Basic Color
There are, genetically speaking, only two colors in Aussies: black and red. Both are alleles situated on the same B locus, with black as the dominant allele and red the recessive. Genes different from the basic colours control both white markings and tan points and merle is controlled by a separate locus.
Red appears to be a somewhat more complicated colour. Variations in the basic colour are so large that red must be under the influence of modifier genes from another locus. Such genes modify, or change, the expression of the phenotypes from another gene (or genes). This can be observed by the different shades, tones and depth of the red colour in different lines: dark crossed with dark results in dark offspring and vice versa. Crossing between such lines will result in an intermediary between the two lines.
The reddish glow some black Aussies have is probably not caused by modifiers, but most likely interactions between different loci.
Hypothetical test cross:
Below is 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.
