In past racial discussions, there were those who claimed that blacks had an extra muscle in their legs to explain the superior performance that black athletes were able to demonstrate particularly in America. Of course I scoffed at such a notion. In addition, past straight dope threads made it clear that we all had the same number of genes though differences could be explained be alleles, differing forms of the gene.
Now I read in the June issue of Discover magazine an article entitled “Useless Body Parts” the following:
Subclavius Muscle: some people have one, some have two, some have none
Plantaris Muscle: 9% of people do not have it
Pyramidalis Muscle: 20% of people do not have it
Palmaris Muscle: 11% of people do not have it
13th rib : 8% have an extra
Can these differences be explained merely by differing alleles rather than missing or extra genes?
There is a fascinating book called {i]Mutants* by Armand Marie Leroi that deals with some of the differences in human bodies - causes range from genes to developmental “accidents” to “we don’t know why”. If the topic interests you check it out.
I saw the same article. As a bit of trivia, I’m one of the 5% who have healthy, functional wisdom teeth. Woo-hoo!
This line doesn’t make much sense genetically, I’m sorry to say. “Allele” is short for “allelomorph” from the Greek “other + form.” Missing or extra genes would just be alleles – deletions and duplications are just other forms of the genes.
And yes, different alleles can easily explain this. Big changes in body plans like this, especially complete absence or duplication of body parts, is pretty commonly duplications, deletions, or other mutations in sets of homeotic gene complexes (an array of tandemly arranged, highly related genes, all encoding DNA binding proteins which sit at the top of large morphogenetic processes). In all examples you have given, there is apparently little stabilizing selection keeping those particular features around, and so those alleles leading to those phenotypes float around populations.
I have not seen a population breakdown of distribution of these particular features or alleles, and PubMed searches come up empty. But I would strongly suspect that these features do not have any particular breakdown with any population, especially not within any particular racial group, which has no real genetic meaning anyway. But this gets into tired territory that we have been through many times before.
In the past it has been stated (as understood by me at least) that an allele is a specific form of a gene. It was my understanding that any difference within the gene, specifically a deletion or addition or change of a nucleotide would constitute a different allele of that particular gene.
Furthermore, it was my understanding that a gene is a particular region along the DNA chain. I would have expected that a missing or duplicated gene would be quite significant.
Are you saying that that all humans (ie normal) do not have the same number of genes?
Interestingly, “Virtual Hospital” does have some ethnic breakdown for another variable muscle - the psoas minor. Not surprisingly, despite a fair degree of genetic isolation between groups, the percentage of those individuals not possessing the muscle is similar in each population.
Further, even among those who possessed the muscle in question, its form was quite variable. I would suspect that the degree of variation in those possessing the traits mentioned in the OP would be proportional to the absence percentage (i.e., the higher the percentage in which it is absent, the higher the degree of variation among those in which it is present).
Your definition of allele is generally correct, but we use the word allele to also talk about deletions and duplications. The actual number of genes (which is unknown despite complete sequencing and large-scale finishing of the euchromatin of the human genome) is unknown and will likely be unknown for a long time to come. It is a pretty trivial point in genetics, and yes, I would expect it to vary by at least a few dozen between different people.
Microdeletions and microduplications are found quite commonly and often without phenotype. Most genes are not exquisitely dosage sensitive – 3 copies versus 1 copy versus the normal 2 usually don’t make a large amount of difference. Gene expression level is pretty pliable and the cell usually makes corrections if there is one normal copy.
Moreover, with 30,000+ genes in the genome, many of which are parts of closely related families, genes often tend to have a level of functional redundancy. So removal of two copies of one redundant gene (or duplicating it to add another level of redundancy) may do absolutely nothing, or have a very minor phenotype (like absence of the palmaris longus muscle or add a rib).
I have a lot of experience with this in mice, where we make null alleles all the time. Many neurologically important genes have little phenotype when deleted. One can argue that the detection in mice is less sensitive (and that mice don’t have 80+ years of wear to develop a phenotype in things like dementia models), but it doesn’t change the fact is that there is a tremendous amount of redundancy, which kinds of acts like a cushion for natural selection.
Even though we spend a lot of time talking about how all humans are more closely related than a single population of chimps, or that there is very little genetic differences between human populations, there is still quite a lot going on in the 3 billion base pairs of our genome, and due to our long lives and our medical technology, we can note many aspects of genetic change. Thus, we can trace things like hair cowlicks to alleles of the gene Frizzled, and we can start worrying about extra vestigial muscles in the calf and forearm. BTW, have you ever seen a psoas minor, plantaris, or palmaris longus muscle? Our cadaver in med school had a palmaris longus and a plantaris, both of which were barely identifiable as muscles. They were about half a centimeter around, and looked like minor tendons or sclerosed veins or something. One would be hard pressed to attribute a sub-10-second 100 meter time to such a thing…
Darwin’s Finch
I found two studies but they suck, and certainly don’t begin to answer any questions…
One other thing – the studies cited at that Virtual Hospital link are really, really old. The newest one is from 1966. PubMed goes back to the 1950s, but doesn’t list any of those studies. I checked my Gray’s and my Moore’s anatomy textbooks. Moore’s mentions that the palmaris longus is absent in 14% of people (no breakdown), but “its actions are not missed.” It mentions that psoas minor is gone in 40 to 50% of people, but may be present on one side.
