What's polygenic inheritance - (Dec/13/2008 )

What does polygenic inheritance mean (the book's (that I use) definition was very confusing - i.e. what's a continuum, etc.)?

Let's see if I can explain this on a post..

There is monogenic inheritance, and this means that for a particular characteristic to be inherited it depends only of one set of alleles (one specific gene). For example, achondroplasia. You only need one gene to cause the phenotype.

After reviewing that, let's move to polygenic. In this case, you need more than one gene (or set of alleles, if you prefer to see it like that) to cause the phenotype. The color of your eyes are a good example. If I remember correctly, there are 3 genes involved in eye color. The resulting color will be determined by those 3 genes instead of only one (old books refer to eye color as a monogenic pattern of inheritance, but it was discovered is not like that). Also, multifactorial pattern of inheritance is also referred as polygenic in some sources. Weight, for instance, depends on several genes and environmental factors as well (nutrition being the most important). However, most sources won't treat multifactorial and polygenic as the same considering that environment has no impact on some polygenic traits.

Think about this:
Mono: one
Poly: more than one
Genic: genes.

About a continuum: picture a timeline. That is an example of a continuum.
100 BC--------------------------------year 0-------------------------------- year 2008.

In genetics:

Monogenic ---------------------------------Polygenic and multifactorial ------------------------------ environmental
traits traits traits

(albinism) (Hipertension) (chickenpox)
one gene several genes and probably environment environment

As you can see in the previous example, if you take 3 diseases and put it in a continuum, polygenic traits go between a disease determined by one gene only and a disease determined by environment only.

I hope this answers your question. Well, I am not sure if that is exactly what you were asking!

Ohh, my genetic continuum was messed up after I placed the post!!!

Well, put one word "trait" (that is repeated 3 times) below "monogenic", one under polygenic and multifactorial and another one under environmental.

Then you got "albinism" that goes below monogenic, "hypertension" goes below polygenic and multifactorial and chickenpox goes below environmental.

Then you got "one gene" (goes, obviously below monogenic column), several genes and environment under polygenic and multifactorial column and finally environment goes below environmental traits column.

Here,

Monogenic traits---------------------------Polygenic and multifactorial traits-------------------- environmental traits
(albinism) -------------------------------------(hypertension) -------------------------------------- (chickenpox)
one gene ----------------------------------several genes and environment ------------------------ environment