Spangles -the Answers

Dr. John Pilkington

In the July edition of The Budgerigar Bill and Christine Heale gave us their interesting account of their experiences with the Spangle variety of Budgerigar. To briefly summarise their observations the Heales found that:

The mating of Normals to Single Factor Spangles produces 70% Spangles, but in the last three years their production of single factor Spangles has diminished to 20 from the usual pairing, which previously produced 3.5 times this figure. It was suggested that the Spangle gene(s) may have been weakened and that outcrossing their own Spangle line to bought in Normals had restored production of the single factor Spangles to over 70%.
Double Factor Spangles are usually smaller in size than Single Factor Spangles
The pairing of a Normal to a Double Factor Spangle sometimes produces Normal offspring when theoretically all the offspring from this pairing should be Normals.

The Answers

The mating of a Normal to a Single Factor Spangle will produce Normals and Single Factor Spangles in the ratio of 1:1 (50% of each type), if the pairing produced an infinite number of offspring. However, when relatively small samples are used for data collection, then severe statistical distortion may occur. I would tend to suggest that the Heale's production of 70% Single Factor Spangles is a typical example of reliance on a small sample giving a distorted figure. To make this point further, when we add the figures for the poor years to the figures for the good years then the production of Single Factor Spangles becomes approximately 50%.
If an event has an even chance of occurring, such as the tossing of a coin to land "Heads", then a representative sample to prove that the event has a 50% chance of occurring would be 250 throws; a lesser sample size than this may show distortions. Try it yourself and you will see what I mean. The production of Spangle Budgerigars is a similar statistical event. If however, a large sample of Budgerigars were produced from the Normal to Spangle pairing (more than 250 live offspring) and, it was found that significantly more single factor Spangles were produced than Normals, then I should conclude that the visual Spangle gene gives a non-visual advantage to the owner of the gene, such as increased vitality or vigour; this would be a very rare example of a beneficial and Dominant gene mutation. It has been suggested that Spangles are more fertile than other varieties. Without the accurate recording of breeding results, a possible project for the Spangled Budgerigar Breeders Association, it is impossible to speculate into fact.
Is it possible for a Double Factor Spangle Budgerigar to be smaller in size than a Single Factor Spangle Budgerigar, directly attributable to the possession of two Spangle genes rather than one. I am always puzzled by the commonly-held view that Budgerigars that possess the dominant colour-influencing genes, such as Green and Grey, are naturally larger than Budgerigars which do not possess such Dominant genes. I have seen many very small Grey Greens that are considerably smaller than some Blue Recessive Pieds. If the Spangle gene is a dominant gene then Double Factor Spangles should be larger than Single Factor Spangles and not smaller. Why do we see large Grey Budgerigars when in reality the Grey is a Blue, not possessing the Green gene?
Violets could be as big as Greys: likewise, the Violet gene is also a Dominant gene, as is the Grey gene, yet breeders consider it impossible to produce Violets of any substance, yet it has been proven time and time again that it can be done with Greys! There are many other similar examples that contradict the afore-mentioned belief.
It is more likely that the breeders of Grey Greens ignore colour as a factor in selecting their breeding teams, and birds are selected in size and conformation only. In the cases of the specialist or rare colours, colour is the main feature, which in turn compromises selection of other features including size; hence the smaller size.
There are several explanations as to why the pairing of a Double Factor Spangle paired to a Normal may appear to contradict current genetic theory.
1. The Double Factor Spangle is a type of Dark Eyed Clear Budgerigar. There are other genetic types of Dark Eyed Clears of very similar appearance. It is possible that we have cases of "mistaken identity" of the Double Factor Spangle parent. In a closely related stud of Budgerigars as described by the Heales, it is possible that somewhere in the ancestry of their Budgerigars, a Dilute gene has been passed on to both Single Factor Spangles ofa pair (which are genetically split for Dilute), and when the Dark Eyed Clear offspring appear, it is automatically assumed that they are Double Factor Spangles. I shall assume that the Heales have not made this mistake.
2. it is possible that there is an impostor in the nest; an egg or an un-rung chick may have been fostered without recording that this has been done. Once again I will assume that this error has not been made.

Explanation Must Satisfy Genetic Theory

Both of these explanations are "human errors". if we now accept that a genuine Double Factor Spangle (Dark eyed Clear) paired to a Normal can produce a Normal, we must search for an answer which complies with genetic theory. There are two possible explanations.

The Spangle mutation is the mutation of a "wild type" gene. Every gene has a mutation rate; since the Spangle mutation is one of the last gene mutations to have appeared in captive Budgerigars, and that it is a dominant gene when present, it would appear that the mutation of the "wild type" gene into the Spangle gene must be of very low incidence. Likewise, mutated genes do mutate back to the "wild type" at a similar mutation rate for each gene in question. It is therefore, possible that the Spangle gene passed on to the offspring by the Double Factor Spangle parent has, in fact, mutated back to the Normal gene, and thus the offspring has received no Spangle genes from the Double Factor Spangle parent. In view of the fact that other breeders have reported the same phenomenon, it seems unlikely that this is the explanation for all of the Normals that have appeared from the Double Factor Spangle to Normal pairing.
At least two Spangle genes: since most of the genes that appear on the chromosomes are repeated in several locations on different chromosomes, it is quite possible that there has been more than one Spangle gene mutation. This has happened in the case of the non sex-linked Lutino and the sex-linked Lutino; also the Australian and Dutch mutations of the Dominant Pied etc. It is possible that there are a least two different Spangle genes in existence, therefore, it is possible that a Double Factor Spangle Budgerigar may have one Type :A Spangle gene and one Type B Spangle gene that occur at different sites on the chromosomes. In this configuration, the Type A and Type B Spangle genes would be acting independently of each other in their mode of inheritance, but in conjunction with each other in their influence. In genetic terms, such a bird would be heterozygous for Type A Spangle, and heterozygous for Type B Spangle. Such a bird would produce gametes (eggs or sperm) in the following percentages;
- Type A gene - 25%
- Type B gene - 25%
- Type A and Type B genes - 25%
- Normal - 25%
As can be seen, 25% of all gametes produced by such a Double Factor Spangle would fail to carry either of the two Spangle genes and would therefore, reproduce Normals when paired to a Normal. If this hypothesis is correct, then such a bird would produce 25% Normals when paired to a Normal plus 25% Double Factor Spangles and 50% Single Factor Spangles (bearing in mind the need to take a sufficiently large sample of offspring described earlier).

Triple Or Quadruple Spangle Could Exist

Since it is unusual to pair two Double Factor Spangles together it would be difficult to prove that other forms of multiple factor Spangles exist. It would be possible to have Triple Factor Spangles of two types e.g.

Homozygous Type A and Heterozygous Type B
Homozygous Type B and Heterozygous Type A

It would also be possible to have Quadruple Factor Spangles that are Homozygous for both Type A and Type B Spangle mutations.

Such Budgerigars would be called inadvertently, Double Factor Spangles and would not produce Normals since the Triple and Quadruple Factor Spangles would be Homozygous for at least one of the Spangle types.

To some breeders my suggestions may seem outrageous, or even ridiculous, but I can assure you that there are many genes, in many other species, which operate along similar multiple gene systems to those I have described.

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