ALBINO, a biological term (Lat. albus, white) for a pigment less individual of a pigmented race. Pigmentation depends upon the presence and interaction in the tissues of colour-bases, chro mogens, colourless in themselves, and ferments or enzymes which, acting upon the colour-bases, yield coloured products. If, there fore, either of these ingredients of pigmentation is absent from the constitution of an individual, it must perforce remain pig mentless, albinotic, even though in the environment in which it lives there are agents, such as sunlight, which are known to stim ulate the production of pigment in the tissues of animals and plants exposed to their action. An albino is an individual whose tissues lack the power to elaborate either the ferments or the colour-bases.
In man the general colour of the skin ranges from the fair, almost pigmentless, condition of the Scandinavian to the almost ebony blackness of the natives of certain parts of Africa. These differences have been employed in the classification of races into white, yellow, red, brown, and black. The colour of the skin proper is creamy white. To this is added a tinge of yellow due to the presence of a yellow pigment in the skin. A further in gredient is black, . due to the presence of minute granules of melanin, itself sepia in colour but, in masses, so completely ab sorbing the light that it appears black. Lastly, the red colour of the blood circulating in the minute vessels of the skin adds its tint. It is possible, by using a spinning top with a combination of these colours, white, yellow, red, and black in different proportions, to match any particular skin colour, and it is found, when this is done, that in all pigmented skins all these four colours can be identified, and that different proportions of the yellow and the black in combination correspond to different racial skin colora tions. The relative proportions of these two pigments vary greatly in different individuals and in different regions of the body of one and the same individual.
Albinism is the condition, as estimated by this test, in which the black pigment is absent and in which the yellow is present only in relatively small amount.
The eyes of the albino are pink, this colour being the red of the blood circulating in the retinal blood vessels, as seen through the transparent tissues in front. The eyes are extremely sensi tive to light and so the eyelids are kept partially closed, while blinking and a general wrinkling of the skin around the eyes are associated conditions which give to the albinotic individual a characteristic appearance. The hair on all parts is white, and all tissues, such as the brain and spinal cord, which in the normal individual are more or less pigmented, are in the albino completely devoid of this melanin.
In the normal individual this black pigment would seem to serve a useful purpose. In the case of the pigmented eye, the intensity of the light falling upon the retina is adequately dimin ished by the absorbing power of the retinal pigment, and this same melanin in the skin and hair, it is assumed with reason, protects the individual from the rays of the sun in tropical climes. The albino is seriously embarrassed in consequence of its deficiency, the albino animal much less so than the plant, however. Commonly in the breeding of corn or sorghum, when the self-fertilized seeds from an apparently normal green plant are sown, it is found that amongst the seedlings a number of spindling white plants make their appearance only to wither. They die because they lack the pigment chlorophyll, so essential in the manufacture of food, and therefore in the life of the plant. When the seeds of such an albino-producing plant are carefully sown and the number of chlorophyll-possessing and chlorophyll lacking seedlings are counted, it is found that the greens out number the albinos by about three to one. From this it is known that the parent plant was heterozygous for its character green, that albinism in plants as in animals behaves in inheritance as a simple Mendelian recessive, and that in the case of these plants this character in the homozygous condition (see MENDELISM) is lethal, since in the absence of chlorophyll the plant cannot assimilate its food.
Albinism has been recorded in the great majority of the species, breeds, and varieties of domesticated animals and of cultivated plants. There are many records of instances of the condition among wild mammals and birds. In the case of man kind it has been observed in most races and probably occurs in all. Its frequency is not known, but it is estimated at perhaps one in io,000. To-day it is possibly most frequent among the Indians of Arizona and Mexico. Whatever may be the exact frequency of the albinotic in a species, it is certain, since the condition behaves as a Mendelian recessive, that there are far more "car riers" than affected individuals in any race in which albinos appear, and that though originally the condition made its appear ance as a "sport" or mutation, resulting from some definite change in the hereditary material of some one individual, since that time its reappearance in the stock is the result of the shuffling and reshuffling of the hereditary factors in their transmission from generation to generation and of the matings of apparently normal individuals who, however, are heterozygous for this particular character, not showing the taint but carrying it and transmitting it to their progeny.
The differently coloured races of a species all have albinotic strains and these may each possess the colour-bases or the fer ments of the particular race to which it belongs. Albinos are all alike in that they are unpigmented, but they differ remarkably in respect of their unexpressed pigmentation. Properly planned breeding experimentation reveals the hidden. Pigmented races are not coloured uniformly : there is always some suggestion of a differential regional distribution of pigment, some indication of a pattern, and breeding experimentation has shown that the hered itary factors for the different patterns are transmitted from one generation to another quite independently of those which relate to the actual colour of the skin or coat. Thus it is that an albino that cannot exhibit a pattern because it has no colour can possess in its hereditary constitution factors for one or other of the various patterns. It is known, for example, that when a pure bred self-coloured mouse is mated with an albino, the pattern of the coats of the offspring is determined by hereditary factors that are known to be absent from the self-coloured stock and to be present in the stock out of which the albino was derived.
So far, complete albinism only has been discussed. Partial albinism is also known, and a study of this condition reveals the fact that the interaction of colour-base and colour-developer is conditioned by such agencies as temperature. The Himalayan rabbit is an excellent example of partial albinism. In this breed the eye is pink and the fur of the body is unpigmented, but the ears, legs, tail, and nose are black. It has been shown that if an area of the white-furred body is shaved and the animal kept in the cold (5-9 ° C) the new fur that grows is black, whereas if the animal is kept in a warmed room, the new fur is white as before. The baby coat of the Himalayan is all white and the pattern develops after a few weeks when the young are moving freely about. But if the naked baby rabbit is taken from the rest for about ten minutes on two successive days and exposed to a temperature of about 17°C, and especially if it is sponged with cold water, the fur when it is developed later is black all over the body. It follows that the essential ingredients for pigmentation are present in all parts of the skin but that ordinarily the pig ment is elaborated only in certain well-defined areas. The Hima layan is a self-coloured race, but the power to elaborate pigment at ordinary temperatures differs in different regions of the body, certain areas being albinotic because normally the temperature of these parts is not that at which the enzyme and colour-base which are undoubtedly present can in their interaction yield pigment.
In certain animals such as the arctic fox, the stoat, ermine, and several hares, and in certain birds, the ptarmigan for example, the pigmented pelage or plumage of the summer and autumn is replaced in the winter by a white one. In the case of the hare, it is held by some that the autumn coat is moulted completely and that the winter coat is a new growth; others hold that there is no moulting but that the autumn coat itself becomes unpigmented still others are of the opinion that both processes are involved, a whitening of the old fibres and a new growth of non-pigmented fibres. In the ptarmigan the pigmented ,feathers certainly become white. It is probable that in the hare and similar forms the white ness is due to the presence of air in the pigmented fibres and con sequently to the irregular reflection of the light from the surfaces of these air bubbles. In the case of the ptarmigan it is possible that the whiteness of the winter plumage is due to a seasonal increase in the activity of the thyroid gland, for it is established that if the active principle of the thyroid gland is administered to pigmented domestic fowls, the new feathers as they grow tend to be non-pigmented. It has not yet been shown conclusively that this seasonal change from the pigmented to the non-pigmented condition is to be regarded as an instance of seasonal albinism.
Occasionally albinotic frogs or axolotls are recorded, but it is doubtful whether all . these cases are true examples of albinism. It is more probable that they are individuals in which the pitui tary gland or hypophysis at the base of the brain is abnormal; for experiment has shown that if this gland is removed from these animals, the pigment cells in their skin no longer expand and that therefore the animals must preforce remain very pale in colour. An injection of the active principle of the gland, pituitrin, turns them coal-black almost at once.
BIBLIOGRAPHY.-G. P. Mudge, "Intravascular Coagulation and Bibliography.-G. P. Mudge, "Intravascular Coagulation and Albinism" (Proc. Phys. Soc., 19o5) ; Pearson, Nettleship and Usher, A Monograph on Albinism in Man (i 913) ; C. B. Davenport, Heredity of Albinism (Jour. Hered., 7, 221-223, 1916) ; J. A. Detlefson, "A Herd of Albino Cattle," (Jour. Hered., i1, 378-379, 192o) ; R. R. Gates, Heredity and Eugenics (1923 bibl.) ; Baur, Fischer, Lenz, Menschliche Erblichkeitslehre (1923, bibl.) ; B. Dean, "The Skin Colors of the Races of Mankind" (Nat. Hist., 26, 44-49, 1926) .
(F. A. E. C.)