Merle
The characteristic pattern of merle is caused by the influence of a dominant allele in the M locus. Being heterozygous this allele will dilute the basic color, either the black or the red, so that patches of original color will appear on a paler background.
Merle is unfortunately a so-called semi-lethal allele, meaning an allele that is harmful for the dog when the dog is homozygous for this allele. In a litter between two merle dogs about 25 % of the puppies will appear very pale or almost white, but also have defects like blindness and/or deafness. Not all homozygous merles will develop problems with hearing; it all depends on whether or not the inner ear is pigmented. If the inner ear isn’t pigmented the nerve cells responsible for hearing can’t develop as they should and the dog will be deaf. All homozygous merles will on the other hand show some kind of abnormalities in the eyes because of improper development of the tissue. The most common defect is an eye smaller than normal; sometimes the eye can be missing all together. Irregular shapes and decentralization of the pupil are other defects which can be seen.
Breeding between two merles is therefore not something that can be recommended. Regarding merles, one should be extra careful with cryptic merles because although they lack the typical merle pattern, they are still in possession of the lethal merle gene.
Homozygous merles are also called excessive whites or lethal whites; though the last term isn’t correct as homozygous merling is not lethal.
The merle gene displays a special dominance relationship called incomplete dominance. In this kind of transmission the heterozygous phenotype will appear as the intermediary between the two homozygous characters. The homozygous merle is theoretically the real merle phenotype and not the beautiful heterozygous phenotype that we acknowledge as merle.
Distribution of the merle pattern, like the shading of the diluted color is highly individual. This suggests that certain modifier genes have an influence on this locus as well.
Harlequin and tweed are two special and very distinct forms of merle. It can often be difficult to distinguish the colors from each other; some therefore speculate that these two colors are in fact caused by one gene instead of two genes from two different loci. The names harlequin and/or tweed are therefore often used to describe both colors.
Both harlequin and tweed are dominant modifiers and reside, for harlequin anyway, on the H locus. The special thing about this locus is that it will only be expressed when the dog is heterozygous (or homozygous merle).
The harlequin pattern arises when the diluted base color of merle is broken up into several shades, sometimes as diluted, almost white patches. These patches though must not be confused with `real´ white or diluted patches that are caused by some other gene interactions. Dogs with diluted or white patches cannot be shown.
Tweed will also break the base color up into several patches of different shades. Unlike harlequin though, these patches must be clear and well bound in colors and never diluted or with white in the color.
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 many litters will center on the genotypes shown below.
