Incomplete Dom versus Codom?

[ThyTempest]

Hey everyone,

Just prepping for my bio exam tomorrow and my prof and my TA still cant explain the difference between incomplete dominance and co-dominance that makes any sense to me, or that falls into place with what I know about bp morphs and genetics.

Heres what I think so far, and someone please tell me if I am wrong...

Incomplete dominance is when two traits are equally dominant and recessive, hence they are incomplete. For instance if XX is long and xx is Short, then Xx would be medium.....another example....RR is red, rr is whiteand Rr is pink. To me this seems to be a blending of the other two traits, but my prof keeps saying no, that the third trait/phenotype is intermediate between the other two, but despite half the class asking wtf he means by intermediate, he just says "look in your book", and I think we all know how textbooks try to teach.

Anyway, on to codom.

Prof says that the difference between codom and inc. dom is that in codom the third (or fourth, fifth, etc) trait is NOT intermediate between the other two, that it is entirely different. I was thinking, ok, this makes some sense....so I asked him how it would fall into a punnett square and he said again..look in your book it has great examples, yet there are no punnet squares for codom in it...so I asked if for instance there were only 3 traits/phenotypes that could be expressed, is it a codominant "trait" or set of traits that is controlled by one allele or does it take multiple alleles and it is how they arrange themselves together that you get the different traits.

So....here is what codom seems to be to me.

RR is red rr is white and Rr is white/red spotted (ie, with flowers and stuff) OR RR is red rr is white and Rr is yellow( or some other color), and that it is has nothing to do visually with the others, but it is just as dominant as the other two traits. If this is the case, in practice/probability, doesnt it present itself as being more dominant b/c having a het genotype is more common than being homozygous for either of the alleles?



I know it was long winded, and I hope someone can help. Thanks

[stangs13]

Ball python people have different meanings than the genetics you learn in school..so dont compair..this saved me alot of trouble.

[RGreen454ss]

I thought co-dom ment incomplete dominant ie. Pastels, Cinnies and all those who had a super (Homozygous) form

and Dominants dont have a super (Homozygous) form ie. Spider, Normal, and Pinstripe.

I think your problem with understanding it is your grouping Co-Dom (Incomplete Dominant) and Dominant together. But then again I was wrong once before

[Chandler reptiles]

From my understanding of it you have to think of codom as the genes coexisting in the same thing, like with your pink flowers and with incomplete dominance it is one or the other either red or white.

Codom somewhere along the lines got mixed into with the snakes and that isn't exactly what it is. All of the visible (not talking any recessive genes here) morphs like pastel and spiders are really just incomplete dominance. Then you can move into boas with the salmons that are dominant genes as where half of your litter will be salmons and half will be normals unlike with the pastels were you have a 50% chance per egg that it will be a pastel. So in theory you could have a clutch of 10 eggs and get no pastels but with the salmon boas you will get salmons.

At least that is the way I understand it.

[andwhy6]

im going to school for a ba in animal science and the way people on here talk bout traits is usually diff than in science class so i just try and leave them separate. ive talked snake genetics with profs and we usually cant relate. its weird. good luck wit class tho

[Rapture]

For some reason the wrong terms are used to describe genetic heritage in the ball python context. What you know as co-dominant in ball pythons is in all actuality incomplete dominance. Vin Russo covers this topic in his book, The Complete Boa Constrictor.

[dprince]

For some reason the wrong terms are used to describe genetic heritage in the ball python context. What you know as co-dominant in ball pythons is in all actuality incomplete dominance. Vin Russo covers this topic in his book, The Complete Boa Constrictor.

Exactly.

[littleindiangirl]

For some reason the wrong terms are used to describe genetic heritage in the ball python context. What you know as co-dominant in ball pythons is in all actuality incomplete dominance. Vin Russo covers this topic in his book, The Complete Boa Constrictor.

Perfect!

We call co-dominants and it gives the impression that it means CO-, (as in CO-ed) which is only a partial explanation of our terms.

Incomplete dominant is the proper term, but we hobbyists like to shorten that to Co-dom.

Incomplete dominance is exactly that, incomplete. The pastel gene is an incomplete dominant, meaning it is only partially expressed when heterozygous (what we call Pastels), and is being completely expressed when homozygous (what we call Super Pastels).

Does that make better sense?

Dominant is a complete expression of the gene whether it is heterozygous or homozygous. It does not need to be a homozygous animal to show the complete expression of the gene.

An example of that is BHB's suspected HOMOzygous pinstripe (carries two copies of the gene).
It will look exactly the same as a heterozygous pinstripe, but when crossed with a normal animal, gives 100% het pinstripes. And remember, a pinstripe is a dominant, so only 1 copy of the pinstripe gene is needed for the full expression of the gene.

Just like if you were to cross a Super Pastel with a normal, you get all Pastels, because each animal receives one copy of the pastel gene.

Well, probably confused you, but hope it helps anyways...

[nwheat]

As has been said - don't let terminology used in herpetoculture influence your answers in biology! I'm sure that many of the traits that are called codominant in ball pythons, leopard geckos, corn snakes, etc are actually examples of incomplete dominance. Incomplete dominance and codominance are very similar in that they both produce visible hets. The difference is in how the various alleles are expressed.

Punnett squares work exactly the same way for simple dominant/recessive, codominant, and incompletely dominant traits. The difference is in the phenotype - the latter two have visible hets. So when considering a one-gene trait, you will get a 1:2:1 genotypic and phenotypic ratio with both codominant and incompletely dominant traits. The difference is only that we usually use two capitol letters R1 and R2 for example instead of R and r. (The 1 and 2 should be superscript - can't figure out how to do that here, though.)

Simple dominance example

R r
R RR Rr
r Rr rr

Genotypic ratio 1RR:2Rr:1rr
Phenotypic ratio 3 red : 1 pink

Incomplete dominance example

R1 R2
R1
R2

I find it easiest to understand the difference by thinking about what the alleles are doing (protien synthesis). The job of any allele is to synthesize proteins - different versions of the allele may synthesize different versions of the protein, or it may fail to synthesize the normal proten.

With a simple dominant/recessive gene, the dominant allele suppress the recessive one. So, you have just the dominant allele synthesizing protein. The single dominant allele can produce enough protein to produce the full affect of the allele, so a heterozygous individual looks just like a homozygous dominant individual.

With Incomplete dominance, the more dominant allele still suppresses expression in the other less dominant allele. However, this time the more dominant one is not able to produce enough protein to produce the full effect of the allele. So, a heterozygous individual will look different than homozygous dominant (and homozygous recessive). In general, the heterozygotes will have an intermediate appearance.

In a codominant trait, neither allele is dominant over the other and both are expressed fully. Both alleles are able to produce enough protein for the full expression of the trait.

[ThyTempest]

No Connie, you are the one that cleared it up. Knowing that all this time when the reptile hobbyists have been saying codom and really meaning incdom changes the whole story, I was trying to get bio to fit into snakes rather than snakes into bio. Either way, genetics was hardly on the test, despite what my prof said the focus would be on....sooooo i screwed up everything on cell replication because I went to such great lengths to know the differences between all the genetic relationships, including pseudodominance which was covered in class....but incorrectly labeled as dominance. I am such a skeptic.

Anyway, thanks for the tips guys, and from now on codom=incomplete dom....if it is really a matter of not wanting to spell out incomplete dominance, call it incdom...it is one letter more than codom.

However, onto the debate.....incdom is present when the "Extra, typically 3rd" phenotype is a blending or "intermediate" of the other two, whereas in codom, the third phenotype is an expression of both of the other two equally. I dont particularly like this example but the blood type in humans is a good one.

There are 3 blood type genes, A, B and O. A and B are Codominant, whereas the O gene is recessive, and hence, more rare. So...A blood types are either AA or AO, B's are BB or BO, and O's have to be OO. Now, because A and B are codominant, AB's or BA's are considered AB blood type. In this type, both the A and B proteins are expressed in the blood.

To contrast this, if this example were considered incomplete dominance (just for teaching's sake, it is not incdom) then the A and B proteins would blend and form another blood type/protein completely, aside from the already recessive O. For example, instead of AB being Just A with Just B, it would be like if they melted together to form some new protein, X or Z, whatever you want to call it.

Another good, perhaps quicker example is with flowers

Codom---Red + White==Red and White spotted
Incdom---Red + White==Pink

Hope this clears it up at least a little, and if someone maybe wants to clear all this up, maybe the topic(not necesarily this thread) is sticky worthy?

Either way, thanks a lot to everyone that chipped in, or has been wondering about this too.


Ps--Another good answer Nancy, didnt get to read it since I was posting too. Thanks