Both single gene and polygene resistance sources have been discovered.

Changing a complex system of genes (or a "polygene") by selectively breeding for some trait like docility is not necessarily beneficial, she said.

The color inhibitor polygene mutated in a Persian cross in 1882 in the UK.

A polygene, multiple factor, multiple gene inheritance, or quantitative gene is a group of non-allelic genes that together influence a phenotypic trait.

Even a slight alteration of a polygene like that presumed to lead to docility "might upset the genetic balance in some animals, causing them to show unusual new traits, most of them harmful to the fox," Dr. Trut wrote.

Two mutations of note influenced the Traditional Persian cats: the color inhibitor polygene (1882) that gave rise to the so-called "chinchilla" coat and the brachycephalic mutation (circa 1942) that gave rise to the so-called "peke-faced" or "snub-nosed" Persian cats.