# Double recessive traits **URL:** \ **Category:** Factual Questions\ **Created:** [November 16, 2015, 4:50pm UTC](https://boards.straightdope.com/t/double-recessive-traits/737712 "2015-11-16T16:50:38Z")\ **Posts on this page:** 12\ **Page:** 1
**Author:** ![dauerbach](https://avatars.discourse-cdn.com/v4/letter/d/c2a13f/32.png) [@dauerbach](https://boards.straightdope.com/u/dauerbach)\ **Post date:** [November 16, 2015, 4:50pm UTC](https://boards.straightdope.com/t/double-recessive-traits/737712/1 "2015-11-16T16:50:38Z")
So after being totally stumped by a pedigree tree that showed a double recessive trait I could not find any example of human disease or even a human trait that shows such inheritance, and yet I am sure many examples exist. Does anyone know of some? ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [November 16, 2015, 6:40pm UTC](https://boards.straightdope.com/t/double-recessive-traits/737712/2 "2015-11-16T18:40:21Z")
A pedigree of what? What do you mean by “double recessive”? A trait that only appears when recessive alleles of two different genes are present (and different from the recessive trait that appears when only one set of recessive alleles are present)? Very few human traits are strictly Mendelian in nature, in that there is a simple dominant-recessive relationship between alleles. Most traits show show partial dominance, codominance, or are polygenic. ---
**Author:** ![Smeghead](https://avatars.discourse-cdn.com/v4/letter/s/f1d935/32.png) [@Smeghead](https://boards.straightdope.com/u/Smeghead)\ **Post date:** [November 16, 2015, 7:09pm UTC](https://boards.straightdope.com/t/double-recessive-traits/737712/3 "2015-11-16T19:09:20Z")
Yeah, I’ve never heard the term “double recessive” in any sense other than Colibri’s in my couple of decades studying genetics and biology. ---
**Author:** ![MLS](https://avatars.discourse-cdn.com/v4/letter/m/919ad9/32.png) [@MLS](https://boards.straightdope.com/u/MLS)\ **Post date:** [November 16, 2015, 7:21pm UTC](https://boards.straightdope.com/t/double-recessive-traits/737712/4 "2015-11-16T19:21:17Z")
If I’m understanding the question, there is one easy example: blue eyes. Dark-eyed parents can have a blue-eyed child if there are blue-eyed ancestors. Two blue-eyed parents can’t have a dark-eyed child unless there is a spontaneous mutation. ---
**Author:** ![dauerbach](https://avatars.discourse-cdn.com/v4/letter/d/c2a13f/32.png) [@dauerbach](https://boards.straightdope.com/u/dauerbach)\ **Post date:** [November 16, 2015, 8:34pm UTC](https://boards.straightdope.com/t/double-recessive-traits/737712/5 "2015-11-16T20:34:48Z")
By pedigree chart I meant the flow chart with squares and circles that show the parents and offspring. By expressed I mean with a clearly identifiable phenotype. This is easiest to think of in terms of disease or not disease, by any phenotypic trait would do. And yes, I mean exactly that, and you put it much better than I did, a trait that appears when recessive alleles of two different genes are present. I suppose that this might be a third trait that is only present when both sets of recessive alleles are present. I am thinking the pedigree example is from real life. I had assumed that it was old and something that McKusick had described in the Amish, but cannot find anything about it. There are numerous examples in the veterinary literature (I think Lab coat color is one) but I can find nothing in H. Sapiens. ---
**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:** [November 16, 2015, 8:34pm UTC](https://boards.straightdope.com/t/double-recessive-traits/737712/6 "2015-11-16T20:34:50Z")
Blue eyes is (mostly) just an ordinary recessive trait, though, not “double recessive” (whatever that means). ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [November 16, 2015, 8:49pm UTC](https://boards.straightdope.com/t/double-recessive-traits/737712/7 "2015-11-16T20:49:58Z")
> [@dauerbach](#): > > By pedigree chart I meant the flow chart with squares and circles that show the parents and offspring. I don’t understand what you mean by “a pedigree tree that showed a double recessive trait”? What trait did the tree show? How do you know it was a double recessive? ---
**Author:** ![Smeghead](https://avatars.discourse-cdn.com/v4/letter/s/f1d935/32.png) [@Smeghead](https://boards.straightdope.com/u/Smeghead)\ **Post date:** [November 16, 2015, 11:32pm UTC](https://boards.straightdope.com/t/double-recessive-traits/737712/8 "2015-11-16T23:32:40Z")
> [@dauerbach](#): > > By pedigree chart I meant the flow chart with squares and circles that show the parents and offspring. Check. > [@](#): > > By expressed I mean with a clearly identifiable phenotype. This is easiest to think of in terms of disease or not disease, by any phenotypic trait would do. Check. > [@](#): > > And yes, I mean exactly that, Exactly WHAT?? > [@](#): > > and you put it much better than I did, WHO did?? > [@](#): > > a trait that appears when recessive alleles of two different genes are present. So just to be clear, you mean a trait that is only seen when the genotype is aabb? A total of four recessive alleles across two different genes? > [@](#): > > I suppose that this might be a third trait that is only present when both sets of recessive alleles are present. You’ve lost me again. > [@](#): > > I am thinking the pedigree example is from real life. I had assumed that it was old and something that McKusick had described in the Amish, but cannot find anything about it. > > There are numerous examples in the veterinary literature (I think Lab coat color is one) but I can find nothing in H. Sapiens. A concrete example, with a source, would help immensely. ---
**Author:** ![Hari\_Seldon](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hari_seldon/32/5173_2.png) [@Hari\_Seldon](https://boards.straightdope.com/u/Hari_Seldon)\ **Post date:** [November 17, 2015, 12:56am UTC](https://boards.straightdope.com/t/double-recessive-traits/737712/9 "2015-11-17T00:56:16Z")
> [@MLS](#): > > If I’m understanding the question, there is one easy example: blue eyes. Dark-eyed parents can have a blue-eyed child if there are blue-eyed ancestors. Two blue-eyed parents can’t have a dark-eyed child unless there is a spontaneous mutation. That would be the case if it were a simple mendelian trait. I don’t think it is, but let me give a slightly different example that a geneticist friend explained to me once. It is possible for two type O parents to have a type A child? Yes it is. The process of making the A antigen is multi-step (same with B). One parent may be genetically AO, but lack a gene required for one step in the process of producing the A antigen and therefore be somatically type O and the other parent could likewise be type A0, but lack a gene that is required for a different step in the process. Put those two together and a quarter of their offspring will have blood type A. I think a similar argument could be made for eye color. My wife has eyes that appear greenish blue, but up close are blue with a few brownish flecks. I have blue eyes. Our daughter’s eyes appear brown, but up close are blue with a lot of brown flecks. ---
**Author:** ![KarlGrenze](https://avatars.discourse-cdn.com/v4/letter/k/dbc845/32.png) [@KarlGrenze](https://boards.straightdope.com/u/KarlGrenze)\ **Post date:** [November 17, 2015, 12:57am UTC](https://boards.straightdope.com/t/double-recessive-traits/737712/10 "2015-11-17T00:57:34Z")
Based on the comment on veterinary species, I’m guessing the talk is about a phenotype that is polygenic and only expressed when it is aabb (or even aabbcc). There are some combinations like that for hair colors in various species and breeds of animals. ---
**Author:** ![John\_Mace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_mace/32/185_2.png) [@John\_Mace](https://boards.straightdope.com/u/John_Mace)\ **Post date:** [November 17, 2015, 1:33am UTC](https://boards.straightdope.com/t/double-recessive-traits/737712/11 "2015-11-17T01:33:45Z")
> [@dauerbach](#): > > So after being totally stumped by a pedigree tree that showed a double recessive trait I could not find any example of human disease or even a human trait that shows such inheritance, and yet I am sure many examples exist. Does anyone know of some? I’m gonna go out on a limb and say that no one can answer you question unless you clarify exactly what you mean. ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [November 17, 2015, 1:57am UTC](https://boards.straightdope.com/t/double-recessive-traits/737712/12 "2015-11-17T01:57:36Z")
> [@Hari\_Seldon](#): > > I think a similar argument could be made for eye color. My wife has eyes that appear greenish blue, but up close are blue with a few brownish flecks. I have blue eyes. Our daughter’s eyes appear brown, but up close are blue with a lot of brown flecks. Eye color in humans is often portrayed in high school biology classes as if it were inherited as a simple Mendelian trait, with brown dominant and blue recessive, but it’s much more complicated than that. Several different genes are involved.