymetrical. So, why vascular tangles woould be an exception?
A genetic determination of handedness could work similarly to a left-right determination pathway. The placement of our internal organs is dependent on a number of things, and several genetic diseases interfere. These generally deal with the signalling cascade involved in left-right determination or in the actual movement of the organs from the midline.
Asymmetry usually results from stochastic differences in gene expression between two cells. Let’s say it is a default pathway to develop into the dominant side of the brain. You need a pathway in order to repress the other half of the brain into the nondominant sections. In lefties, this pathway is somehow broken – both sides develop the capability to become dominant. Half of these people become left handed.
Just a guess.
More likely, it is polygenic with a large environmental influence. You know, the baby hurts his hand at an early age just inside of a critical window where handedness is determined. He uses his left hand preferentially during this period, and bingo.
Most attributes of our bodies are not tied to one single gene. For instance, eye color is determined by 6 genes (a number I’m pulling out of my ass, but it sounds right- maybe it’s six pairs. anyway.)
If a lot of genes are expressing protein, those proteins are going to interact with one another (and the body) in subtle and complex ways. Take out one protein, or change it, and the relationship of the whole mix changes, so the effects change.
I’m guessing that handedness, being intimately tied to brain structure, has some pretty complex genetic mamajamas.
jb
My understanding is the same. As far as I know the hypothesis does not yet address how a single dominant gene would result in obligate rightsidedness. I think that the researchers first want to address the question of whether handedness actually follows the inheritance pattern they have conjectured. After that I guess the next step would be to try and isolate the gene. Only then could they examine the mechanism by which it might work.
My own speculation had been thatif such a dominant gene actually existed it might code for a protein directly effecting neurons somehow. Peace’s question about blood flow made me realize it could just as well effect brain vascularization.
I realize many traits are polygenic in origin, or arise from a mixture of genes and environment. However, this particular hypothesis, as I understand it, does postulate a single dominant gene.
The implication, if it is a single autosomal dominant gene, is that the penetrance is quite low. I find this hard to believe, however. I deal in human genetics every day and I haven’t ever heard handedness attributed to one gene. It would also be extremely easy to clone such a gene – to clone a gene in humans, you need to start with a large pedigree and work downwards. I would assume it would be easy to get giant pedigrees with left- and right-handed people. This would make cloning the gene quite easy.
Edwino, I’m certainly in no position to argue genetics with someone of your experience, so having presented my understanding of the hypothesis I will step down. The researchers who are conducting the study I mentioned are Drs. Klar and Sabl of the National Cancer Institute. They are gathering the pedigree data you talk of (although it is not just “left and right handed people” they need- it is people who are left dominant for any function vs. people who are completely right dominant).
I’m not sure I get your point about low penetrance- as I understand the hypothesis, it estimates ca. 75% of the population has the gene. Of the ca. 25% without the gene, approximately half would be lefthanded, which is close to the observed number.
nebuli
I did a PubMed search because I would be legitimately interested in reading any research by these guys. I find it fascinating, and I honestly should know it for a PhD…
A search for sabl[au] pulled up 8 references, none about handedness, none from the NCI. Two guys came up – one Czech guy, and another Drosophila chromatin guy from Harvard/Seattle/New Brunswick. Nothing has been published since 1998.
A search for clar[au] pulled back 81 hits, mostly about anesthesia and cardiology and dentistry. A search for clar[au] AND hand pulled out 1 hit, a clinical report of a peculiar intracranial tumor masquerading as a radiculopathy, from my institution.
Where did you read of the research? I am very interested in this type of study.
Just to cover the ‘what about me, I do X right-handed, and Y left-handed’ folk… There’s something called ‘mixed-dominant’.
I did some research on this a while ago (because my son is coming up mixed-dominant, and I was curious).
Righties are technically better at ‘dexterity’ (no pun intended) tasks with their right hands. Lefties are more nimble with their left hands. Ambidexterous are equally good with both hands. Mixed-dominants have better dexterity with one hand than the other on a task-by-task basis.
IIRC, the rates are something like this:
leftie: 9%
mixed-dominants: 19%
ambi: oohh… shoot, I think it was slightly fewer than leftie
rightie: remainder. About 60-65%?
HOWEVER, there is a reporting issue, because mixed-dominants who write right-handed may self-report as righties (since people tend to associate writing with handedness). Also, lefties who were ‘trained’ rightie may report as mixed-dominants. AND, mixed-dominants who write rightie may have been encouraged not to go leftie on other tasks (another training issue, but more subtle), etc. So the rate of true right-dominance is probably LOWER than we think, and the rate of mixed-dominance (in a spectrum) probably takes up more of the middle of the scale than we think, too. And there could even be fewer lefties than we think, since some of those are bound to be mixed-dominants, too.
That there is such a range of expression of handedness supports the multi-gene trait concept. The more genes involved, the more variations in outcome.
My son has been switching sides over time - started out very strongly leftie (first few months), then has shifted back and forth, with some tasks finally ‘settling’ on one side or the other. He bats, swings a golf club, and kicks (granted, that’s feet) leftie, and uses pencils and hammers better rightie (and now is throwing both rightie and leftie), and some tasks are still flip-flopping (he’s 3). My DH (his dad) is the other way around - writes leftie, does a lot of other stuff either rightie or leftie or ambi. My DH was just told to try things with each hand and see which one felt better - and then use that hand for that task. This is working pretty well with our son, too.
Anyway, just wanted to add in the concept of mixed-dominant, because it is pretty common, but hadn’t been mentioned explicitly yet.
WillGolfForFood
Your friends (and you) have been brainwashed by the Right-Handed Conspiracy to write everything with the right slant. That’s why you turn your hand. Reject them!!! Be proud of your left-leaning letters. Place your hand below the line of text and write like a normal person. Yea verily!
Also a big shout-out to my wife hedra, who manages to post before me in threads.
Edwino,
I don’t know that Drs. Amar J.S. Klar or Joy F. Sabl have published anything on this topic yet in a peer-reviewed journal. As I hope I made clear, it is only a hypothesis at this point, and they are currently collecting samples and analyzing them.
I read of Dr. Klar and his idea in several general public sources. I can’t recall all of them, but three of them are:
-New York Times, July 30, 1997, p. C8 [in Local ed., p. B10 in Nat’l ed.]; “In New Theory, Single Gene Makes a Left-Hander”;
-Washington Post, December 29, 1998, “Washington Post Health/December 22-29”, p. WPH-6; “Left-Handed? Right-Handed? Geneticist Offers Clue”, by Jamie Talan, reprinted from Newsday;
-New York Times, May 16, 2000, “Science Times”, pp. 1,6; “On Left-Handedness, Its Causes and Costs”, by David E. Rosenbaum.
The Post/Newsday article mentions a presentation that Dr. Klar made the preceding week at a neurobiology seminar sponsored by the Whitehead Institute in Cambridge MA, in which he discussed preliminary findings from a three-generation study of fifty families. Sorry I don’t have more of a lead for you.
In case I haven’t made myself really clear on this, I should make the following disclaimer: I’m not advocating or endorsing this hypothesis, I’m merely presenting it as an interesting possibility for consideration. If the data fail to support it, it’s no loss to me. (And the two Times articles cited above do quote other researchers skeptical of the hypothesis). The one point about Dr. Klar’s proposal which I think is of most significance is the reminder that when looking for a genetic component (whether single or multiple) we must consider the absence of “rightsided” gene(s) as well as the presence of “leftsided” one(s).
For whatever it is worth, I’m not even sure how original this idea is with Dr. Klar. For some reason I seem to have a very vague recollection that I read of a similar proposal from someone else a few years before Dr. Klar came forward.
Finally, to close on a completely trivial tangent- thanks, Edwino, for making me look up the cites for you. While doing it, I coincidently discovered that the candidate who I wrote in this year was, unbeknownst to me, a southpaw, thus keeping intact my 7 election string of voting for lefties for President. 
I have to lean to the right. I’m a Republican.
Reagan was a lefty.
Admittedly, this is not a scientific survey but… In college, it seemed that in the more advanced math classes the percentage of lefties was higher than the general population. More recently, the same applies to the people in my profession (planning) who use “gravity models” to predict future transportation patterns.
However, whenever I get too lefty-proud, my wife, who teaches special education in elementary school - kids with ADD and/or behvior problems - points out that more of her students are lefties than the general population.
Don’t know what this means, but thought it interesting.