White Markings
The color white and markings of white are controlled by alleles on the S locus. The dominant allele in this locus causes a plain, solid colored dog with no white markings. Markings of white are genetically a bit more complicated. The reason for this is that first of all, several recessive white alleles exist and second because other loci seem to influence the distribution of white.
Irish Spotting (the typical Lassie markings) is caused by the allele si. This is the only type of white markings recognized in the Aussies. Almost all show Aussies are homozygous for this allele.
The allele sp, called piebald or pattern white, results in highly irregular white markings. A homozygous dog will be piebald, like is seen in some horses.
The last of the recessive alleles is sw and a homozygous state results in a pure white or extremely piebald dog. The Samoyeds are genetically homozygous for this allele, but in contrast to Aussies that are pure white, they seldom have problems with hearing. The reason for this is that these dogs aren’t true white, but extremely diluted. The skin on these dogs is black, not pink as it will be on a white Aussie.
Colors and color combination not recognized in the Aussie
There does not seem be a clear dominance/recessive relationship between these alleles. A heterozygote dog will express both characters in the phenotype. Normally though, you will see that the allele that causes the lesser-white phenotype will be more widely expressed than the other allele. When the characters from both parents appear in the offspring’s phenotype, the alleles are called co-dominant, meaning that neither of the two alleles are more dominant than the other.
To make things a bit more difficult, the above-mentioned alleles are under influence of so called modifier genes. These special genes produce a secondary effect on the phenotype by altering the characters that the alleles from other loci express. They can either increase or decrease the characters in question. These genes act by altering the distribution of white markings and they are the primary reason why such individuality in white markings exists.
Hypothetical test cross:
Below is shown what is called a hypothetical test cross, a theoretic layout for what the offspring between two specific genotypes can look like. The litters in reality will often look quite different, but the average of several litters will center on the genotypes shown below.
-Credit: Anne-Lotte Prohl