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The PE Theory for the Crested Budgerigar
by George Clarke Western Australia

This article summarises the basics of the new PE theory for Crested Budgerigars presented in my recent and more comprehensive article "CRESTS – Towards a Better Understanding" published in Dec 2004 by The Crested Budgerigar Club of Australia. The original article describes the theory, its derivation and application, comparisons with the Initiator theory, and includes a commentary, acknowledgements, references and validation of the PE theory. Readers seeking a wider understanding of the theory are recommended to consult the original article.

The Crested Budgerigar is often described as the "Ultimate Challenge". The challenge is threefold;

The new PE Theory is a very different approach to Crests. It alters and enhances our understanding, giving breeders new opportunities to meet these challenges through more predictable breeding results.

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One of the most popular matings is Crest x Crestbred. From this mating breeders are usually able to produce on average about 30% visual crest progeny. The PE theory, when used together with appropriate breeding strategies, can double this percentage to 59%. The theory also demonstrates how inappropriate strategies can reduce Crest production from this mating to as low as 9%.

The PE Theory indicates that improvement in the quality of crest and other exhibition features is best achieved through the use of the same strategies we apply when improving other polygenic (i.e. controlled by multiple genes) characteristics like Size, Type, etc. There is no magic prescriptive formula to follow, only guidelines and strategies.

Analysis of breeding results comprising 1,759 progeny including 570 visuals, shows the predictions of the new PE Theory to be in good agreement with actual breeding results. In particular the predictions of the PE Theory are proven to be far more accurate than the Initiator theory (our guide since 1970), and are free of all the well-known and significant anomalies of the now out-dated Initiator theory.

The basic genetics involved in the PE Theory are not difficult to grasp. Some terms will be new to readers so the following few short paragraphs give a brief background on genetic terminology applicable to Crests. The PE Theory can be used without the need to think too much about genetics but it will be more useful to breeders having a basic understanding of the underlying genetics. Readers approaching the PE Theory need to set aside old ideas particularly ideas created by the Initiator theory.

Each budgerigar inherits pairs of genes from its parents; each parent contributing one gene to each gene pair. An inherited gene can be either a mutant gene or a non-mutant (wild-type) gene. When two mutant genes are inherited the budgerigar is described as Double Factor (DF) for that gene. When only one mutant gene and one wild-type gene are inherited the budgerigar is described as Single Factor (SF) for the mutant gene.

It is usual for any bird carrying a Dominant gene in either SF or DF form to express the gene visually. The Crest gene is Dominant, but with a very unusual twist. All DF crests are visual crests, but only 15%-20% of SF crests are visual crests instead of the expected 100%.

This effect is known as "Penetrance". The Crest gene is said to have a penetrance of 15%-20%.

Most genes responsible for colour varieties cause effects which have only a very limited range of variability in expression. Some genes, such as the Pied genes, exhibit wide variability in expression. The Crest gene is said to have highly variable "Expressivity" because of the many visual forms of Crest expression ranging from single feathers to multi-crests.

Hagoromo Budgerigar

The PE Theory

The new PE Theory is based on the concepts of Penetrance and Expressivity and states,

The exact number of E genes and the nature of their action is unknown.

Definitions of the PE Theory

Symbols. The crest gene is given the symbol "Cr", while the symbol for the wild type allele is "cr"

Normal. A Normal in the context of the PE Theory is any non-crested variety.

Crest. A Crest is a bird which exhibits a crest phenotype and carries at least one Crest gene. The definition includes birds which exhibit crest characteristics temporarily as nestlings but do not do so as adults.

There are two basic genetic forms of the Crest, visually indistinguishable from one another,

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Crestbred. A Crestbred is a bird having at least one Crest or Crestbred parent but at no time exhibiting any permanent or temporary visible crest characteristics. Crestbreds exhibit a Normal phenotype but carry either one or zero crest genes. A Crestbred cannot carry two crest genes because all DF’s are visual crests.

There are two basic genetic forms of the Crestbred, visually indistinguishable from one another

  • Single Factor (SF) Crestbred, having one crest gene and one wild type gene, (Cr,cr)

  • Zero Factor (ZF) Crestbred, having two wild type genes, (cr,cr)

Table 1 Summary of Crest Phenotypes and Genotypes

Description Phenotype Genotypes
Crest Crest Cr,Cr Cr,cr
Crestbred Normal Cr,cr cr,cr
Normal Normal cr,cr

Penetrance. The penetrance of the crest gene determines the percentage of birds, carrying the crest gene, which will at some stage exhibit visual crest characteristics.

Expressivity. Expressivity determines the type of crest which will be exhibited.

  • Crest phenotypes comprise a continuous spectrum ranging from a single feather to multiple crests.

  • Any bird penetrant for the Crest gene will exhibit one of the many crest forms, depending upon the modifying expressivity genes (E genes) present.

  • Any pairing in which at least one parent carries a crest gene, expressed or not, may produce any form of visual crest depending upon the E genes present. The frequency of occurrence of some forms may be low.

  • The expressivity of the Crest is a polygenic characteristic and cannot be predicted theoretically for individuals.

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Predictions of the PE Theory

Theoretical expectations for all possible Crest, Crestbred, and Normal pairing combinations are summarised in Table 2. [ Note. Table 2 is based on a Penetrance value (Psf) = 17%]

Table 2 PE Theory – Theoretical Breeding Expectations

Parents Progeny




Visual Crests


Cr,Cr Cr,cr cr,cr Total DF SF Total SF ZF


DF Crest DF Crest 100%     100% 100% 0% 0% 0% 0%
DF Crest SF Crest 50% 50%   59% 50% 9% 42% 42% 0%
SF Crest DF Crest 50% 50%   59% 50% 9% 42% 42% 0%
SF Crest SF Crest 25% 50% 25% 34% 25% 9% 67% 42% 25%
Average         63% 56% 6% 37% 31% 6%


DF Crest SF CB 50% 50%   59% 50% 9% 42% 42% 0%
SF Crest SF CB 25% 50% 25% 34% 25% 9% 67% 42% 35%
DF Crest ZF CB   100%   17% 0% 17% 83% 83% 0%
SF Crest ZF CB   50% 50% 9% 0% 9% 92% 42% 50%
Average         29% 19% 11% 71% 52% 19%


DF Crest Normal   100%   17% 0% 17% 83% 83% 0%
SF Crest Normal   50% 50% 9% 0% 9% 92% 42% 50%
Average         13% 0% 13% 87% 62% 25%


SF CB SF CB 25% 50% 25% 34% 25% 9% 67% 42% 25%
SF CB ZF CB   50% 50% 9% 0% 9% 92% 42% 50%
ZF CB SF CB   50% 50% 9% 0% 9% 92% 42% 50%
ZF CB ZF CB     100% 0% 0% 0% 100% 0% 100%
Average         13% 6% 6% 87% 31% 56%


SF CB Normal   50% 50% 9% 0% 9% 82% 42% 50%
ZF CB Normal     100% 0% 0% 0% 100% 0% 100%




4% 96% 21% 75%

Empirical rules for making approximate predictions of expressivity can be developed by analysing a sufficiently large number of breeding results.

The author has analysed a sample of breeding results comprising 1,411 progeny including 403 visual crests. Based on these results Table 3 shows approximately, how visual crest progeny may be expected to be split between less complex and more complex Crest forms, for various Matings.

Table 3 Approximate Expressivity Predictions


Group T
(Less complex
Crest forms,
Tufts and
Weak Tufts)

Group C
(More complex
Crest forms,
Half Circular,
Full Circular and Multiple)

Crest x Normal



Tuft x Tuft



Crest x Crestbred



Crest x Crest



Crest x FC or HC



Use of the PE Theory to improve breeding results

A new and more accurate theory is all very well but can it help breeders to make better pairing decisions and faster, and more certain, progress?

Table 2 allows breeders to predict the % of visual Crests and Crestbreds expected from any mating and Table 3 to predict the % split of Crest forms expected within those visuals.

There are really only three matings to adopt when upgrading crests. Crest x Crest, Crest x Crestbred or Crest x Normal. (Crestbrd x Crestbrd and Crestbred x Normal produce too few Crests to be useful.)

Crest x Crest will usually be slow to improve general exhibition characteristics and in many cases may even diminish them, but needs to be used to strengthen Crest characteristics and maximise production of DF crests.

Crest x Normal produce too few visuals to provide a good selection for the next generation, but this mating is the vehicle by which good non-crest exhibition characteristics can be introduced.

Crest x Crestbred is the optimum mating to make steady progress on both fronts.

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From Table 2, the best Crest x Crestbred pairing is DF Crest x SF Crestbred, producing 59% visuals. The best pairing to produce SF Crestbreds is DF Crest x Normal. This produces 83% Crestbreds and all are SF. Are DF Crests identifiable? Intuitively it seems that the birds with the more extreme crest characteristics, the full circular and multi-cresteds, would be the DF Crests. The PE theory does not imply any such direct relationship, different genes being responsible for the presence of the crest, and its crest expression. However there is an important indirect relationship. The production of higher forms of Crest requires the concentration of favourable "E" genes. This is best achieved by mating Crest x Crest. Coincidentally this is also a mating which produces a high proportion of DF’s. Hence DF’s and the higher crest forms can frequently, but not always, be found together in the one bird. Since Crest x Crest matings (Table 2), produce on average 56% DF, and only 6% SF visuals, identification becomes a less critical issue for crest breeders when this mating is used to produce DF crests.

The optimum breeding program is therefore to firstly mate Crest to Crest to produce DF Crests, and next to mate these DF Crests to high quality Normals, and finally to mate the best of the Crest or Crestbred offspring (all SF) from this mating, back to the DF Crests to produce visual crests (59%).

The author claims no particular originality for this breeding program. However the PE theory provides a sound rationale for the program, and a guide to its implementation and breeding expectations. Breeders can now proceed with greater confidence, avoiding pitfalls and optimising results through having a better understanding of the underlying genetics.

As far as the expressivity of the crest is concerned, apart from the guidance of Table 3, the only proven tools available to the breeder seeking to improve the quality of the crest, or to introduce more complex forms of crest, are the same simple breeding strategies he would use when dealing with other polygenic traits (size, type, etc). These strategies, aimed at gradually concentrating desirable genes by selective breeding, are well documented and understood by experienced budgerigar breeders and will therefore not be discussed further here.

One point to be considered by breeders, is that it is probable that when the Crest mutation appeared the modifying expressivity genes were already existing in the normal population in both heterozygous (SF) and homozygous (DF) forms. It is highly improbable that the expressivity genes came into existence as new mutations at the same time as the Crest gene. This means that while "Normals" have no active crest genes they should not be regarded as being free of the active forms of the expressivity genes (E genes). The expressivity genes simply do not act to determine a crest form until an active (penetrant) crest gene is present. I believe this to be the reason that it is possible, contrary to the predictions of the Initiator theory, to produce Half circular and the occasional Circular from Tuft x Normal pairings. (e.g. Ref 1 reports Tuft x Normal matings producing 247 Normal, 29 Tuft, 8 Half circular, and one Circular progeny).

Similarly it is reported in Ref 1 that Full Circular x Normal can produce all forms of crest, but any one particular pairing may only be able to produce one or two types of crest, while another pair produce different types. This variability in results from identical pairings is probably due to different concentrations of E genes in the individual normals used in the pairings. The normals of course show no indication of their E gene content and when mated to Crests can produce unanticipated Crest forms.

Variability in breeding results

Along with general comments on variability of results it is frequently reported that some breeders seem to be able to produce high numbers of visual crests compared to other breeders.

An examination of Table 2 reveals the reasons behind these observations.

Firstly there is a large difference in the average number of visuals produced by each of the following matings; Crest x Crest (63%), Crest x Crestbred (29%), Crest x Normal (13%).

Secondly within each of these mating combinations there is a wide range of results depending upon the genetic make-up of the parents. Crest x Crest (34%-100%), Crest x Crestbred (9%-59%), Crest x Normal (9%-17%).

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Care and luck is needed when selecting pairings. Eg when upgrading using the Crest x Crestbred pairing it makes a big difference if the pairing involves a DF Crest x SF Crestbred (59% visual progeny) compared to a SF Crest x ZF Crestbred (9% visual progeny). The difficulty is that visually we cannot reliably differentiate between a DF and SF Crest, or between a SF and ZF Crestbred. Fortunately for a breeder armed with the PE theory and Table 2, there are breeding strategies which can minimise this problem.


1. Handbook of the Crested Budgerigar Club of Australia 2002 (Based on UK Handbook) CBCA

2. CRESTS—Towards a Better Understanding George Clarke 2004 CBCA

Note: The unabridged version of this article (Ref 2) contains a full list of references consulted.


This work would not have been possible without the extensive breeding records kindly provided by experienced Australian Crest breeders Rob Hugo and Ken Yorke.

Additional breeding results are needed to further refine expressivity aspects of the theory. If you are able to assist, the author would be pleased to receive them. A Reporting Form is attached.

Copyright – George Clarke 2004

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