The crested is one of the most unusual budgerigar mutations. This phenomenon is the only structural trait that is controlled by a few readily identifiable factors. Unfortunately, most fanciers do not understand the genetics of this variety. There is no reason for this lack of knowledge. The genetics of crest breeding are no more mysterious than is that of breeding violets. Again, we are dealing with complimentary genes.
The crested budgerigar is available in three phases:full crest, one-half crest, and tufted. The full crest possesses a daisy crest very similar in appearance to that of the Gloster canary. It will soon be shown that this similarity in appearance implies no similarity in the mechanisms of inheritance. The one-half crest budgerigar has, as the name leads one to believe, half the adornment of the full crest. Here the bird only has the crest forward toward the crown. This leaves the viewer with the impression that the bird has had its feathers done up in bangs! The tufted is the most striking form of the crest. This budgerigar has the reverse of the cockatiel crest. The best examples show a strong “horn” of feathers rising from the crown. Poor specimens show only a few disturbed feathers.
The three crest phenotypes are manifestations of the interactions of two genes:crest-determining, D, and crest-initiating, I. The corresponding wild, normal, gene is designated by the plus symbol, +.
A bird with one crest-determining gene and one crest-initiating gene will be tufted. The notation for the genotype of the tufted is I/+ D/+. A bird that has one crest-initiating factor and two crest-determining factors is a one-half crested. The notation for the genotype of the one-half crested is I/+ D/D. A bird that has two crest-initiating genes andeither one or two crest-determining genes is a full crest. The notation for the full crest is either I/I D/+ or I/I D/D.
Table one gives a full listing of all the possible crest genotypes and the manifested phenotype. If the crest-determining and crest-initiating genes are found to be independent, many of these permutations will be found to be equivalent. If they are linked, the order of the genes is important.
It should be noted that crest-bred birds may posses very different genotypes. This term is meant only as a category of convenience for it does not denote any particular genotype. The exact genotype of these birds of normal appearance may often be ascertained through careful records. Otherwise, test mating is necessary.
The crossing a two factor, full crested bird with a normal will lead to the following result:
I D X + + I D + + YIELDS 100% I D TUFTED + +
This formula represents that the two factor full crest produces only the gametes I and D. The normal bird, of course, only produces the gametes + and +. The pairing of a two factor, full crested bird to a one-half crest bird gives the following yield:
I D X + D I D I D 50% I D FULL-CREST 50% I D HALF-CREST I D + D
Crossing a one factor full crest with the crest-bred bird from entry number four in table one gives us the following result:
I + X I + I D I + 50% I + ONE FACTOR FULL-CREST 50% + + CREST-BRED I D I +
These results are according to the Initiator theory of crest inheritance as posited by Dr. J.E. Fox. There is strong evidence that the crest-determining and crest initiating gene are linked, located on the same chromosome. Further experiments are in order to validate this hypothesis.
The continued pairing of crest to crest is not to be suggested. It must be stressed that there are no lethal factors involved in budgerigar crest inheritance. The young from double crest pairings will generally lack type. The crossing of a crest bird to a high quality normal will lead to an initial advance in type. Continued crossings to the normal exhibition bird brings about a degeneration of the crest, possible due to an accumulation of multiple allele inhibitors. The best method is a judicious crest to normal pairing, followed by a series of crest to crest-bred matings.
I hope that this short article will show that crests are produced in exactly the same manner as various colors. All pairings involving crested birds may be analyzed through fairly simple tables.
The Crested Budgerigar Club Handbook,ed. Fullilove