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Comb Genetics

All About Combs
 
***This page is still in construction stages...  Please check back soon for additional information and images.***
This part is crazy-hard to write in a way that is easy to understand.  If you see serious mistakes, PM me.
Also, information is missing because it's not done.  :)
I'm still working on this part and I don't have it anywhere near perfect yet...
 

 
Anatomy and Function      History, Food and Medicine      Common Problems     Genetics and Standards
 

 
Genetics and Standards
 
Chicken combs come in an incredible range of colors, shapes and sizes.  Where did all these different shapes come from, and how were they developed?

To understand the origins of different comb types, its important to look at genetics.  There are two terms, dominant and recessive, that are used when talking about genes for a particular characteristic.
 
dominant

A genetic trait is considered dominant if it is expressed in a creature that has only one copy of that gene.  (Only one copy of that gene is necessary for that trait to appear in the living creature).  Dominant genes are represented by capital letters in genetics discussions.

"Expressed" means that the creature displays that characteristic.

   
recessive

A genetic trait is considered recessive if it is expressed only when two copies of the gene are present.  (Two copies of that gene are necessary for that trait to appear in a living creature).  Recessive genes are represented by lowercase letters in genetics discussions.

 
The diversity of chicken combs in the world today can be traced back to only two types of genes, pea comb genes and rose comb genes.
 
These genes are located on two different chromosomes, one chromosome for the pea comb genes and one for the rose comb ones.  Each chromosome can hold two genes.  Therefore, every chicken has two genes expressing the appearance or lack of a pea comb, and two genes expressing the appearance or lack of a rose comb, for a total of four genes related to comb type.
 
The pea comb genes and the rose comb genes can either be dominant (P or R) or recessive (p or r). 
 
So, how do you know which genes a chicken probably has?
 If a chicken has a pea comb, then it has at least one copy of the dominant gene for the pea comb (P) in its genome.  Since a bird can have two pea comb genes in its genome, it can have two dominant genes (P, P) or one dominant and one recessive gene (P, p) and still have a pea comb.

If a bird doesn't have a pea comb, it most likely lacks the dominant gene for pea comb.  Instead, it will have two recessive genes for pea comb (p, p).
 
Possible Combinations
for the Pea Comb Gene
(P, P)
(P, p)
(p, p)
Result
Dominant for pea comb
(pea comb possible)

 Dominant for pea comb
(pea comb possible)

 Recessive for pea comb
(rose comb or ?)
 
The rose comb genes work in the same way.  If a chicken has a rose comb, then it has at least one copy of the dominant gene for rose comb (R) in its genome.  The combinations of genes that will produce a rose comb are (R, R) and (R, r).
 
If a bird doesn't have a rose comb, it most likely lacks the dominant gene for rose comb.  Instead, it will have two recessive genes for pea comb (r, r). 
 
Possible Combinations
for the Rose Comb Gene

(R, R)
(R, r)
 (r, r)
 Result  Dominant for rose comb
(rose comb possible)
 Dominant for rose comb
(rose comb possible)
Recessive for rose comb
(pea comb or ?)

 
Since chickens carry both pea comb and rose comb genes, there are nine possible combinations of the pea comb and rose comb gene pairs. 
 
(P, P) (R, R)
    (P, p) (R, R)
    (p, p) (R, R)
(P, P) (R, r)
    (P, p) (R, r)
    (p, p) (R, r)
(P, P) (r, r)
    (P, p) (r, r)
    (p, p) (r, r)
 
If you have a bird with a pea comb, it will either have the combination (P, P) (r, r) or (P, p) (r, r).  A rose combed bird will either have the combination (p, p) (R, R) or (p, p) (R, r).  The other five combinations will have different results.


Wait -- different results?  I see that a chicken could have more than one dominant gene or more than one recessive gene in its genome!  What happens then?
The way these gene pairs interact and express themselves in the living bird depends on the combination of dominant and recessive pea and rose comb genes that a particular chicken has.


In everyday language, the way dominant genes and recessive genes interact is kind of like a game of rock, paper, scissors.  Dominant genes (like P or R) will always beat recessive genes (like p or r) in a match-up for a particular characteristic.  That's why, for example, the combinations (P, P) (r, r) or (P, p) (r, r) will produce a bird with a pea comb. The dominant gene P "wins."
 
However, if there's a tie -- two different dominant genes on different chromosomes (P vs. R) or two different recessive pairs (p, p vs. r, r) going head-to-head in a match-up -- weird and unusual results will occur!

"Weird and unusual"?  Is that where the different types of combs come from?  I was just about to ask about single combs...
That's exactly right.  Both single and walnut combs result from the interaction of a "tie" in dominant or recessive genes.  Here are the pair combinations that produce each type:
 
 Comb Type
Formed because of...
Chromosome pairs that will produce this type:
Walnut
...interaction between at least one dominant pea comb gene and one dominant rose comb gene
(P, P) (R, R)     (P, P) (R, r)
(P, p) (R, R)     (P, p) (R, r)
Single
...interaction between two recessive pea comb genes and two recessive rose comb genes.
(p, p) (r, r) only

What happens when two parent birds are bred together?

A chicken will inherit one gene from each chromosome from its father and one from its mother.  Because a chicken will receive two pea comb genes and two rose comb genes, each chicken will hatch with four genes that work together to determine what comb type the bird will have.

How can I predict the chance of a certain type of comb when I'm breeding my birds?
Some knowledge of the genetic background of your birds is necessary to make a 100% accurate prediction of probability.  Since the majority of us don't have access to genetic testing, we have to make a guess about what kind of genes each parent bird we want to breed is carrying.

A single-combed bird is the only bird that you can be 100% sure about without a genetic test.  This is because the only genetic combination that will produce a single comb is (p, p) (r, r).  If a bird has a single comb, it's carrying (p, p) (r, r) for sure.

The other comb types have multiple genetic combinations that will cause the same result to be expressed.  A bird with a pea, rose, or walnut comb could be carrying any one of those combinations!  Here is a chart that shows the four basic comb types and the possible gene combinations for each:

Comb Type
Possible Gene Combinations
 Pea (P, P) (r, r) or (P, p) (r, r)
 Rose (p, p) (R, R) or (p, p) (R, r)
 Walnut (P, P) (R, R)     (P, P) (R, r)
(P, p) (R, R)     (P, p) (R, r)
 Single (p, p) (r, r)

Fortunately, the other three comb types breed true almost every time.  If you breed two pea comb, two rose comb, or two walnut comb birds together, you can accurately predict that you will almost always get pea comb, rose comb, or walnut comb chicks.  How is this known?  Punnett squares.
 
A Punnett square is a chart which predicts all possible gene combinations in a cross of parents whose genes are known.  It is read similarly to a multiplication table.
 
Here is a Punnett square predicting the combinations for a pea comb rooster bred with a pea comb hen.  Let's assume that the rooster carries the combination (P, P) (r, r) and the hen carries the combination (P, p) (r, r).  The chicks will receive one gene on each of the two chromosomes (remember, there is a chromosome for pea comb and a chromosome for rose comb!) from each parent.
 
   P 
 P 
 r 
 r 
 P 
(P, P)
(P, P)
   
p (P, p)
(P, p)
   
r     (r, r)
(r, r)
r     (r, r)
(r, r)
 
To complicate this even further, a chick can receive ANY of the Punnett square possibilities predicted for the pea comb chromosome with ANY of the Punnett square possibilities predicted for the rose comb chromosome.  (In fact, there are sixteen different ways that the chromosomes can be matched together).  You can tell what will happen without figuring out all sixteen possibilities, however.  By examining the Punnett square, you can see that, no matter how the chromosomes match up, a chick will always receive at least one dominant gene (P) for pea comb and two recessive genes (r, r) for rose comb.  Any chick that hatches from this breeding is guaranteed to have a pea comb.
 
In case you're curious, the sixteen possible combinations from that example are:
1. (P, P) (r, r)

2. (P, P) (r, r) 

3. (P, P) (r, r)
4. (P, P) (r, r)
5. (P, P) (r, r)
6. (P, P) (r, r)
7. (P, P) (r, r)
8. (P, P) (r, r)
9. (P, p) (r, r) 10. (P, p) (r, r) 11. (P, p) (r, r) 12. (P, p) (r, r)
13. (P, p) (r, r) 14. (P, p) (r, r) 15. (P, p) (r, r) 16. (P, p) (r, r)    
 
 
What if each pea comb bird I want to breed carries a recessive gene for pea comb?
If the two pea comb birds you're breeding both have recessive genes for pea comb (P, p) (r, r), then there is a 25% slight chance their chicks will have a single comb.  Here is the Punnett square for that situation.
 
    P  
  p  
  r  
  r  
  P  
(P, P)
(P, p)
   
p
(P, p)
(p, p)
   
r     (r, r)
(r, r)
r     (r, r)
(r, r)
 
If you were to figure out all sixteen possible chromosome pairs for this breeding, you would find that four out of the sixteen possibilities, or 25%, are the match of (p, p) with (r, r).  (p, p) (r, r) expresses a single comb.

***These same techniques will work for figuring probability for birds with any of the four basic comb types (pea, rose, walnut or single).  You may have to complete several Punnett squares just to get a complete picture of the possibilities for two birds if you have no idea what kind of genes they carry for the trait they are expressing.***
 
I have pea comb birds from a (breeder/hatchery) that I want to breed!  Will some of their chicks end up with single combs?
Although it is theoretically true that some birds with pea combs can carry the recessive gene for pea comb, it is highly unlikely that birds you buy from a breeder or hatchery will have it.  This is due to selective breeding for specific traits that has taken place over the years to produce the birds closest to standard.  (It is generally agreed that birds from a breeder are closer to standard than hatchery birds).  The birds that were kept in a breeding program are likely to have dominant genes for pea comb since a breeder would probably only continue to mate birds that produced chicks that were true to the standard.

But I'd really love to have a single-combed Sebright!  How can I make one?
For this specific situation, you would need to introduce recessive genes for both pea comb and rose comb into the gene pool.  The best way to do this is to mate another breed of bird with an excellent single comb to the Sebright.  Let's pretend that the Sebright has the chromosomes (P, P) (r, r).  The single-combed bird would have the combination (p, p) (r, r).

If you did the Punnett square for this breeding, you would see that every bird from this breeding would end up with the chromosomes (P, p) (r, r).  Of course, that combination still expresses a pea comb.  However, the important thing is that recessive "p" and "r" genes are now present.

From this (P, p) (r, r) generation, you should choose a hen and rooster to breed that exhibit the best feather trait for Sebright.  From their mating, you would then have a 25% possibility of getting a bird that looks very much like a Sebright with a single comb.
 
 ***Send The Chicken Lady a PM via the BYC Forums
if you have additional comb genetics questions. 
I will do the research and add your question here if I can.***
 

For more information and further reading...

Hutt, F. B.  Genetics of the Fowl:  The Classic Guide to Chicken Genetics and Poultry Breeding.  Norton Creek Press, 2003.
 
Somes, Jr., R.G.  "Duplex Comb in the Chicken:  A Multi-Allelic Trait."  Journal of Heredity.  1991.  82:  174-176.
 
 

 
 

 

 

 




 
 
 

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