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?
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.
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.
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.
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.
Blue eyes is (mostly) just an ordinary recessive trait, though, not “double recessive” (whatever that means).
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?
Check.
Check.
Exactly WHAT??
WHO did??
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?
You’ve lost me again.
A concrete example, with a source, would help immensely.
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.
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.
I’m gonna go out on a limb and say that no one can answer you question unless you clarify exactly what you mean.
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.