# Extra fingers is a dominant genetic trait? **URL:** \ **Category:** Factual Questions\ **Created:** [August 9, 2006, 2:22am UTC](https://boards.straightdope.com/t/extra-fingers-is-a-dominant-genetic-trait/367950 "2006-08-09T02:22:31Z")\ **Posts on this page:** 15\ **Page:** 2
**Author:** ![Lasciel](https://avatars.discourse-cdn.com/v4/letter/l/e79b87/32.png) [@Lasciel](https://boards.straightdope.com/u/Lasciel)\ **Post date:** [March 27, 2012, 4:28pm UTC](https://boards.straightdope.com/t/extra-fingers-is-a-dominant-genetic-trait/367950/21 "2012-03-27T16:28:36Z")
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! ---
**Author:** ![Malleus\_Incus\_Stapes](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/malleus_incus_stapes/32/3293_2.png) [@Malleus\_Incus\_Stapes](https://boards.straightdope.com/u/Malleus_Incus_Stapes)\ **Post date:** [March 27, 2012, 8:58pm UTC](https://boards.straightdope.com/t/extra-fingers-is-a-dominant-genetic-trait/367950/22 "2012-03-27T20:58:19Z")
> [@Blake](#): > > 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? 😃 ---
**Author:** ![Ludovic](https://avatars.discourse-cdn.com/v4/letter/l/7ab992/32.png) [@Ludovic](https://boards.straightdope.com/u/Ludovic)\ **Post date:** [March 28, 2012, 1:42am UTC](https://boards.straightdope.com/t/extra-fingers-is-a-dominant-genetic-trait/367950/23 "2012-03-28T01:42:25Z")
> [@Malleus\_Incus\_Stapes](#): > > 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. ---
**Author:** ![hogarth](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hogarth/32/1773_2.png) [@hogarth](https://boards.straightdope.com/u/hogarth)\ **Post date:** [March 28, 2012, 2:18pm UTC](https://boards.straightdope.com/t/extra-fingers-is-a-dominant-genetic-trait/367950/24 "2012-03-28T14:18:39Z")
> [@Lasciel](#): > > 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. ---
**Author:** ![Smeghead](https://avatars.discourse-cdn.com/v4/letter/s/f1d935/32.png) [@Smeghead](https://boards.straightdope.com/u/Smeghead)\ **Post date:** [March 28, 2012, 2:47pm UTC](https://boards.straightdope.com/t/extra-fingers-is-a-dominant-genetic-trait/367950/25 "2012-03-28T14:47:47Z")
> [@hogarth](#): > > 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. ---
**Author:** ![awldune](https://avatars.discourse-cdn.com/v4/letter/a/c77e96/32.png) [@awldune](https://boards.straightdope.com/u/awldune)\ **Post date:** [March 28, 2012, 3:03pm UTC](https://boards.straightdope.com/t/extra-fingers-is-a-dominant-genetic-trait/367950/26 "2012-03-28T15:03:59Z")
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. > **[Maux de tête Douleur](https://ehf2019.com)** > [@](#): > > 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. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [March 28, 2012, 5:52pm UTC](https://boards.straightdope.com/t/extra-fingers-is-a-dominant-genetic-trait/367950/27 "2012-03-28T17:52:51Z")
Huh, if I’d been born with an extra finger, I’d want them to let me keep it as a spare. ---
**Author:** ![hogarth](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hogarth/32/1773_2.png) [@hogarth](https://boards.straightdope.com/u/hogarth)\ **Post date:** [March 28, 2012, 6:39pm UTC](https://boards.straightdope.com/t/extra-fingers-is-a-dominant-genetic-trait/367950/28 "2012-03-28T18:39:49Z")
> [@Smeghead](#): > > 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: ---
**Author:** ![Smeghead](https://avatars.discourse-cdn.com/v4/letter/s/f1d935/32.png) [@Smeghead](https://boards.straightdope.com/u/Smeghead)\ **Post date:** [March 28, 2012, 10:11pm UTC](https://boards.straightdope.com/t/extra-fingers-is-a-dominant-genetic-trait/367950/29 "2012-03-28T22:11:49Z")
> [@hogarth](#): > > Sorry, apparently I didn’t find it slightly cryptic. > > :rolleyes: I was just clarifying in case someone else was confused. Nothing personal. ---
**Author:** ![SticksAndString](https://avatars.discourse-cdn.com/v4/letter/s/3bc359/32.png) [@SticksAndString](https://boards.straightdope.com/u/SticksAndString)\ **Post date:** [March 29, 2012, 2:15am UTC](https://boards.straightdope.com/t/extra-fingers-is-a-dominant-genetic-trait/367950/30 "2012-03-29T02:15:55Z")
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? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [March 29, 2012, 2:36am UTC](https://boards.straightdope.com/t/extra-fingers-is-a-dominant-genetic-trait/367950/31 "2012-03-29T02:36:54Z")
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. ---
**Author:** ![SticksAndString](https://avatars.discourse-cdn.com/v4/letter/s/3bc359/32.png) [@SticksAndString](https://boards.straightdope.com/u/SticksAndString)\ **Post date:** [March 29, 2012, 2:56am UTC](https://boards.straightdope.com/t/extra-fingers-is-a-dominant-genetic-trait/367950/32 "2012-03-29T02:56:47Z")
Yeah…see all of that when (whoosh!) right over the old noggin. Science was never one of my best subjects. ---
**Author:** ![Smeghead](https://avatars.discourse-cdn.com/v4/letter/s/f1d935/32.png) [@Smeghead](https://boards.straightdope.com/u/Smeghead)\ **Post date:** [March 29, 2012, 1:30pm UTC](https://boards.straightdope.com/t/extra-fingers-is-a-dominant-genetic-trait/367950/33 "2012-03-29T13:30:18Z")
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. ---
**Author:** ![awldune](https://avatars.discourse-cdn.com/v4/letter/a/c77e96/32.png) [@awldune](https://boards.straightdope.com/u/awldune)\ **Post date:** [March 29, 2012, 2:59pm UTC](https://boards.straightdope.com/t/extra-fingers-is-a-dominant-genetic-trait/367950/34 "2012-03-29T14:59:58Z")
> [@SticksAndString](#): > > 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. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [March 29, 2012, 5:35pm UTC](https://boards.straightdope.com/t/extra-fingers-is-a-dominant-genetic-trait/367950/35 "2012-03-29T17:35:14Z")
> [@](#): > > 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. [Previous page](https://boards.straightdope.com/t/extra-fingers-is-a-dominant-genetic-trait/367950.md?page=1)