Through the years many avicultural magazins have published articles about avian albinism. I have noticed that there are many misunderstandings about this phenomenon and especially in canaries [4,5,12,14,23,27,36,41,42,43,44].
Scientific research conducted in the past century, showed that in birds at least two main types of albinism do excist, namely autosomal recessive (NSL) albinism and sex-linked recessive albinism. This goes for chickens, quails, budgerigars, love birds, Indian ringnecks, elegans as well as canaries, other finch-like birds and many other avian species [2, 6, 11, 13, 17, 26, 28, 30, 38, 46, 47, 48]. However, the problem is that in psittaciformes (parrot-like species) which do not posses any phaeomelanin, albinism is more easier to recognize than in finch-like- and in gallinaceous birds which do have phaeomelanin in their plumage.
The traditional idea many people have about albinism is a mammal with white hair and red eyes or a white bird with red eyes. Scientifically spoken this is the wrong idea and we must see this in a wider perspective.

After careful examination of scientific literature about this subject, it becomes clear that albinism knows many colour shades and is more complex than we think. Albinism is divided in two main groups in literature; tyrosinase positive (Ty-pos) and tyrosinase negative (Ty-neg) albinism.
Tyrosinase is a copper containing enzyme composed of more than 500 different amino acids and catalyses the initial steps in melanin synthesis (melanogenesis).
In mammalian tyrosinase the number of amino acids is determined at 533 and in fishes at 540, even fishes need tyrosinase for the production of pigment [19]. The number of amino acids in avian tyrosinase is 529 [29].
The gene which encodes for tyrosinase synthesis, inherits autosomal recessive in mammals as well as in birds and is scientifically called the c-locus [6, 33]. This is known for a long time in scientific literature and the canary is no exception to the rule.
The difference between Ty-pos and Ty-neg albinos is that in Ty-pos albinism an even more than normal tyrosinase activity can be demonstrated in contrast to Ty-neg albinism where a lower or even an almost absence of activity has been found [3,15,33].
The question which is bothering us is how it is possible that an individual, in this case a bird, demonstrates a more than normal tyrosinase activity and in spite of this shows an albinotic phenotype. In order to answer that question we must make a summary analysis of the complete pigmentation process (melanogenesis).

The melanocyte, or pigment cell, is the most studied cell in cell biology. Pigment synthesis is a process which is completely under genetic control and, that makes it most interesting for scientists because in this way they are able to study heredity in relation to enzymatic processes at the same time.
Hundreds of scientific papers are published on a yearly basis concirning pigment synthesis and related subjects. Also international congresses are held about this subject because as a consequence of this, pigmentation is associated with certain types of cancer.
Through years of studying scientific papers, with regards to pigment synthesis, it is now possible to obtain a better understanding of the processes leading to normal and abnormal pigmentation of which the latter can manifest itself as a type of albinism.

A pigment granule, also known as melanosome, is basically made of a colourless matrix composed of a number of different proteins [51]. These matrices are produced by the endoplasmatic reticulum which is a specific part within the melanocyte. The production of these colourless matrices is controlled by more than one gene, each of them concerned with the production of one of these proteins.
A gene exposes itself by mutating, in other words, if by mutation a pigmentation gene completely or partly fails to deliver its gene product to a certain process, the process can only partly proceed or cannot proceed at all and as a consequence of this it will show dramatically in the phenotype of the individual.
If during the production of the protein matrix of a melanosome (pigment granule) one of the necessary proteins is lacking by mutation of a gene, the result will be underdevelopment of the matrix and it will be deformed and much to small. As a consequence this will disturb the formation of normal pigment in the next step because almost no black colouring of the matrix will take place and the effect is a type of tyrosinase positive (Ty-pos) albinism.
Tyrosinase is present but the complete process has been desturbed by defective incomplete matrices caused by mutation of one of the many genes involved in pigment synthesis.
Howmany genes these are in the canary goes without saying. The phaeo and its allele the topaz, the opal and its allele the onyx, recessive white, dominant white, ivory, cinnamon, satinette and its allele the agate are important candidates and all involved in the realization of the most succesful phenotype namely the wild-type.
Certain factors in canaries such as pastel, opal, onyx or e.g. dilute in budgerigars shoult be considered as melanin distribution factors and act independent of melanin production (melanogenesis). They affect this process in an indirect way by which pigment clustering and often the formation of macro melanosomes (giant granules) arise which is possibly the result of the obstruction of melanin deposition.
In canaries the originally ruby red eyed "Van Haaff" mutant, named after its discoverer, could be the primarely candidate for being the mutation of the gene locus involved in protein matrix production. Equivalent in the budgerigar is the pale fallow [4, 23].
The almost complete disappearance of phaeomelanin in the "Van Haaf" mutant, nowadays known as "eumo", is explicable. phaeomelanin granules have been described in scientific literature as being amorf, i.e. they do not show a specific shape or model. That means in the first place phaeomelanin granules (phaeomelanosomes) are much smaller than eumelanin granules (eumelanosomes) and that their matrices have an indiscriminate model and thus an indistinct shape, in contrast to eumelanin granules which are recognizable pheric (round) or oval, rod-shaped and sometimes even needle-shaped.
It is understandable that a mutation which interferes in the formation of the melanin matrices, affects the already untidy produced matrices of the phaeomelanin granules the most and, the much larger matrices of the eumelanin granules to a somewhat lesser degree. The result of this is the "eumo" phenotype in canaries and the pale fallow types in parrot-like species. Even in chickens a similar mutation excist [7,46].
In the phaeo canarie the situation is reversed. The formation of eumelanin is severely affected whereas the formation of phaeomelanin is almost unaffected. This is also explicable.

Already in 1962 Cleffmann [10] showed that the production of phaeomelanin is considerable less dependent on tyrosinase activity than the production of eumelanin. Decreased tyrosinase activity associated with the presence of high cysteine levels results in phaeomelanogenesis; however, even at lower cysteine levels, decreased tyrosynase concentration favors the synthesis of phaeomelanin [49]. The "phaeo" in canaries is in fact the autosomal recessive (NSL) ino and we know nowadays that this mutant represents the so called c-locus (gene-locus which encodes for tyrosinase production) in finch-like species. The appearance of the "topaz" allele has confirmed this opinion.
Also in chickens several alleles are known of the NSL ino locus [6] and also in these birds the eumelanin is primarely affected. Because it concerns a mutation affecting the activity of tyrosinase, the key enzyme in melanogenesis, we speak of tyrosinase negative (Ty-neg) albinism.
In sex-linked albinism on the other hand, we speak of tyrosinase positive (Ty-pos) albinism. The satinette canary makes a clear example of this type of albinism. These birds are not completely pigmentless but phaeo- as well as eumelanin are severely reduced.
Electron microscopical examination of wingcovert feathers taken from sex-linked inos have shown that there are actually very small amounts of abnormal eumelanin to be found in the cortexes of these feathers [35c]. However, it struck me that the amount of granules is reduced with approximately 95% and that the still excisting granules are to small and moreover severely deformed but still showing a normal black colour.
The present of these pigment traces causes ghost barring in the plumage of e.g. budgerigars, chickens and quails but also in satinette canaries. Indistinctly stripe markings can be observed in the plumage of the wings and the back.
The agate, an allele of the satinette, is much less affected and the melanins still present, are clearly visible, predominantly eumelanin and phaeomelanin to a lesser degree.Talking about these mutants the comparison with the sex-linked ino in the budgerigar forces itself upon us.
In the budgerigar also an allele of the sex-linked ino arose, nowadays known as "SL clearbody" (pallid in other species). In this species the ino arose first and much later the clearbody [26]. According to the books, in canaries the opposite is the case; at first the agate arose and much later the satinette. However, this is actually an unusual order because it is known that the sex-linked ino-locus has a pretty high mutation rate which is proved by the many of these mutants seen in bird species held in captivity. It is however possible that the satinette canary excisted much earlier but was not recognized as such or was sorted out because of the colour.
By the way, I once have hypothesized that the SL clearbody budgerigar could be considered a possible "back" mutation of the SL ino locus [34], this applies possibly for agate canaries too.

After some literature research I found that "agate" is on naming lists since 1712 and described as "agate with red eyes". I seriously doubt whether this is about true agates or not because a true agate does not have red eyes but dark eyes. These were probably already satinettes which were called agates in the absence of a better name.
In 1853 Jules Janin wrote about this colour: "the agate-coloured are usually of a uniform colour, however there are some speciments which are of a lighter or darker tint".
This could have been one of the first describtions of crossing-overs between cinnamon and satinette which makes the difference between so called "bleached" satinettes and "full coloured" satinettes. What did people know about the phenomenon of crossing-over in 1853?
Probably nothing because it was not discovered and described until 1915 by T.H. Morgan [31] and the excistance and deciphering of DNA was not done until 1953 by Watson and Crick [47].
Let us go back to basic, the pigment synthesis also known as melanogenesis.

There are at least two other major enzymes involved in pigment synthesis. Since the nature and composition of these enzymes is not completely understood, they are adressed as "Tyrosinase Related Proteins" or TRP-1 and TRP-2.
These might be indefinite gene-loci at first sight but they play a major role in melanogenesis. Both gene loci are identified in mammals and if one of them mutates, the result will be brown eumelanin instead of black.
In canaries cinnamon is sex-linked just as in other avian species [9]. However, in the budgerigar brown eumelanin inherits sex-linked as well as autosomal recessive. There are autosomal recessive brownwings known in Australia and the USA [8, 16, 34] although they are extremely rare. This could be prove that pigment development runs analogously in mammals and birds.
In humans a mutation of the TRP-1 or TRP-2 locus leads to a type of albinism called "brown" albinism and has been described in Africans. These people have milk chocolate brown hair and brown skin instead of black [24, 25]. Cinnamon in birds could therefore be taken as a type of albinism.

The recurrence of a crossing-over between sex-linked cinnamons and satinette canaries, has lead to cinnamon-satinettes and by the recurrence of a crossing-over between cinnamon and agate has lead to cinnamon-agates also known as "isabel". In budgerigars the same crossing-over produces cinnamon-inos (aka lacewings) which also can be found in many other psittaciformes [13].
By studying many breeding records [35a] we now know that the sex-linked cinnamon locus in canaries resides far apart from the satinette locus (app. 46 cMorgan) in contrast to the cinnamon and ino loci in the budgerigar and other psittaciformes (app. 3 cMorgan). It is remarkable and most obvious in budgerigars, that the sex-linked ino factor is not capable of masking the cinnamon factor completely. On the other hand we could say that two types of sex-linked albinism residing on one and the same chromosome are unable to mask each other in the presence of very high tyrosinase activity levels possibly initiated by the action of the ino-locus [1, 3].

In canaries two distinct types of albinism occur namely tyrosinase positive (Ty-pos) and tyrosinase negative (Ty-neg). Ty-neg inos in canaries are considered to be the phaeo and its allele the topaz. The phaeo represents a mutation of the gene-locus which encodes for tyrosinase named the c-locus in mammals and renamed in aviculture as the a-locus (a stands for autosomal albinism).
The phaeo as a formula is a / a, the topaz as a formula is a ^tz / a ^tz, crossing the phaeo and the topaz will result in a proven intermediate phenotype with the formula a / a ^tz.
The sex-linked satinette is the ty-pos ino in canaries. The formula is Z ino / Z ino in cocks and Z ino / W in hens. Agate is an allele of satinette and as a formula Z ino ^ag / Z ino ^ag in cocks.
I hope to have broadened the view of some readers with regard to the problem of understanding avian albinism.

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