Expert: Dana Krempels, Ph.D. - 12/7/2008

Question
I know you can get single Bankiva Type combs when breeding peacombed oriental type fowl if there is single comb bankiva type genes already present in the bloodline ,My question is why cant you get the pea comb type from a mating of single comb parents with the peacomb type in their bloodlines???

Answer
Dear James,

The wild type, single comb in jungle fowl (aka, chickens) is encoded by a recessive allele, and so its expression is masked if a dominant version of the allele is present.

Let me back up a bit, and give a wee genetics primer.  You can skip this part, if you already know it.  :)

All animals have two copies of the genome--one from each parent.  The genome is the complement of all the genes encoding the proteins that produce physical and other genetic characteristics.  This means that every animal has two copies of every gene.  In most cases, there can be more than one "version" of any given gene, and these different versions encode different traits.

Thus, the  versions of each gene (called *alleles*) that an animal gets from each parent may be the same in a single individual, or they may be different.  For example, if there's a gene we'll call X, and the father's alleles are XX and the mother's are xx, then the offspring will get one from each parent, and have the genotype Xx for that particular green.

Note that I've used capital and lower case letters to represent the different alleles of the same gene.  This is because some alleles are *dominant* to others, which are said to be *recessive.*  A dominant allele will mask the expression of a recessive allele.  In the paragraph above, X would be the dominant version of the gene, and x would be recessive.  Let's say that X codes for curly hair, and x for straight hair.  That means a person with the genotype XX or Xx will have curly hair (the dominant condition).  The only way a person could have straight hair would be to inherit two copies of the recessive gene, and have the genotype xx (with respect to the hair texture gene).

How does this affect the physical appearance of comb morphology in chickens?  There is are two genes, each with two alleles, that encodes the shape of a rooster's comb.  These two genes are abbreviated "P" (for pea comb) and R (for rose comb).  

If a rooster has the genotype PP or Pp, he will have a pea comb.  If he has the genotype pp, he will have a wild type comb.

If a rooster has the genotype RR or Rr, he will have a rose comb.  If he has the genotype rr, he will have a wild type comb.

If you breed two chickens of genotype Pp (a cross of Pp x Pp), then--on average--25% of the offspring will be PP (pea comb), 50% will be Pp (pea comb), and 25% will be pp (wild type, single comb).  This means that no matter how often you breed two pp individuals together, they cannot produce any babies with pea combs.  All their offspring will have the genotype pp, even if the grandparents had pea combs (and a copy of the dominant allele).  

As an interesting sidelight, the P and R genes give an interesting result when both are inherited in dominant condition.  A rooster with genotyp e RRPP, RRPp, RrPP or RrPp will have a "walnut" comb because of the interaction of both genes.  But a rooster with genotype rrpp will always have a single comb, and will produce sperm carrying only the recessive alleles.  To get the pea or rose type combs to show up again, he must be bred to a hen carrying those dominant alleles.

Hope that helps!  :)

Dana