Tan
An Aussie with tan points is homozygous for the recessive allele at. This allele is apparently under the influence of several modifier genes. This is shown in the variation seen in the shade and tones of tan as well as the highly individual distribution of tan points. The at allele is furthermore influenced by some special gene interactions called suppression. A suppressor is a gene that eliminates the phenotype of another gene from another locus. This means that a dog that genetically should have been in possession of tan points could be lacking these by being in possession of such suppressors.
Another recessive allele at this locus, ay, results in a sable dog when homozygous (you can read about sable in the Non-Recognized Colors text). The dominant allele at this locus, A, results in solid, dark pigmentation of the hair, and therefore the dog has no tan markings.
The three alleles at this locus show complete dominance, with A being dominant over ay which is dominant over at. A sable dog can therefore have no tan markings, but can transmit it to its puppies.
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