Tribbles are hermaphrodites -both male and female in one individual. This means that, unlike an amoeba, which splits itself, producing an exact clone, fertilization does take place, and a limited gene pool is available. Still, tribbles will inter-breed whenever possible.
The most important factor in selection and breeding of wild tribbles is reproductive capability, but in the tribble used as pet the size, color, and hair coat are valued higher.
The natural colors of the tribble range from black, brown, gray and white to almost yellow, in solid, spotted, and roan. However, through selective breeding and the more controversial genetic engineering, breeders have come up with tribbles of more exotic colors: green, blue, yellow, red and even leopard.
True white is dominant, then black, and roughly down to lighter and lighter with cream and light yellow being the most recessive natural colors in tribbles. Spotted is dominant over solid, roan is codominant, and leopard is recessive to all.
Many colors will be codominant to some extend, so a black crossed with a natural red will produce some offspring with a more liver-color, or deep brown-red, rather than just pure black offspring.
Genes are identified for true white, black, brown, grey, red and cream, these are often called 'true' colors; liver, mouse-grey, dark red (bay) and infinite other shades are various combinations of 'true' colors, working together to produce that certain shade.
Natural colors tend to be dominant even over darker exotic colors. The exception is albino, which is naturally occurring, but is recessive to all other colors. The reason for this is that albino is not a real color, but rather an absence of color. Thus, as soon as any color does come into the picture, the tribble will not be an albino but instead show that color.
Albino tribbles are rare but can hardly be discerned from true white tribbles. An albino will throw no color when self-fertilization has taken place, while a true white will throw a variety of colors. With non-breeding tribbles it is only possible to tell by genetic testing.
It is of course possible to ascertain the exact genotype of any tribble in a well-equipped laboratory, but unless there is a special reason for it, like the breeding of a new color, the time and cost involved are not worth it.
The tribbles sold as pets are all genetically altered and do not breed. The exact process is too complicated to elaborate on at this time, but for those interested I would recommend "Reproduction and Genetics in Tribbles", by Drs. Prahyr & Taylor, Rigel IV, s.d. 25013.