Yes, it is more common in inbred birds; I didn't want to flog this after babbling on so much about inbreeding in the other thread, but there it is.
In the simplest case of recessives, if you mate two birds who carry the same recessive allele (and the more closely related two birds are, the more likely they are to carry the same allele), then approximately 1/4 of the offspring will display the trait, even though both parents appear normal. If other genes mediate expression, the numbers can be more variable and complicated, but the tendency will still be the same.
This also brings uo the subject of breeder responsibility - if a breeder knows that a given mating produces a high proportion of defective birds, they should, at the VERY least, not be selling hatching eggs from that mating! Some would say that both parents should be culled, as they clearly cary the defective allele.
There are two ways of dealing with defective alleles -
1) work very hard at exposing them, through back-crossing, test mating, etc., and then removing (culling) all birds that are known to carry that allele (which will mean getting rid of a lot of apparently healthy birds, simply because they carry one copy of a recessive but deleterious allele), or
2) outcross to a relatively unrelated bird.
Last edited by ipf on Sun Jul 10, 2011 9:27 am; edited 2 times in total (Reason for editing : add info)
Wheee! 