Thursday, 4 August 2011

genetics and skin colours: Human pigmentation: genetics and biology and cat coat colors

Skin colours are determined by genetics. Genetics of skin colours although quite complex. Racial skin colours are rather crude determination of human skin colours. This article will talk about skin colours in disease: albino and their underlying genetic mechanisms. The link of skin colours in human and albino might entertain the reader here.

There are large variations of cat coat colors which make cat breeders, cat lovers and scientists intrigue about the nature of these different shades, colors and patterns of their skin and fur for centuries. Until recently, that the comparative molecular genetics and newly developed techniques can uncover the genetic basis of these interesting characteristics of cats. These article series will talk about cat coat, color characters, and patterns and their genetic mechanisms and linking to human skin pigmentation understanding.

Skin and hairs or fur colors of mammalians including humans and cats are determined by many types of pigments. The most important one is called melanin. The cell produce melanin is called melanocytes. These cells are normally situated in the layer of skin cells or keratinocytes. Melanocytes will produce melanin by changing tyrosine, an amino acid, by the enzyme called tyrosinase through multiple steps.

There is constitutive production that is different among races and can be generally classified into three human races: Caucasian: white skin, Mongoloids: yellow skin and Negroids: dark skin. In cats, there is no such major groups. So we can see solid cats with color ranging from light brown, grey, to pure black. (Other characters and pattern will be discussed later) The production will be increased in some situations such as light, friction and injuries, or chemical agents and some disease states that can be generalized or localized such as freckles, melanoma, or malignant melanoma. Some genetic syndromes has increase incidence of these pigment disorders such as neurofibromatosis, McKune-Albright syndrome, Noonan syndrome and its spectrums.

In contrary, the rate of production can be reduced from various physical agents, chemical and disease conditions too. The disease with lowering production of melanin in human is called vitiligo that is usually localized but sometime can be generalized. A disease that has multiple small area of depigmentation causes only cosmetic problem. The disease as a fancie name: Hypomelanosis of Ito. The normal rate of production is primarily determined by racial difference in human and breed in cats and also the genetic mutation might switch off the production of melanin pigment at all which is so called albino. In the cat with complete no production of melanin pigments, the skin and fur are white, eyes will be in blue color. In human, all the hairs will be white and skin is also creamy white. Eyes are also blue like the cat eyes. This albino is called Oculocutaneous type 1a. Both cats and humans with this type of albino will have high incidence of skin cancers unravel the importance of melanin pigment for prevention of photodamage.

If the process is incomplete disruption, there will be some melanin produced leading to various level of dark color range from very light skin and hair to somewhat near normal pigment production. Clinical recognition of this mild form of albino is called Oculocutaneous type 1b. Normally, there will be no pigment production from birth like type 1 but lately produce some amount of melanin pigments that make light-brown or hazel color iris and dark eyelashes with some tanning of the skins. Oculocutaneous type I is autosomal dominant inherited. That means that the patients usually have one parent that also has the same condition. There is about 50% chance for transmission of the disease to the offspring and both male and female can be equally affected.

Recently, we found some families with typical type I albinism but with autosomal recessive like inherited pattern. There is no parent with the diseases but siblings. The parents are both carriers of the disease and normally, there will be some degree of consanguineous marriage. Genetic mutation underlying of this type of autosomal recessive albino is unknown.

There is some interesting feature of the pigment producing that leads to cat fanciers interest. It is called temperature sensitive albino. Some mutations in the tyrosinase genes make them sensitive to temperature change. It stops producing melanin when the temperature is high. So the terminal parts of their body that is colder will be darker than the body that is the characteristics of Siamese pointed cat breed, mink Tonkinese and Sepia Burmese breed. It is more interesting that we also found many families with show the features like this temperature sensitive albino cat breeds. Some areas that are less warn: facial and pubic hairs develop slightly pigmentation and arms and leg hairs which is cold will be normally pigmented.

Albino Chimpanzi and his normal friend

Temperature sensitive albino cat (Siamese cat - pointed breed)
Source: A Tyrosinase Gene Missense Mutation in Temperature-sensitive Type I Oculocutaneous Albinism
The same document with more demonstrative picture can be found at

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