Albinism
Albinism is the congenital absence of melanin in an animal or plant resulting in white hair, red hair, yellow hair, feathers, scales and skin and red, pink, purple, or blue eyes. Individuals with the condition are referred to as albinos.
Varied use and interpretation of the terms mean that written reports of albinistic animals can be difficult to verify. Albinism can reduce the survivability of an animal; for example, it has been suggested that albino alligators have an average survival span of only 24 years due to the lack of protection from UV radiation and their lack of camouflage to avoid predators. It is a common misconception that all albino animals have characteristic pink or red or violet eyes ; this is not the case for some forms of albinism. Familiar albino animals include in-bred strains of laboratory animals, but populations of naturally occurring albino animals exist in the wild, e.g., Mexican cave tetra. Albinism is a well-recognized phenomenon in molluscs, both in the shell and in the soft parts. By definition albinism is a genetic condition, however a similar coloration could be caused by diet, living conditions, age, disease, or injury.
Oculocutaneous albinism is a clearly defined set of seven types of genetic mutations which reduce or completely prevent the synthesis of eumelanin or pheomelanin, resulting in reduced pigmentation. Type I oculocutaneous albinism is the form most commonly recognised as 'albino' as this results in a complete absence of melanin in the skin, hair/fur/feathers, and pink pupils, however this has led many to assume that all albinos are pure white with pink pupils, which is not the case.
In plants, albinism is characterised by partial or complete loss of chlorophyll pigments and incomplete differentiation of chloroplast membranes. Albinism in plants interferes with photosynthesis, which can reduce survivability. Some plant variations may have white flowers or other parts. However, these plants are not totally devoid of chlorophyll. Terms associated with this phenomenon are "hypochromia" and "albiflora".
The persistence of albinism in humans is generally the result of a mutation-selection balance in which the tendency to be reduced due to a small lack of fitness is counterbalanced by a low rate of mutation in the genes responsible for melanin production and regulation.
Biological colouration
are substances produced by living organisms that have a colour resulting from selective colour absorption. What is perceived as a plant or animal's "colour" is the wavelengths of light that are not absorbed by the pigment, but instead are reflected. Biological pigments include plant pigments and flower pigments.Animal colouration
Animals can appear coloured due to two mechanisms, pigments and structural colours. Animals may have both biological pigments and structural colours, for example, some butterflies with white wings.Pigments
Many animal body-parts, such as skin, eyes, feathers, fur, hair, scales and cuticles, contain pigments in specialized cells called chromatophores. These cells are found in a wide range of animals including amphibians, fish, reptiles, crustaceans and cephalopods. Mammals and birds, in contrast, have a class of cells called melanocytes for colouration. The term chromatophore can also refer to coloured, membrane-associated vesicles found in some forms of photosynthetic bacteria. Chromatophores are largely responsible for generating skin and eye colour in poikilothermic animals and are generated in the neural crest during embryonic development. Mature chromatophores are grouped into subclasses based on their colour under white light:- xanthophores : contain yellow pigments in the forms of carotenoids
- erythrophores : contain reddish pigments such as carotenoids and pteridine
- melanophores : contain black and brown pigments such as the melanins
- cyanophores : limited taxonomic range but found in some fish and amphibians
Structural colours
- iridophores : sometimes called "guanophores", reflect light using plates of crystalline chemochromes made from guanine
- leucophores : found in some fish, utilize crystalline purines to produce a reflective, shiny, white colour.
Plant colouration
Definition
Definitions of albinism vary and are inconsistent. While they are clear and precise for humans and other mammals, this is because the majority of mammals have only one pigment, melanin. Many animals have pigments other than melanin, and some also have structural colours. Some definitions of albinism, whilst taking most taxa into account, ignore others. So, "a person or animal with very pale skin, white hair or fur, and pink eyes caused by a medical condition that they were born with" and "a person or animal with white skin and hair and pink, red and violet eyes" do not include feathers, scales or cuticles of birds, fish and invertebrates, nor do they include plants. Some definitions are too broad to be of much use, e.g. "an animal or plant with a marked deficiency in pigmentation".Other definitions of albinism encompass most of the major animal taxa, but ignore the several other pigments that non-mammalian animals have and also structural colouration. For example, "Absence of the pigment melanin in the eyes, skin, hair, scales, or feathers." refers only to the pigment melanin.
Because of the various uses of different terms applied to colouration, some authors have indicated that the colour of the eyes is the defining characteristic of albinism, e.g. "This leads to a good diagnostic feature with which to distinguish leucistic and albino individuals – the colour of the eye." However there are several forms of albinism – currently seven types recognised for humans – most of which do not result in red or pink pupils.
The term "partial albino" is sometimes used in the literature. However it has been stated that "A common misnomer is 'partial albino' – this is not possible since albinism affects the whole plumage of a bird, not just part" and the definition of albinism precludes the possibility of "partial albinism" in which a mostly white bird shows some form of melanin pigmentation. "It is simply impossible, just like being 'partially pregnant'". Conditions that are commonly termed "partial albino" include neural crest disorders such as piebaldism, Waardenburg syndrome, or other depigmentation conditions such as vitiligo. These conditions result from fundamentally different causes to the seven types of oculocutaneous albinism that have been identified in humans and the use of the term "partial albino" is therefore misleading.
One definition states that "albinism, hereditary condition characterized by the absence of pigment in the eyes, skin, hair, scales, or feathers", However this does not encompass invertebrates, nor does it include plants. Furthermore, it could be interpreted that "...absence of pigment..." does not include an absence of structural colours.
The lack of clarity about the term is furthered when the name of an animal includes the term "albino" although the animals do not have the condition. For example, the albino gaur has this name because it is ash-grey whereas other gaur are almost black.
A clear definition appears to be – "Congenital absence of any pigmentation or colouration in a person, animal, or plant, resulting in white hair and pink eyes in mammals." Whilst this does not state specifically that non-mammalian albino animals are white, this can be inferred from "...absence of any pigmentation or colouration..." Due to the varied use and interpretation of the term "albino", written reports of albinistic organisms can often not be verified.
Mechanism and frequency
Melanin is an organic pigment that produces most of the colour seen in mammals. Depending on how it is created, melanin comes in two colour ranges, eumelanin and pheomelanin. The dark and light melanins have their influence either alone or in conjunction, making either plain or multi-coloured coats. Sometimes, in a condition called agouti, they make multi-coloured individual hairs. The production of melanin occurs in melanocytes in a complex process involving the enzyme tyrosinase. Mammals have a gene that codes for the presence of tyrosinase in cells – called the TYR gene. If this gene is altered or damaged, melanin cannot be reliably produced and the mammal becomes an albino. Besides the TYR gene, several other genes can cause albinism. This is because other hormones and proteins are involved in melanin production, the presence of which is genetically determined. In mice, a total of 100 genes are known to affect albinism.Most forms of albinism follow a recessive pattern of inheritance. However, this is not always the case. In palomino horses, genes coding for coat whiteness are dominant, and this is also true for several arctic mammals who possess dominant white colors which are pseudo-albinistic. However, these mammals differ from truly recessive albinos in that they still produce tyrosinase, and have normal eye pigmentation. In Japan, research has identified a dominant albinism in the rainbow trout.
Albinism occurs throughout the animal kingdom. The condition is most commonly seen in birds, reptiles and amphibians, but more rarely seen in mammals and other taxa. It is often difficult to explain occasional occurrences, especially when only one documented incidence has occurred, such as only one albino gorilla and one albino koala. In mammals, albinism occurs once in every 10,000 births, but in birds, the rate is once in every 1,764 births.
Some species, such as white peacocks, swans and geese, are not believed to be true albinos, as they do not have red eyes, rather, their colouration is suggested to be the expression of a white fur or feather gene, not a lack of melanin.