Introduction to the site
This site reports on those of the writer's breeding programmes which have generated fresh information about the genetic mechanisms involved in breeding white canaries . Two articles are listed -
- The dominant-white mutation and its supposed lethal effect.
- The recessive-white mutation and its likely epistatic character.
Background reading
http://petcraft.com AVIAN GENETICS
SCROLL to access to the two articles
The dominant-white mutation and lethality
Introduction
Most books dealing with canary breeding claim that the double-factored dominant-white bird does not exist because, in combination, the dominant alles have a lethal effect. There is no clear evidence to support this widely held belief, and it is worth noting that the eminent author, Geoff Walker, in his manual 'Colour, type and song canaries', goes on to say of the lethality notion – 'one is left to conject whether there is any truth in the statement or whether it is another example of the folklore which persists throughout the fancy and which has no scientific basis'
Breeding programme
This article descibes a breeding programme which supplies hard evidence that the dominant-white mutation is NOT in fact lethal and that the double-factor bird does indeed exist. The procedure used was to self-cross a goodly number of dominant-whites and then, the following season, to check the progeny for the presence of double-factors by pairing all of the whites with yellows. If a white bird parented a yellow chick it meant, without any doubt, that the white parent was single-factored, and could therefore be dismissed from further consideration. On the other hand, if the white bird parented only successive white youngsters the probability of its being double-factored would increase with every white chick hatched. This procedure does not of course prove absolutely that a bird is double-factored but it has the potential to deliver a persuasive probability of it being so.
Analysis of results
The first self-crossed whites produced 12 youngsters and these were crossed with yellows the following year. Of the twelve, nine parented yellow chicks and were set to one side, and three parented 11, 8, and 6 white youngsters consecutively. These three parents were of considerable interest because their performance tended to contradict the genotype usually ascribed to them -
[DOM /yellow] x [DOM / yellow] = 2 [DOM / yellow] +2 [yellow / yellow]
That is to say, that half the youngsters should be yellows.
If we can work out the probability of consecutive white youngsters happening by chance in that scenario, we can then work out the probability of the only alternative scenario, which is where the parent is double-factored. -
[DOM / DOM] x [yellow / yellow] = 4 [DOM / yellow] always a white phenotype
It is plain that with a single-factored bird the odds in favour of parenting a white chick are 50 : 50, or 1/2 ( if certainty = 1), the same as they are for showing a 'head' when coin tossing. The odds in favour of tossing a series of consecutive 'heads' decrease with the number of tosses made and, similarly, the odds in favour of producing consecutive white chicks from a single-factor white parent decrease also.
Maths people tell us that the probability of tossing two heads in a row is the probability of showing a head in the first toss multiplied by the probability of showing the same again with the next toss. Further probabilities are set out below -
Number of Probability of % Probability of
consecutive coin being fair coin being unfair
heads ( head on one side only) ( head on both sides)
1 1/2 = 0.50 {1- 0.5}x100 = 50 %
2 1/2 x 1/2 = 0.25 {1-0.25}x100 = 75 %
3 1/2 x 1/2 x 1/2 = 0.125 {1-0.125}x100 = 87.5 %
We can see that with an increasing number of successive 'heads' our confidence increases that the coin is unfair ie double-headed.
Now continue the table with experimental results, ie successive white youngsters instead of successive 'heads'.
Number of white Probability of bird % Probability of bird
canaries in a row being single-factored being double-factored
6 1/2^6 = 0.0156 {1 - (1/2^6)} x 100 = 99.44 %
8 1/2^8 = 0.0039 {1 - (1/2^8)} x 100 = 99.61 %
11 1 /2^11 = 0.00049 {1-(1/2^11)} x 100 = 99.95 %
We can see that, as with the coin, an increasing number of successive white youngsters increases our confidence that the parent is double-factored.
The confidence level never reaches 100 % but it gets as close to it as makes no difference; in one of our cases it's 99.95 %. Imagine if a player in the local pitch-and-toss school turned up 11 'heads' in a row, would anyone believe the coin wasn't double-headed ?
Conclusions
- The evidence provided by the parent of the eleven consecutive white chicks, supported by the 6 and 8 from the other two parents, is a compelling argument in favour of the double-factored bird's existence.
- There is, it must be said, no great advantage to be found in keeping double-factors. They have nothing to offer the show breeder over and above the single-factor, and maintaining a line of double-factors would probably involve the average breeder in an unacceptable level of in-breeding.
- However, the breeding exercise described above does throw some light on canary genetics, and it demonstrates once again that the canary fancy is riddled with old wives' tales and notions unsupported by evidence.
In the writer's own case the exercise was satisfying in that it established the double-factor birds as a fact, and this turned out to be a very useful piece of information during an investigation of the recessive-white mutation - as is described in the next article..
THE RECESSIVE WHITE MUTATION
and its epistatic effect.
Background
Here we have a white bird with a vitamin A deficiency and a complete absence of yellow pigmentation. These characteristics also occur in some domestic fowl and have been investigated by veterinarians. Evidence has emerged showing that a genetic mutation of a hitherto unknown gene leads to an inability to absorb food-borne carotenes through the gut wall. The carotenes are known to be precursors of both vitamin A and the yellow lipochrome pigment, xanthophyl.
The consequences of having this gene in its double-factor mutated form are, apart from a vitamin deficiency, that there will be no xanthophyll available for the yellow-pattern gene to work on, and an epistatic effect is therefore introduced immediately.
NOTE: Where the expression of one particular gene disturbs or denies the expression of a second unrelated one, the feature is described as an epistatic effect.
It is argued here that to explain the outcome of a dominant-white / recessive-white cross epistasis must apply.
Allelic configurations
The labelling of the gene's alleles, and the expression of the three possible allelic configurations, are set out below.
Simple labels are here applied to the alleles using 'NORMAL' to suggest normal processing of vitamin A and xanthophyll precursors at the gut wall, and 'epistat' for the abnormal mutated blocking condition. This mutated form, being recessive with respect to the 'NORMAL' form, requires both alleles (double-factor) to be present before the effect of the mutation expresses itself through a starving of the bird of vitamin A and xanthophyll. The three possible allelic configurations are set out below.
[NORMAL / NORMAL] This is the normal gene with unmutated alleles.
Proper gut absoption of carotenes occurs,
Vitamin A synthesis is facilitated.
Yellow lipochrome pigment is generated
Yellow patterning as appropriate.
[NORMAL / epistat] The gene carries (split for) the mutated allele. The gene's expression is as above.
[epistat / epistat] The gene has both alleles mutated.
Gut absorption of carotenes fails.
No Vitamin A is generated
No yellow lipochrome pigment produced.
All plumage completely white
To test the idea that the above gene actually impacts on the expression of the other yellow pigmenting gene, a well-recognised technique used by geneticists to test for just such a state of affairs can be applied. Many examples of the technique in action, with diagrams, are to be found on-line by googling 'dihybrid cross epistasis images' . The technique calls for a bird with the TWO relevant genes split for the recessive allele, as follows :-
[DOM-WHITE /yellow)] // [NORMAL / epistat]
and then pairing it with another bird of exactly the same genotype.
When the writer applied the technique, the breeding outcome was found to be a good match for the one which might be expected from a two-gene model which includes epistatic effect.
Theory
The Punnet square on the left illustrates the genotypic and phenotypic outcome from a dihybrid cross.
With sixteen progeny, the ratio of phenotypes is-
9 : 3 : 4
::
dominant-white : yellow : recessive-white
EXPERIMENT.
This writer set up a series of suitable dihybrid crosses and the breeding programme produced 35 youngsters whose appearance was as follows:
dominant-white yellow recessive-white Total
phenotype phenotype phenotype Progeny
21 6 8 35 actual result
When the 9:3:4/16 base is adjusted to a base of 35 (ie 16* 2.2) the ratio reads
19.6 6.6 8.8 35.2 result in theory
It will be seen that the match between the actual result and the theoretical one is very close.
CONCLUSION
Admittedly, the sample size is on the small side, nevertheless the near parity of the actual result with the theoretical one argues that the model used is highly plausible, and suggests that -
- Epistasis is central to explaining the complete lack of colour in the recessive-white bird.
- No breeder carrying out a dominant-white x recessive-white cross should be surprised to find any or all three of the possible phenotypes in the nest.
- The cross is not a promising one in that it is likely to generate a mish-mash of 'splits' unreadable by the breeder.
Those breeders who do not accept that the double-factor dominant-white bird exists may still use the Punnet square to predict the outcome of a dominant-white x recessive-white cross, as set out below, provided they allow the epistatic effect model to apply and that neither bird be split for the feature.
In the square the the alleles of each bird are represented by single letters e.g. -
Dominant-white = D, Yellow = y, Normal = N, Epistat = e, in black type in one case, in red in the other, ie
First parent, Dominant-white [D / y] / [N / N] and
Second parent Recessive-white [y / y] / [e / e]
Yield : 8 Dominant-white : 8 Yellow ground
It will be seen that, of the 16 progeny, 8 are phenotypically dominant-white, and 8 are phenotypically yellow. All birds are split for the recessive-white factor. A number of other allelic combinations are possibe, but the one used above will be the more common one.
