Taeniopygia guttata internationalis

Semi-Lethal Dominant Dilutes

Reprinted with permission from the Zebra Times, October 1995

Double Factored Dilute Blues don't exist!

The mutations called Dilute Blue and Silver in this country have been around for about fifty years, yet their genetics are still misunderstood. Early in their development the mutation spread overseas where a lot has been written about the so-called Dominant Silvers. The desired bird there has been the 'Silver,' a light silvery grey body with white cheek patches. These birds fit our Silver show standard. The Dilute Blue is not recognised overseas, but is darker overall with orange tinged cream cheek patches, and has been a popular show bird in Australia for the past twenty years. I've grouped both these mutations together in this article because anyone who has ever bred these birds knows that nearly every possible shade between the Normal Grey and White is possible from Dilutes, and there is no defining borderline between these two mutations. Therefore they cannot be distinguished for the purposes of a genetic study. Birds that fit the correct Silver standard are extremely rare and it could be argued that the true Silver is a different bird from the Dilute Blues I've bred with, but until new information becomes available, I'll treat them as one bird.

It should be noted before continuing that overseas there is a Recessive Dilute mutation described as being darker than the dominant mutation. Photographs of this bird are rare, but it is not recognised in this country and I have never seen any breeding results indicating its existence. But if no one knows how to recognise it, then it could be floating about amongst all the variations of the Dilute Blue. Some people have suggested that this mutation may be similar to our Normal Blue (Slate). Maybe, but the standard in the U.K. calls for a slightly diluted cheek patch as opposed to our birds' normal cheek patch. This suggests the mutations are different.

A lot of problems in Australia have developed in our understanding of Zebra Finch genetics because all books to date have been written overseas and therefore deal with their birds not ours! For many years the tendency of breeders in this country was to try to fit our birds into their information. Finally over the last decade or so, with the formation of Zebra Finch Societies across the country and the more recent Federation of Zebra Finch Societies, we are finally giving ourselves some credit and paying less attention to the overseas people.

The accepted method of inheritance for the Dilute Blue has therefore been Dominant with both single and double factored birds indistinguishable. This belief is FALSE ! There are no double factored birds because the mutation is LETHAL in the double factored state. These chicks never form or hatch and probably fail to develop because the gene for the mutation destroys an important internal mechanism when in the double factored state.

How can this be proven? If double factored birds cannot be distinguished, how can I make this categorical statement? The answer lies in the use of statistical analysis of breeding results involving sufficient young bred from Dilute Blues. I was originally awakened to the truth when given a copy of the breeding results of a Queensland breeder (Mr. R. Robinson). Ray had collected results over many years and thought they may be useful to my research. His records provided results for about one hundred and eighty offspring from over twenty matings of birds. These immediately showed a trend I had noted within my own birds - that far too many Normal (Non-Dilute) birds are bred from these matings. Using a statistical test called a Chi-squared test, I have shown that this trend is true. I have now accumulated breeding results for Dilute Blues involving over four hundred and filly youngsters from nearly sixty matings. This is sufficient to produce a significant result. Within genetic terminology, normal means non-mutant with regards the mutation being investigated; i.e. not just Greys are normals but also Fawns, Pieds and Yellow-Bills; but Creams are still a dilute mutant carrying the Dilute Blue gene.

These results are outlined below in a summarised form.

Dilute Blue X Dilute Blue matings

If double factored birds exist then the expected breeding results would be:

This means three Dilutes for every Normal bird. If the double factored birds die then this would result in only two Dilutes for every Normal bird.

The third column is the Chi-square test value called X². This value should be as close to Zero as possible, and if it's above 3.84 it's significant. This means the hypothesis is wrong and you need to start again. It is therefore clear that the first option that double factored birds exist, is clearly disproved with a X² value of 19.33. In fact the second option also has a high X² value as well. This is because of the high numbers of Normal birds and could indicate that even some of the single factored birds fail to develop. If we assume that one in four single factored young die then the expected young would be 101.4 Dilutes and 67.6 Normals - giving a X² value of 0.00887! Could be something in this. To investigate further, consider another mating:

Dilute Blue X Normal matings

 If 25% of Dilutes die then the ratio would be 43% Dilutes, and 57% Normals.

It can be seen from this that while far from significant, the results support the theory that some Single Factored Dilute Blues fail to develop beyond the embryo stage. Doesn't this ring a bell, with breeders often remarking about how many Greys are produced from Dilutes?

If double factored birds occurred then the ratio would be pushed the other way, increasing the number of Dilutes produced, as a double factored bird could not produce any Greys. Over thirty matings make up these results so by random chance some would have to be double factored if they occurred. Since the ratio goes the other way then this can be used as further proof against the existence of double factored birds.

What does all this mean? Firstly whilst mating Dilute Blue to Dilute Blue will produce more Dilutes, you will not get as many as expected. In fact you nearly get as many from mating Dilutes to Normals, and if good quality Greys are used then this would have to be the best way to improve Dilutes. But what about colour? Obviously Greys are not going to affect the colour of the Dilute youngsters, this being inherited from the Dilute parent. So why not mate Dilutes together to change the shade? But I've just shown that a youngster only inherits dilute genes from one parent otherwise it dies, therefore the youngsters only inherit what one parent is or the other. The confusing breeding results occur here because hens do not carry all the cock bird markings and therefore it is impossible to determine what they will throw. 

This raises the question - what causes all the variations we see in Dilute Blues, and how do we know if one strain of Dilute is the same as another? You cannot prove that one dominant strain is identical to another with any ease. Recessives can be test mated together, and if they are different, Normals split for both will be produced. But with dominants, one may overpower the other or both may be expressed without alteration to the appearance of the young. This means that maybe not all Dilute Blues in this country follow those of Ray's and mine (two unrelated flocks) but to date all breeding results published in club magazines have yielded the same results when analysed in this way.

So how do I account for the observed variation in Dilute Blues? I have evidence to suggest there are 'modifying genes' inherited separately (on another chromosome) to the Dilute Blue gene. These genes would only express when combined with the Dilute Blue gene in the one bird. Otherwise a Grey bird could carry these without being apparent. It's therefore possible in a mixed breeding flock that hidden genes are floating about to interfere when least expected and cause strange results. What evidence? Dilute Blues can be used to change Marked Whites into Chestnut Flankeds, yet these birds do not normally carry the Dilute Blue gene. This theory would require years of selective matings to investigate and would be better described more fully in another article.

Finally although I don't have the same number of breeding results, the inheritance of the Black Face/Black Bodied mutation seems to follow the same pattern. That is - no double factored birds, and a modifier to increase the amount of black on the bird. No wonder people have so much trouble breeding Black Bodied Silvers - combining two semi-lethal dominant mutations eliminates a large percentage of the desired results! - Another research area and another article.

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