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View Full Version : Extra fingers is a dominant genetic trait?


XaMcQ
08-08-2006, 10:22 PM
So says 'The Cartoon Guide to Genetics', by Larry Gonick and Mark Wheelis. Right after Mendel, while they are explaining dominant and recessive traits, they list some examples, blue eyes vs. brown, tongue curling, baldness, and wham- "extra fingers is dominant over five fingers(weird but true)"(paraphrased-the messy living room ate the book, but that's what it said).

How the heck can that be true? Wouldn't there be more six-fingered men(and women) walking around if it were?

easy e
08-08-2006, 11:03 PM
This site (http://www.hmc.psu.edu/healthinfo/pq/poly.htm) says that polydactyly (extra digits) is an autosomal dominant trait. The reason there's not more people with it is that the prevalence is incredibly low. Also, there could have been negative selection for it as superstitious people may not have wanted their sons/daughters marrying someone they knew to have extra digits.

The same is true with Huntington's disease (http://en.wikipedia.org/wiki/Huntingtons_disease). It's autosomal dominant, but luckily rather rare.

Smeghead
08-08-2006, 11:49 PM
It's a common misconception that dominant = common and recessive = rare, and it's completely untrue. Barring selective pressure for or against, the prevalance of any allele, dominant or recessive, will remain roughly the same from generation to generation. Prevalence and dominance are completely unrelated.

WhyNot
08-09-2006, 12:14 AM
Myopia (nearsightedness) may be (http://hubel.sfasu.edu/courseinfo/SL2000/myopia.html) due to a dominant gene, as well:
Since a definite cause for myopia has not been determined, researches are divided as to whether the disease is caused by genetic factors or environmental factors. The debate for the genetic theory begins with the thought that myopia may be due to a dominant gene that is inherited by the child from the parent (Young et.al., 1998a). There is evidence to suggest that genetic heterogeneity attributes to high myopia ( Young et.al., 1998b). This implies that the child who inherits diversified genes from his or her parents is more likely to develop myopia. However, it is also thought that myopia may be caused by influences from the nervous system that are not normal and have a negative impact on the developing eye. Thus, it may, as well, be genetically determined (Kolata, 1985). The basis for the genetic theory is supported by the fact that myopia tends to run in families. The genetic theory has ample evidence for support; however, it has not been directly linked to myopia as a definite cause for its development.

Chronos
08-09-2006, 12:40 AM
Barring selective pressure for or against, the prevalance of any allele, dominant or recessive, will remain roughly the same from generation to generation.And if there is selective pressure against some trait, it'll act a lot faster on a dominant trait. If everybody were to decide that extra digits were unattractive, and refused to mate with anyone with extra digits, the trait would be completely gone (aside from new mutations) in the next generation, because a kid can't end up with extra digits unless at least one parent has them. On the other hand, if (say) blond hair (a recessive trait) were deemed similarly repulsive, you would nonetheless see blonds showing up for several generations afterwards, since many blond genes would still be in the pool hiding behind brown genes. This is why most genetic disorders are carried by recessive genes, because if they were dominant, the selection against them would be more efficient.

WhyNot
08-09-2006, 12:51 AM
And if there is selective pressure against some trait, it'll act a lot faster on a dominant trait. If everybody were to decide that extra digits were unattractive, and refused to mate with anyone with extra digits, the trait would be completely gone (aside from new mutations) in the next generation, because a kid can't end up with extra digits unless at least one parent has them. On the other hand, if (say) blond hair (a recessive trait) were deemed similarly repulsive, you would nonetheless see blonds showing up for several generations afterwards, since many blond genes would still be in the pool hiding behind brown genes. This is why most genetic disorders are carried by recessive genes, because if they were dominant, the selection against them would be more efficient.
*light goes on*

OOOOOoooooohhhhhh!

Thanks, Chronos, for putting that in Captain Dummy Speak! I finally get it!

First Amongst Daves
08-09-2006, 09:52 AM
This site (http://www.hmc.psu.edu/healthinfo/pq/poly.htm) says that polydactyly (extra digits) is an autosomal dominant trait. The reason there's not more people with it is that the prevalence is incredibly low. Also, there could have been negative selection for it as superstitious people may not have wanted their sons/daughters marrying someone they knew to have extra digits.

The same is true with Huntington's disease (http://en.wikipedia.org/wiki/Huntingtons_disease). It's autosomal dominant, but luckily rather rare.

Possibly not "incredibly low". A fantastic and very readable book by Armand Marie Leroi, Mutants: On the form, varieties and errors of the human body published in 2003, says that 1 in 3000 Europeans are born with extra fingers or toes, and 1 in 300 Africans.

I first came across this in speaking to a senior government official in Cambodia. I had never seen it before and found watching him gesticulate fascinating.

Zsofia
08-09-2006, 12:19 PM
Well, not every instance of polydactwhatever is a perfect extra finger. I'm sure plenty of people get their finger-stub taken off as a baby and you'd never know they had one.

I did know a girl once with an extra toe - not a complete one, a little half stumpy one that kind of stuck out from her foot. I only found out when we went to the beach.

Ethilrist
08-09-2006, 12:23 PM
Plus, people keep hunting down and killing six-fingered people, and that's gotta cut into the numbers.

Malienation
08-09-2006, 01:05 PM
Whynot, see my post here. (http://boards.straightdope.com/sdmb/showthread.php?p=7661964#post7661964) The source is "The Axemaker's Gift", by James Burke (the guy who did the popular series Connections and is a columnist for Scientific American. Being nearsighted, I found this fascinating.

Bear_Nenno
08-09-2006, 01:17 PM
Plus, people keep hunting down and killing six-fingered people, and that's gotta cut into the numbers.
Well if they would just stop killing everyone's fathers...

Quercus
08-09-2006, 02:14 PM
> Plus, people keep hunting down and killing six-fingered people, and that's gotta
> cut into the numbers.

Well if they would just stop killing everyone's fathers...
Is it just coincidence that I opened this thread right next to the 'Building up an immunity to poison' one?

Glynis
03-26-2012, 09:18 PM
It's a common misconception that dominant = common and recessive = rare, and it's completely untrue. Barring selective pressure for or against, the prevalance of any allele, dominant or recessive, will remain roughly the same from generation to generation. Prevalence and dominance are completely unrelated.

I get that if we socially decide 6 fingers is creepy, then that poor schmuck just isn't gonna get laid. Duh.
But if you could take that choice totally out of the situation, I do NOT get how the number would not grow to MORE common (than 5 fingers) over enough time.

How is it not a ratio of 2 to 1?
Three options. SIX SIX... SIX five... five five. That looks like 2 to 1 to me !

I read Smeghead's comment over and over and over. I just MAYBE can grasp the math of it - how it stays the same. Something like how throwing snake eyes with dice has exactly the same chance of coming up, even if you haven't thrown one in 300 tries, or how it is theoretically possible for someone on earth to throw 100 snake eyes in a row. Thinking that it can't happen, is just superstitious.

Is it like that?

If so, then I have to ask...... How did SIX SIX and SIX five ever GET to be so rare in the first place - (If we were to imagine a theoretical world wherein no one had ever thought they were freaky looking and refused to date them, I mean.)

So I can ALMOST grasp the homeostatic concept, like the dice, that the polydactyly would remain AT its present rate over generations, like Smeghead said, but I CAN'T understand how it happened from way, way, way back.

As the brighter (mathematically) of you future readers might guess, I also struggled severely with the Monty Hall Problem - http://en.wikipedia.org/wiki/Monty_Hall_problem

Glynis
03-26-2012, 09:48 PM
I get that if we socially decide 6 fingers is creepy, then that poor schmuck just isn't gonna get laid. Duh.
But if you could take that choice totally out of the situation, I do NOT get how the number would not grow to MORE common (than 5 fingers) over enough time.

How is it not a ratio of 2 to 1?
Three options. SIX SIX... SIX five... five five. That looks like 2 to 1 to me !
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Ahh, a little more light shown here: http://www.enotes.com/topic/Polydactyly

After reading that polydactyly is far more than SIMPLY a finger/toe thing and quite often a visible symptom of quite of few other things going wrong with the body, I can see more reasons these folks aren't more prolific. I clicked on every one of the syndromes listed, and gave them a quick scan. Some of these syndromes shorten life (which may prevent procreation entirely, or cut short the fertile years), cause enough other medical issues that cut into a person's health enough to limit or preclude procreation entirely, reduce a person's wish to procreate either by the effort needed to deal with the medical issues, or by the person choosing not to risk passing along the entire syndromes (worse than just the creepy finger look).
Ah.... So it's NOT just that people are so shallow that they won't date a polydactyl because of looks.

heathen earthling
03-27-2012, 04:42 AM
Is polydactyly dominant in other species too? Thinking of other animals that have digits, it is far more common for evolution to delete digits in a lineage than to add them. Even if it is negatively selected in humans, you'd kind of think there would be at least one mammal out there that could benefit from six or seven fingers.

Blake
03-27-2012, 05:05 AM
Is polydactyly dominant in other species too?

It's common enough in dogs that it hardly even rates a mention outside of show dogs. Dogs normally have four toes and a dewclaw on the front foot and four toes on the hind. But dogs with no dewclaws or dewclaws front and back are common as muck.


Thinking of other animals that have digits, it is far more common for evolution to delete digits in a lineage than to add them.

That's because it is much easier to remove digits than to add them. Once you lose a digit, the genes associated with it cease to be subject to selective pressure and so accumulate mutations. So you can't simply add a digit back again, you need to re-evolve the whole structure from scratch: nerves, tendons, bones, connections to the ankle, the whole lot.

In contrast, deleting a digit is fairly easy. You just have to prevent the damn thing from growing. That doesn't usually even have any great effect on the rest of the foot.

Even if it is negatively selected in humans, you'd kind of think there would be at least one mammal out there that could benefit from six or seven fingers.

There are plenty, most notably pandas, roosters and iguanodons but several other species as well. The odd part is that these species had to evolve their extra digit from totally unrelated structures because its isn't that easy to produce a novel digit.

Floater
03-27-2012, 05:38 AM
The odd part is that these species had to evolve their extra digit from totally unrelated structures because its isn't that easy to produce a novel digit.
Which, according to Stephen J Gould, is a very good example of how evolution works.

heathen earthling
03-27-2012, 05:47 AM
That's because it is much easier to remove digits than to add them. Once you lose a digit, the genes associated with it cease to be subject to selective pressure and so accumulate mutations. So you can't simply add a digit back again, you need to re-evolve the whole structure from scratch: nerves, tendons, bones, connections to the ankle, the whole lot.

Yet there still exist people with fully formed sixth digits.

There are plenty, most notably pandas, roosters and iguanodons but several other species as well. The odd part is that these species had to evolve their extra digit from totally unrelated structures because its isn't that easy to produce a novel digit.

I'm only asking about true digits, not "pseudo-digits" as in pandas. I think the silkie chickens do have true extra digits, but that's an artificially selected animal. Iguanodonts didn't have any extra digits or pseudo-digits (the thumb spur is just a modified digit I).

Ludovic
03-27-2012, 08:16 AM
How is it not a ratio of 2 to 1?
Three options. SIX SIX... SIX five... five five.
SIX! Six Options! Ah HA HA HA HA HA HA!

Smeghead
03-27-2012, 10:28 AM
How is it not a ratio of 2 to 1?
Three options. SIX SIX... SIX five... five five. That looks like 2 to 1 to me !


I have honestly no idea what your argument is in this post, but let me update my 6-year-old post and see if I can clarify things.

First of all, like I said, whether an allele is dominant or recessive has nothing whatsoever to do with how common it is. Nothing. They're completely independent properties. There are two forces that can alter the prevalence of an allele (that is, how common it is): selection and genetic drift.

The easiest way to think of this is as a probability question. You can Google "Hardy-Weinberg equilibrium" if you want a full treatment, but I'll give a summary. Let's say we have a gene pool. That is, some number of individuals who each have two copies of a gene. Let's represent each copy as a marble in a bag. Let's say the polydactyly allele are represented by red balls, and the "normal" allele is represented by green balls. Each individual can have two red balls, two green balls, or one of each, but only passes one on to each child.

So, to model our current human population, you'd have lots and lots of marbles, and almost all of them are green. That's the parental generation. Now, let's make the next generation, which will have the same number of individuals as the parental. To do this, for each individual, we reach into our bag of marbles and grab out two to determine the individual's genotype. We then put the marbles back in the bag (selection with replacement), because each parent can have more than one child. If you do the probability math, you'll find that you would expect the second generation to have very close to the same ratio of red to green balls. If the parental generation had 2% red, the second generation should have very close to 2% red, as long as the population sizes are large. This should be obvious, because each ball is equally likely to be picked.

Now, just due to random chance, there could be a slight deviation from the parental ratio. This is genetic drift, and it's far more prevalent in small populations. If you're only picking 100 balls, you're much more likely to deviate from the expected 2% than if you picked 10 million balls.

Note that none of this is affected at ALL by whether green or red is dominant or recessive. The only difference that has at all is on the appearance of heterozygotes. In other words, if an individual has one red ball and one green ball, does he look red or green? That's it. As long as people that look red are just as likely to have kids as people that look green (which is generally the case for polydactyly in particular), the ratio of red to green will stay unchanged (or very very close to unchanged) generation after generation.

Now, if you want to get into selection, that's a different matter. Let's say red people are twice as likely as green people to reproduce. You'd model this by doubling all the balls that go into the bag from all of the red people in the population, while leaving the green peoples' balls the same. If you do the math on that, you'll find that within not very many generations, your population will be entirely red (if red is recessive) or almost entirely red (if red is dominant).

Lasciel
03-27-2012, 12:28 PM
I'm not the target of that post, Smeghead, but I wanted to say that I greatly appreciated the marbles-in-a-bag summary. I think that's the clearest explanation I've ever seen of how traits get passed on in a more-or-less consistent rate through generations.

Kudos!

Malleus, Incus, Stapes!
03-27-2012, 04:58 PM
It's common enough in dogs that it hardly even rates a mention outside of show dogs. Dogs normally have four toes and a dewclaw on the front foot and four toes on the hind. But dogs with no dewclaws or dewclaws front and back are common as muck.
Cats too.


There are plenty, most notably pandas, roosters and iguanodons but several other species as well.

Wait, inguanodons and roosters are mammals?

Wait another second, there are igaunodons? Where do I get one?

:D

Ludovic
03-27-2012, 09:42 PM
Wait, inguanodons and roosters are mammals?Inguanodons are mammals: they're Mafia bosses who got tossed in a shithole. Because they're so pissed at this, inguanodons flip out and kill people all the time.

hogarth
03-28-2012, 10:18 AM
I'm not the target of that post, Smeghead, but I wanted to say that I greatly appreciated the marbles-in-a-bag summary. I think that's the clearest explanation I've ever seen of how traits get passed on in a more-or-less consistent rate through generations.
I found it slightly cryptic, since it seemingly ignores the idea every person has two copies of the gene rather than one. But in conjunction with the Wikipedia article I managed to figure it out.

Smeghead
03-28-2012, 10:47 AM
I found it slightly cryptic, since it seemingly ignores the idea every person has two copies of the gene rather than one. But in conjunction with the Wikipedia article I managed to figure it out.

Not at all. Each ball represents one allele. Each individual contributes two balls to the bag.

awldune
03-28-2012, 11:03 AM
I don't think this has been mentioned, but the gene for polydactyly has incomplete penetrance. That is to say, even if you have one copy of the dominant polydactyl gene, you might still not express the trait.

http://www.geneticsofpregnancy.com/Encyclopedia/Polydactyly.aspx?pid=37&cid=113

Penetrance is not full. The probability that the polydactyly gene will be expressed in a person who carries it is about 65%. In other words, there may be instances where members of a family with many cases of polydactyly will be born with a normal number of digits, but will give birth to children with polydactyly - these family members carry an abnormal gene for the condition but do not express it.

This is of some interest to me because my father was born with six fingers on his right hand*, but I was born with no supernumary digits.

*Ironically, he lost a finger in a wood shop accident. The extra finger was removed at birth, so he only has four fingers on that hand today.

Chronos
03-28-2012, 01:52 PM
Huh, if I'd been born with an extra finger, I'd want them to let me keep it as a spare.

hogarth
03-28-2012, 02:39 PM
Not at all. Each ball represents one allele. Each individual contributes two balls to the bag.
Sorry, apparently I didn't find it slightly cryptic.

:rolleyes:

Smeghead
03-28-2012, 06:11 PM
Sorry, apparently I didn't find it slightly cryptic.

:rolleyes:

I was just clarifying in case someone else was confused. Nothing personal.

SticksAndString
03-28-2012, 10:15 PM
Honestly I don't see how it can be a DOMINANT trait..recessive maybe. But not dominant.

Nobody in my family (that I'm aware of) or in DH's family (again, nobody he is aware of) have had extra digits, yet our son was born with what the doctors were calling a digit by technicality (it was a flap of skin about 1/8th of an inch wide by 1/8th inch long with a fleshy bit on the end that was complete with a bone inside and a fingernail which grew normally on the outside). So how is it that our kid had an extra digit?

Chronos
03-28-2012, 10:36 PM
I think that gets into what awldune was saying about incomplete penetrance. Real gene expression, and real biology in general, is messier than the simple binary dominant vs. recessive we like to talk about in freshman biology classes.

SticksAndString
03-28-2012, 10:56 PM
Yeah..see all of that when (whoosh!) right over the old noggin. Science was never one of my best subjects.

Smeghead
03-29-2012, 09:30 AM
Think of having the gene giving you a tendency toward extra digits, rather than giving you extra digits. It greatly increases the chance, but it doesn't always happen.

awldune
03-29-2012, 10:59 AM
Honestly I don't see how it can be a DOMINANT trait..recessive maybe. But not dominant.

Nobody in my family (that I'm aware of) or in DH's family (again, nobody he is aware of) have had extra digits, yet our son was born with what the doctors were calling a digit by technicality (it was a flap of skin about 1/8th of an inch wide by 1/8th inch long with a fleshy bit on the end that was complete with a bone inside and a fingernail which grew normally on the outside). So how is it that our kid had an extra digit?

A couple of things:

Sometimes supernumary digits were(are?) removed at birth without the parents' knowledge. In a case like your son's (and my father's), the scar is quite small and might be overlooked or mistaken for a birthmark/mole/wart by adulthood.

Also there is the "penetrance" issue as mentioned above. If my father has polydactyly and my mother does not carry the gene, then I had a 50% chance of receiving the gene. If I carry the gene and don't express it (further 65% chance) I could pass that gene on to my child (50%) who would then have a 65% chance of presenting supernumary digits. So one would expect that roughly 1/3 of the descendants of a polydactyl would express polydactyly.

I should say all of this is a layman's "As I understand it" and open to correction by someone who more about genetics.

Chronos
03-29-2012, 01:35 PM
So one would expect that roughly 1/3 of the descendants of a polydactyl would express polydactyly.1/3 of the immediate children, but it becomes less and less likely with each subsequent generation, since grandchildren, great-grandchildren, etc. have a much lower chance of getting that gene from that particular ancestor.