UNDERSTANDING DOMINANT INHERITANCE
by Michael Stuart Christian
Dominant inheritance is undoubtedly, the mode of variety transmission most readily understood. After the first few experiences with breeding birds, enthusiasts soon begin to appreciate the intricacies of mendelian Genetics by experience and observation. However, there are a number of misunderstandings, concerning dominant inheritance, and these require to be addressed with appropriate elaboration to explain both the nature of the ambiguities and the cause of the various phenomena. The information contained herein is applicable to any species where the particular characteristic in question is governed by Mendelian Inheritance.
Dominance in Brief
Enthusiasts recently entering into the hobby of bird breeding may not be fully conversant with the basics of dominance, therefore, some value will be gained by a simple explanation of the "Law of Dominance" before we proceed further. When an individual bird carries a single gene for a dominant characteristic, 50% of its offspring will receive that gene, and it will be absent in the other 50%.
Each dominant characteristic is completely independent of another, the gene for gray has no effect on a gene for green, nor will the gene for green have an effect on a gene for pied. The genes affect the expression, not the cause of the expression. In Budgerigars, the dominant expressions are:
Dominant pied, Dutch dominant Pied, dark factor, violet, spangle, Australian yellow-face, and English yellow face.
Result of a Dominant Gene
Dominance is the consequence of a gene action where a given gene overrides the instruction on its allele and, consequently, is expressed.
Genes, Loci and Chromosomes
As with recessive, genes for dominant expression are carried on chromosomes and occupy loci. Chromosomes, or more appropriately, "autosomes", come in homologous pairs, and each gene has an allele occupying an identical locus on the opposite chromosome. Some dominant expressions may occupy the same chromosome, whereas others will occupy completely different chromosomes.
Among budgerigar breeders, it has only been in recent years that the hobby has formally recognised that many of the expressions perceived as "dominant" are, in fact, incompletely dominant. Any variant on an expression resulting from the individual being either heterozygous or homozygous for a character means the expression is incompletely dominant. For example, the dark factor in budgerigars, the heterozygous expression (dark green and cobalt) is significantly different from the homozygous expression (olive and mauve).
True dominance is where the expression, regardless of whether the bird is heterozygous or homozygous is constant. Among budgerigars, with Australian dominant gray, there is no observable distinction in the phenotype occurring from the two genotypes.
Among budgerigars, as among the breeders of some other species of birds, "single factor" relates to the genotype rather than the phenotype. It means heterozygous for a character (e.g., SF dominant pied).
As with single factor, double factor relates to the genotype and means homozygous for a particular expression.
There are two distinct dominant pieds: the Australian and the Dutch. Both can be had in heterozygous and homozygous versions. I am not experienced with the Dutch version, but I have been led to believe by breeders who are that these are completely dominant, the expression being consistent regardless of the relevant genotype. Of the Australian dominant pied expression, to distinguish between the two genotypes visually can be next to impossible. Homozygous is theoretically far more variegated than heterozygous, but this geno-type, too, can be quite heavily variegated.
Some breeders and writers claim that yellowface is also recessive, typically with a rider asserting that the expression is recessive to green, but dominant to blue. This is most definitely not the case; yellowface, whether the Australian "Golden face" or the English "lemon face," is dominant. These two expressions are quite different genetically, but they are, nonetheless, both dominant.
Misinterpretation of Data
I believe the claim of recessiveness with these varieties (for there are two) is based on the observation that when yellowface (regardless of version) is bred to green, all the resulting green offspring are visually non-yellowface. Where the yellowface is bred to blue, the offspring carrying yellowface, of course, express it. This may sound a little peculiar, but with green birds carrying yellowface, the yellowface is still expressed in the genetic sense, but visually the expression is concealed by the green. Nonetheless, it is still there and in no way becomes recessive. It could be said that green is masking the yellowface. Blue is the result of a recessive gene, and as already stated, genes do not affect each other, but they may affect one another's expressions. Blue is expressed as a recessive character, yellowface is expressed as a dominant character, and when both these characters are on the same individual, the bird is, in fact, a blue-yellowface composite! To illustrate the discussion, I have included the two following examples:
green x yellowface=
50% green yellowface (can't be seen)
50% green (non-yellowface)
blue x yellowface=
50% blue yellowface (visual yellowface)
50% blue (non-yellowface)
This is of course, dominant transmission, if the expression were recessive, the result would be as shown below. But it isn't. The following example is purely hypothetical, yellowface is inherited as above:
green x yellowface = 100% green/yellowface
blue x yellowface = 100% blue/yellowface
In the not so recent past, I heard two breeders discussing gray greens. Such an event would not be of any particular note if it were not for the insistence of one that "all gray greens are split blue! They have to be gray is blue, and if a green is gray, it must be split for blue!" This is most certainly not the case. The fact that a bird is carrying a dominant gene for a factor that to us appears gray is not testimony that it is likewise carrying a recessive gene to facilitate blue. The dominant gene for "gray" has absolutely nothing whatsoever to do with blue. This is purely a matter of how we speak about things. In this regard, it is unfortunate that we have chosen to call that mustard or steel-colored expression gray, for it is in naming it such that newer enthusiasts perceive the expression to be blue. Such confusion would, undoubtedly, have been substantially reduced if the hobby had elected to call the expression gray mustard or possibly even better steel. This would adequately distinguish the gray expression as something innately unique from blue. I did initially prefer the term "mustard," as "mustard green" was quite appropriate, but I felt unhappy with it as "mustard blue," since it had an unreal sound to it--whereas "steel blue" and "steel green" appear to be comfortable terms. Note: Gray is not an ideal expression to be used with harlequin pied and clearwing because of its dulling effect. Likewise, it may not be desirable in fallow either.
Autosomal: Any chromosome occurring in pairs in diploid cells that are not "sex (gender determining) chromosomes."
Cross-over: A phenomenon where sections of a chromosome cross one another and exchange portions and, in consequence, genes. The more closely genes are linked, the greater the chances of them being passed together.
Diploid Cell: Any somatic cell in which each chromosome is represented twice bar for the "Y" sex chromosome.
Gametes: Sperm or ova.
Genotypes: The genetic constitution of an individual as determined by the genes it carries.
Heterozygous: An individual in whom the members of a pair of genes determining a characteristic are dissimilar.
Homologous: A Pair of chromosomes of similar shape and size with identical gene loci.
Homozygous: An individual in whom members of a pair of genes determining particular characteristics are identical.
Linkage: The phenomenon where the loci for different characteristics are close together on the same chromosomes---the closer they are, the greater the degree of linkage.
Phenotype: Visual characteristics of an individual determined by genes it possesses and the environment.
Somatic: All body cells except Gametes.
More information on GENETICS can be found here.
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