its about 4 cM(centi Morgan or m.u map unit) its about 1% chance of recombination per cM, so about 96% chance of getting a blue/egger if one starts with a Pea Comb breed like Araucana or Ameraucana.The best information I have says the oocyanin gene is located about 80 centimorgans up from the allele that controls pea comb/straight comb. From work I've done with plants, a crossover occurs about 3 times per 1000 when the genetic distance is 80 centimorgans. I can't say that this is applicable to chickens, but it is a good starting point. If you read up on the history of the Whiting Blues, he was able to find a crossover and develop a straight comb version. I don't think the straight comb birds have been released.
One of my goals with the DNA tests is to nail down the pea comb allele location on chromosome 1. It is a separate project from the blue egg tests. Said another way, we could use a good test that could differentiate straight comb from pea comb on chromosome 1 and could ID the rose comb inversion on chromosome 7. It should be relatively simple to set up a DNA chip to detect rose comb, pea comb, straight comb, and blue egg.
One of the aspects of pea comb that causes problems is that it is dominant over straight comb. You can still usually tell that a bird is homozygous pea comb by the size and structure of the comb. Given the low crossover rate, always breeding from pea comb birds is one way to select for blue egg layers. Unfortunately, if you want rose comb - as i do - you have to breed away from pea comb. This is because combining the rose comb inversion with pea comb results in a walnut comb, not a rose comb.
There is a third allele of the same gene that controls straight and pea comb. It causes an inverted tip on the comb looking somewhat like a short walnut comb. I have not looked up cushion comb to see if it also is an allele. This is just speculation on my part, but given the location on chromosome 1, there is a very high probability of several more mutations affecting the straight comb gene.
https://reader.elsevier.com/reader/...egion=us-east-1&originCreation=20210427001714
Once the linkage is broken, the recombinant is linked to the single comb p+ allele(P an p+ are allelic).
Breeds like the Cream Legbar and Isbars can trace their lineage to Dr. Punnett line of chilean stock, they were single comb.
Also, Pea Comb has already been maped to Chromosome 1 on the chicken genome, its been given the name SOX5
Last edited: