Codominance

[sally baker]

hey i was away the day codomiance was covered i  read the text book , but dont understand it very well  and it uses examples such as blood groups. And how is codomiance different fro incomplete dominance ?

Thanks

[Russ]

I think of it like discrete/continuous numbers.

Codominance - you get two distinct types expressed because they can't be merged
Incomplete - you get one, that's located partway between the two options (on the continuum)

Skin colour is the easiest one to describe, usually in horses.
Black x White = Grey is incomplete dominance
Black x White = Spotted black/white is codominance

[thushan]

Remember that codominance and incomplete dominance are relationships between PHENOTYPES.

Co-dominance is where TWO phenotypes are simultaneously expressed in an organism to their full extent.
Eg. blood groups ->

I(A) corresponds to production of antigen A
I(B) corresponds to production of antigen B
i corresponds to production of no antigen

These two phenotypes are codominant, because a heterozygote with I(A) I(B) genotype have BOTH the phenotypes "production of antigen A" and "production of antigen B."

Incomplete dominance is where TWO phenotypes are only PARTIALLY expressed, with one inhibiting the expressio of the other.

Made-up example:

R corresponds to production of red flowers
r corresponds to production of white flowers

These two phenotypes are incompletely dominant because a heterozygote with genotype Rr has NEITHER the "red flowers" NOR "white flowers" phenotype, but an intermediate; "pink flowers".

Now, to emphasise the fact that we talk about PHENOTYPES, let's look at the same gene locus:

R corresponds to production of red PIGMENT
r corresponds to production of white PIGMENT

These two phenotypes, although they come from the same gene locus as the above case, are CODOMINANT because a heterozygote with genotype Rr has BOTH the "red pigment" and "white pigment" phenotypes. The mixture of red and white pigment leads to pink flowers (a DIFFERENT type of phenotype, not related to the codominant ones here).

Make sense?

[sally baker]

Thankyou So much Russ & Thushan , makes sense .