The GENETIC HYPOTHESIS  formulation may be explained as follows:

The gene controlling under coat expression has two alleles:

The wild allele “yuc” recessive producing a normal thick under coat
The mutation allele “Yuc” dominant producing a total under coat lack

The initials of the following words York chocolate, under coat obtain the Yuc acronym

Another interesting York Chocolate physic expression surly is the particular white displacement on the coat of the bicolor subjects.  Janet Chiefari, the first York breeder and Anna Baldi first European breeder just fixed a particular white displacement on the coat maintaining the same configuration in all successive generations. The white displacement on the York’s coat is called Spotting Particolor. In literature, several genetic experts kept an open mind about the existence of a allele called “particolor” inside the White Spotting gene.

The White spotting gene control presence and pattern of white masking the normal coat pattern, and has four alleles:  non-spotted, "s", spotted, "S", particolor, "Sp", and Birman, "sb". 
The presence of the parti- color and Birman alleles of this gene is still subject to argument at this time:

- The spotted allele,  “S”,  is mutant, is dominant, and produces white spotting which masks the true coat color in the affected area. This is a variably-expressed allele with a very wide expression range: From a black cat with one white hair to a white cat with one black hair.
- The non-spotted allele, “s” , is wild, is recessive, and produces a normal coat without white.
- The Birman allele, “Sb”, if it exists, is a variation of the spotted allele producing white feet. Variable expression ranges from white legs and feet to white toes only.
Unlike the white gene or the albinism gene, the white-spotting gene does not affect eye color: if your all white cat has green eyes, it is most definitely a colored cat with one big white spot all over.
- Finally the particolor allele, “Sp”, if it exists, is a variation of the spotted allele affecting the pattern of white. The classic particolor pattern is an inverted white "V" starting in the center of the forehead and passing through the centers of the eyes. The chin, chest, belly, legs and feet are white. Variable expressions of this allele range downward to a simple white locket or a white spot on the forehead.

The York chocolate breed would totally confirm the Particolor allele existence. Observing the “Sp” frequency expression of white pattern transmission on several bicolor York chocolate of the Janet  Chiefari and Anna Baldi breeders would confirm perfectly the inherit factors of the Mendel lows.
On conclusion, York chocolate is the exclusive breed fixing the particolor white displacement on the bicolor subject as well as the Birman is the only cat breed having the typical white gloves.

Genes and polygenes transmission controlling the singular phenotypes of the York Chocolate breed may be considered perfectly stabilized. To day, a perfect and original standard describing exhaustively the body conformation and a complete genetic map consisting of all possible allele combinations of the York has been performed.

This very meticulous genetic work about classification of all York chocolate phenotypic expressions had two advantages:
·        To have a detailed scientific description of the breed
·        To help the breeders in order to transmit and to preserve the typical physic expressions of the York on all the next cat generations.

  Legend:

The Scottish-fold gene
The Japanese Bobtail gene
The Manx gene

The Sphinx gene
*The Longhaired gene à short or long coat. The wild allele, "L", is dominant and produces a normal shorthaired coat. The mutation, "l", is recessive and produces the longhaired coat of the Persians, Angoras, Main Coons, and others.
The Cornish Rex gene
The Devon Rex gene
The Oregon Rex gene
*The York under coat geneà1)The wild allele “yuc” is recessive and produces a normal thick under coat 2)The mutation allele “Yuc” is dominant and produces a total under coat lack

*The Color Gene à this gene controls the actual color of the coat and comes in three alleles
1)The black allele, "B", is wild, is dominant, and produces a black or black-and-brown tabby coat;  2)The dark-brown allele, "b", is mutant, is recessive to black but dominant to light brown, and reduces black to dark brown;  3)The light-brown allele, "bl", is mutant, is recessive to both black and dark brown, and reduces black to a medium brown.
The Orange-Making Gene
*The Color-Density Gene à controlling the coat color is the color-density gene. This gene controls the uniformity of distribution of pigment throughout the hair and comes in two alleles: 1)dense, "D", and 2)dilute, "d".
*The Albinism Gene à controls the amount of body color and comes in five alleles: 1)The full color allele, "C" is wild, is dominant, and produces a full expression of the coat colors. This is sometimes called the non- albino allele. 2)The Burmese allele, "cb", is mutant, is recessive to the full color allele and produces a slight albinism, reducing black to a very dark brown, called sable in the Burmese breed, and producing green or green-gold eyes. 3)The Siamese allele, "cs", is mutant produces an intermediate albinism, reducing the basic coat color from black/brown to a light beige with dark brown "points" in the classic Siamese pattern and producing bright blue eyes. 4)The blue-eyed albino allele, "ca", is mutant, is recessive to the full color, Burmese and Siamese alleles and dominant to the albino allele, and produces a nearly complete albinism with a translucent white coat and very washed-out pale blue eyes. 5)The albino allele, "c", is mutant, is recessive to all others, and produces a complete albinism with a translucent white coat and pink eyes.
*The Agouti Gene à controlling the pattern of the coat and comes in two alleles: 1) The agouti allele, "A", is wild, is dominant, and produces a banded or ticked (agouti) hair, which in turn will produce a tabby coat pattern. 2) The non-agouti allele, "a", is mutant, is recessive, and suppresses ticking, which in turn will produce a solid-color coat.
The Tabby Genes
*The Color-Inhibitor Gene à controls the expression of color within the hair and comes in two alleles: 1) The non-inhibitor allele, "i", is wild, is recessive, and allows expression of the color throughout the length of the hair, producing a normally colored coat. 2) The inhibitor allele, "I", is mutant, is dominant, and inhibits expression of the color over a portion of the hair.
*The Spotting Gene à color expression is the white-spotting gene and has four alleles: 1) The non-spotted allele, "s", is wild, is recessive, and produces a normal coat without white. 2) The spotted allele, "S", is mutant, is dominant, and produces white spotting which masks the true coat color in the affected area. 3)The particolor allele, "Sp", if it exists, is a variation of the spotted allele affecting the pattern of white 4)The Birman allele, "Sb", if it exists, is a variation of the spotted allele producing white feet.
*The Dominant-White Gene à expression of white coat 1) the white allele ”W” is dominant and produces white coat with blue or orange eyes  2) the recessive allele ”w” produces the total expression of color 

The columns (composed by two cells) classify the genes expressing the physical conformations.
On the first row we have alleles controlling the phenotypic expressions
Second row showing recessive alleles.