Thread: pigments
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Old 10-26-02, 02:31 AM   #5 (permalink)
Bryan Self
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Join Date: Aug-2002
Location: So Cal
Age: 46
Posts: 148
Assume a male with tyrosinase-negative albinism with the genotype (actual gene make-up) nnPP. The lowercase n represents the recessive mutation for tyrosinase-negative albinism. Assume a female with tyrosinase-positive albinism with genotype NNpp. The lowercase p represents the recessive mutation for tyrosinase-positive albinism. Using a simple Punnett square analysis we can expect the genotype of all offspring to be NnPp. All offspring would be heterozygous and their phenotype would therefore be normal.

Assume a male with tyrosinase-negative albinism heterozygous for tyrosinase-positive albinism with genotype nnPp. Assume a female with tyrosinase-positive albinism heterozygous for tyrosinase-negative albinism with genotype Nnpp. A cross between these two snake would result in the following genotypic frequencies: NnPp:nnPp:Nnpp:nnpp. These genotypes would result in the following phenotypic frequency: 1 normal:3 albino.

The last aspect to mention concerning albinism is that it only affects melanin production. Xanthophores in an albino snake are functional and are still capable of producing pigment. This is why some technically albino snakes have yellowish or reddish coloration. Axanthism is a term that was unfamiliar to me until I researched this topic. Axanthism as defined by Bechtel is a hereditary defect of xanthophore pigment metabolism. The result is a decreased amount or absence of red, yellow and the intermediate colors they form. In this defect melanophores and iridophores function normally. The most typical example of an axanthic snake is what is referred to as an anerythristic (the literal translation of this word is "without red") corn snake. Normal cornsnakes are very colorful including reds, oranges, yellow, brown and black. A corn snake with a xanthophore defect is mostly black and grey. Axanthism, similarly to albinism, can also be the result of a number of different mutations. A mating between two axanthic individuals will not always produce axanthic offspring.
Leucism is commonly confused with albinism with good reason. Both defects will produce a so called white snake. The difference is that while I have shown that not all albinos are white, all leucistic animals are pure white. Leucism is a defect that affects all chromatophores including melanophores and xanthophores. They produce no color whatsoever. The defect is caused by a recessive trait in its homozygous state. This is the only known mutation that causes leucism. So, if you have a white snake, how do you know if it is albino or leucistic? The answer to this is surprisingly easy: an albino snake will have red eyes while a leucistic snake will have darkly colored eyes, probably blue or black. Please use caution when using this to identify a snake! While I have received this information from a book by a well respected herper, I imagine there are always exceptions to the rule.
Piebaldism is the defect I find most fascinating. Piebald means spotted or patched, especially in black and white. The example that stands out for me and probably most of you is a piebald ball python. Unfortunately, little is known about the genetics of piebaldism, except that it is most probably hereditary. Vitiligo is a condition similar to piebaldism. Vitiligo is when a normally colored hatchling loses color as it ages resulting in a snake that looks piebald. The cause of this ontogenetic (acquired during life) condition is unknown. I keep hoping that my ball python will undergo this pigment change (I know the chances are slim, but I can dream, right?)
Melanism is a color variation that results in black snakes that are normally characterized by a color pattern. Melanism is the phenotypic opposite of leucism. Like piebaldism, melanism can occur congenitally or ontogenetically.
The various color anomalies I have discussed are commonly found in the herp trade. We find these snakes fascinating and are willing to pay big prices for them. In our hands these snakes will most likely lead a long healthy life. In nature, on the other hand, these snakes have low chances of survival. Imagine that you are an albino snake, what are the chances you will be able to sneak up on prey unnoticed? What are the chances that a hawk flying overhead will notice you before he notices your normally colored sibling? The fact is that albino and leucistic snakes stick out like a sore thumb in most environments, seriously decreasing their chances for survival. Another factor that can affect snakes with little or no melanin is harmful exposure to ultraviolet radiation. The absence of melanin in these snakes magnifies the detrimental affects of the sun.
I have always considered snakes with color defects interesting. The mechanisms that cause these variations, while somewhat complicated and confusing at times, are interesting too! I have received most of the information from a book by Bernard Bechtel entitled "Reptile and Amphibian Variants." I hope that I have managed to relate some of the information that I have gained about this subject to you.

Bechtel, H. Bernard (1995). Reptile and Amphibian Variants: Colors, Pattners, and Scales. Krieger Publishing Co., Malabar, FL. 206 pp.
Bryan Self
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