THE MODIFIERS

    Up to now the traits with in the genetic make up of the dogs coat color are reasonably predictable. They are characters controlled by one or two genes with fairly clear differences between the genotypes and phenotypes. Continuous Variation or Polygenic Inheritance has the same Mendilian principles with the exception of having a relatively larger number of genes each of which have a very small effect individually but in unison could have very marked effects.

     The basics of Polygenic inheritance are made up of 5 different levels of light dark to. You might call them plus or minus modifiers. The levels are White-Light-Medium-Dark-and Black. It follows the same principles of combining genes but becomes more complex because there is more of them and the combinations become much greater

It looks like this:
Very Light 1 Light 2 Medium 3 Dark 4 Very Dark 5
If we apply a letter to each level we can combine them in the same manner as we combine the genes for coat color. The levels apply to what shade of color might appear. The darker the modifier the darker in theory the color will be irregardless of what color.

Very Light = wwzz
Very Dark = WWZZ
Light = wwZz
Medium = wwZZ
Dark = WWZz

   If we think of it in terms of four small letter equals White color, four capital letters equals Black, three small letters equals Light color, two small letters equals Medium color, and one small letter equals Dark color we can begin to see the boundaries between them.
Essentially, if you breed a parent carrying wwzz which is all white modifiers to a parent carrying WWZZ black modifiers you will get a complete array from White to Black. This is because each off spring will be carrying one set of a possible 16 combinations as shown in the following table.
 
 

   They play a significant role in determining the lightness and darkness of the coat color. For example, if a Black dog carries a single dilute gene, the blue dilution may show through easier if the dog carries more light modifiers as opposed to dark modifiers.  And so if carrying light modifiers and the dog is an (Aat) dog which is a black dog with recessive tan markings, the tan may show through the coat more.

Mendel's Law Defined
     The principles of heredity of sexually reproducing organisms formulated by Gregor Mendel, now usually summarized in three laws: 
a. Law of Segregation: Certain paired characteristics, one from each parent, do not blend with or each other in the offspring, thus accounting for contrasting traits in successive generations. 
b Law of Independent Combination: The genes determining such pairs of traits combine in the offspring according to the statistics of chance. In practice, the manifestation of Mendel's laws is seen by characteristic ratios of phenotypic classes, such as 3:1 and 9:3:3:1.
c. Law of Dominance: If one of a pair of genes is dominant and the other recessive, the recessive trait may appear in an offspring only if both genes of its pair are recessive.
There are extensions to Mendelian principles, including the fact that some alleles are incompletely dominant, that some genes are sex-linked, and that some pairs of genes do not assort independently because they are physically linked on a chromosome.

Summary

     There are some things in coat color that just cannot be explained mostly due to lack of research.  It is probable that more than the genes that are listed exist but there hasn’t been clear cut evidence. There may be more than one or two genes controlling the colors black and white in that a recessive and dominant may exist in each color. This could account for color patterns sometimes "bleeding" into the white color and not in other instances. The variation in shade of black may also be due to a dominant/recessive nature. One must also consider Modifying genes that contribute to variations in coat color and also mutations from time to time. 

     The tan (ay) gene can be questioned as to its dominance over (at) when at times the gene can push through the coat so hard when in combination with (ay) that the Tan dog appears to have tan points. This incomplete dominance can occur with many other gene pairs and at times making it difficult to conclude exactly what genes a dog carries without mulitple breedings or knowing the full history..