Dr John Pilkington answers questions on Genetics

Answer: Although I have also read of the same speculation that both Violet and Crested genes may be lethal, to be perfectly honest, I don't know the answer to this question, but I do have an opinion. The budgerigar as a species has no true commercial value, unlike poultry, cattle and sheep etc., and as such, it is extremely unlikely that such esoteric study of lethal genes in budgerigars will ever be made; even the vast sums of money that are involved in horse racing and the bloodstock industry do not promote such studies in thoroughbreds.

For me, the violet budgerigar is the most beautiful variety of all. I have a number of them and I have not noticed anything unusual in the breeding pattern that has hinted that the Violet gene may be lethal. However, it is possible that there are several different Violet gene mutations, one of which may well be lethal.

To discuss the mechanics of lethal genes is very complex, but I'll try to give you a simple analysis. There are many different classes of lethal genes, and there is a spectrum of lethality, ranging from lethal to sub-lethal to super-vital. Lethal effects may occur at any stage in development from conception through all stages of development up to adult maturity. True Lethality causes mortality before birth, whereas Sub-lethality often causes low fecundity and causes premature death.

For ease of explanation, imagine that I have just discovered an Orange mutation and it is dominant to the Green gene. If the dominant Orange gene is represented as O and the recessive Green is represented as o then there are three possible genotypes as in Diagram 1.

The Dominant Orange gene may also have lethal effects which may exhibit their lethality in a dominant or recessive form. If the O gene is lethal in the dominant form, then if it is lethal at any stage in development prior to breeding age, no adults of breeding age will ever live long enough to pass this gene to the next generation and so the O gene mutation will not be perpetuated and the mutation would not survive. However, if the lethality of the O gene occurs only after sexual maturity (sub-lethal), then the O gene can survive long enough to be passed on to the next generation, therefore both OO and Oo genotypes will be found in the species population.

If on the other hand, although the O gene has a dominant effect on colour, it may also have a recessive effect as a lethal gene, in which case, the heterozygous Oo genotype would not prove lethal at any stage in development,, and such an individual would have both normal health and normal longevity. However, if the O gene is a lethal recessive, then the OO genotype would not occur in the surviving population. If the O gene was sublethal, then some adults may survive long enough to reproduce. If a gene is lethal then the breeding results will be different to those expected. If two Orange budgerigars are paired together, then they can only be the heterozygous genotype Oo and they will never produce Homozygous Orange offspring. As such, the numbers of offspring produced will be less than average, but more important, the ratio of surviving offspring will be different to the Mendelian ratio expected.

If lethality is ignored, with reference to Diagram 2, the expected genotype ratio of the offspring is:

Table 1
Pairing	Offspring
OO	1
Oo	2
oo	1

Table 2
Phenotype	Offspring
Orange	3
Green	1

However, since the OO genotype does not hatch, the observed ratio of offspring is:

Table 3
Phenotype	Offspring
Orange	2
Green	1

The 2:1 ratio does not occur in the expectations of non-lethal genes and so it is obvious that something is going wrong - lethality is the answer.

So far, I have explained a dominant gene with a recessive lethal effect; there are of course recessive genes with both dominant and recessive lethal and sub-lethal effects. For reasons already explained, a dominant lethal effect in the earliest stages of development, would prevent the gene from surviving to birth, and so will fail to produce any living offspring, but if the recessive gene also has recessive lethal effects, then the heterozygous form will survive and the homozygous recessive will not survive. The most obvious effect of such a gene will be the reduced numbers of surviving offspring, which appears as low fertility - does this sound familiar? Say a recessive Red gene mutation has already occurred in the budgerigar population, and this gene is also a recessive lethal, no examples of the Red phenotype will ever be seen but the Red gene would be passed from generation to generation in the heterozygous form. However, if the recessive Red gene were also a recessive sub-lethal gene the homozygous Red genotype would survive and would also produce Red phenotypes which may or may not survive to reproduce.

I have great suspicion that the poor fertility seen in exhibition budgerigars these days is attributable to the prevalence of sub-lethal recessive genes. The preoccupation with size for some breeders is almost a disease or a religion; since the skeletal size of the budgerigar has a finite limit (otherwise it would be a parrot), the increase in visual size (as opposed to real size) is achieved by producing longer feathers, i.e., Buff feathering. The development of such a Buff gene has other effects upon the bird. Displaying such a phenotype, the poor creature now must wear a thicker insulating coat, which prevents easy thermo-regulation. As a consequence, the whole of the metabolic processes (all chemical reactions), including sperm and egg production, are slightly affected from their evolutionary designed critical temperatures, and thus have a generally detrimental effect upon the survivability of the individual. In poultry, there is a gene mutation known as Frizzle which produces curly feathers, and this has an adverse effect upon thermo-regulation causing early death (a self-cooking chicken!). Is not the Buff mutation similar? I digress.

With specific regard to the Violet gene, this is a gene with a dominant effect for violet colouration, and since Violet budgerigars appear to have normal longevity, it must be assumed that the Violet gene is not also a dominant lethal or sub-lethal gene. Therefore, if it has any lethal potential, then it would be as a recessive lethal or sub-lethal, as in the case of the Orange budgerigar example.

The difficulty in assessing the Violet gene as a lethal gene is the fact that the visual Violet budgerigar is, at best, produced in small numbers. It is a Blue series bird possessing one Dark factor gene and one or two violet genes - it is in fact a Violet Cobalt. The Violet gene is easy to identify on the Visual Violet, but a Violet Skyblue is also a Violet budgerigar, although not visually violet in colour. This can easily be confused with a Cobalt budgerigar, which does not possess the Violet gene. I have seen several birds in shows that are described as Cobalt which, in my opinion, are Violet Skyblues. I suspect that the difficulty in identifying the presence of the Violet gene, other than in the Visual Violet budgerigar, has added to the confusion over the lethality of the Violet gene. The production of Visual Violet phenotypes is, in my opinion, the hardest variety to produce because the mating of Visual Violet to Visual Violet never produces 100% Visual Violet offspring, although the Violet gene is a dominant gene. I have read an article saying that a Homozygous Violet Skyblue (two Violet genes) is also a Visual Violet; such a bird is produced by mating together a pair of Violet Skyblues - this contradicts conventional wisdom, and as far as the expectations of pairings goes, only adds to the confusion. The colour expectations of the Violet can be seen on the Violet Breeding Chart This chart is the most complicated of all expectations of any variety of budgerigar, and of course the Violet gene can also be possessed by any colour of bird besides Blue series birds.

The genetics of the Crested varieties is discussed by Ghalib Al-Nasser in his article The Crested Budgerigar.In view of the difficulty in determining the true operational genetics of the Crested varieties, it is clear that there is plenty scope for speculation that perhaps this gene is also a lethal gene - which would explain why the expectations of any breeding pattern have so far confounded observers. Until you raised your question, I have never had any interest in Cresteds, because I do not like them; perhaps I will now study the genetics and later, throw my hat into the ring on the subject.

One advantage that budgerigar breeders have over breeders of mammals is that an egg can be "candled" and fertilisation can be determined from the fifth day. The obvious tell-tale sign of lethality is "dead-in-shell", and whilst budgerigar breeders like to claim that it is the bacteria which pass from their hands, budgerigar droppings, litter etc., that causes dead-in-shell, it is my opinion that most of the causes are in fact many lethal genes that breeders have unwittingly sustained in the Exhibition budgerigar population.

IfEditors' note: I regret that Dr Pilkington is unable to answer any further questions.

Question to Dr John Pilkington from Royan Webb