Albinism - Testudoalbino

Albinism - Testudoalbino

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Albinismo
Albinism is usually considered as a deficiency or partial absence of melanin production by a living organism. The most common mistake is to think that albino animals or people do not have any coloring. Nothing could ever be more wrong. Actually, there exist several types of albinism. In humans traditional albinism comes alongside with ocular albinism (only the eyes appear affected by this mutation), however it is recognized that over the years a certain amount of pigment is deposited, probably introduced with the food, or produced by the body  itself although  in exceptionally low amount, which require long periods of time to become visible.

In reptiles, the specimens that completely lack coloring are defined leucistic (they do not have red eyes because they not suffer from a deficiency of melanin production, but of chromatophores). True albinos, who totally or partially lack melanin (the black pigment only) retain the typical pattern of the species they belong to, and if they have colors that stem from metabolic processes different from pigment melanin’s, they keep them. Albinos are divided into two groups: tyrosinase-negative (red eyes and coloration totally or almost absent) and tyrosinase-positive (red eyes at birth soon turning into brown and a typical ivory or caramel colored staining pattern), according to the metabolic process involved in melanin production deficiency. A typical example is offered by those black and red livered snakes, whose black pattern at some time disappears.  

Anyone who observed an albino python has surely noticed that it exhibits the typical pattern of the species, only in a very strong yellow color. The same happens with Testudo marginata albino. Our selected variant is tyrosinase - positive: this peculiarity implies that the eyes, red at birth, soon turn to  hazel, so as to make these tortoises as strong as normal specimens when they are exposed to sunlight.

Albinism is found throughout the animal and plant kingdom with often very low incidence because in nature this feature is negatively selected. Usually this mutation is associated with other deficits that affect the normal   survival , therefore the albinism-affected animals have little chance of surviving in the wild. The main reason, however, consists in  the decreased ability to camouflage and the augmented sensitivity to sunlight.

In some animals, albinism occurs with an exceptionally high frequency. Let us think, for instance, about certain urodeles living in underground caves where the camouflage has relative importance due to the lack of light.

With the tortoises, the frequency is very low and usually disadvantageous in terms of camouflage. The case of Testudo marginata albino is the exception that proves the rule, being therefore even more extraordinary.

Albinism is a recessive mutation: it requitres that both copies of the gene that produces melanin are affected. This, like other recessive mutations, is transmitted by carriers in which it is present, even in a single copy dominated by another normal and fully functional copy, such as to lead to a normal coloring. These carriers are defined heterozygous, while an albino is considered as recessive homozygous and both copies of the gene are mutated.
A bit of genetic
For each character, there is a copy of the maternal and paternal copies of the gene or genes (called alleles) that encode them. In higher organisms, the genome is 2n, with n = the number of chromosomes typical of the species. This bears a great evolutionary significance.

A gene can have characteristics of dominance and manifest even if present in a single copy, of codominance when both copies (different) equally appera ( blood groups ), or recessive. In the latter case it may manifest only if both copies, maternal and paternal, are like that. This is the case of the albino mutation.

This means that even from two normal copies that are both carriers of an albino allele ( heterozygous ), albino specimens can result. In this case the probability (observed only on large numbers) will be 1 albino (25 %), 2 heterozygotes ( 50%) and a homozygous normal (both normal alleles) out of 4 newborn (25 %), respectively .

If a heterozygous for the albino gene breeds with an albino homozygous recessive, the probability that albinos are generated is typically1out of 2 hatchlings (50 %). The specimens with normal coloring will be in turn heterozygotes (carriers) due to the unexpressed albino gene because recessive.

If  an albino mates with a specimen not carrying the albino gene, all the newborns will be carriers of the mutated gene (heterozygous), but  will show normal coloring .




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