White Spotting & Merle
Merle is considered an incomplete dominant with multiple alleles. The merle gene deletes pigment to white, like many of the white spotting genes. The longer the merle allele, the more pigment is deleted. A dog can carry a short merle allele without appearing outwardly merle. The gold (ee) color can also hide merle.
The deletion of pigment associated with longer merle alleles can lead to eye and ear problems. Deletion of inner ear pigment causes deafness. A dog can have solid colored ears, but still be deaf due to deleted inner ear pigment. Merle is also associated with blindness, vision impairment, and microphthalmia. These health conditions can be easily avoided with appropriate testing and knowledge of merle and its inheritance. The leading researcher of merle is Tilia Laboratory in Czechia. They have a comprehensive guide to the inheritance and expression of merle, including photographs illustrating each of the merle types.
Dogs that are mm do not carry or express merle. they cannot produce merle offspring unless bred to a dog that carries merle.
Harlequin is the longest merle length and therefore, the merle that deletes the most pigment. The result is a dog with a white coat and large, dark spots. Harlequin is most common in Great Danes.
Occasionally, a dog possesses three merle alleles, instead of two.
The genetics behind many of the white spotting genes is still unknown. There are thought to be many untestable white modifiers that affect how white spotting patterns present.
This is the classic border collie spotting pattern. It usually consists on a white blaze, feet, chest, ruff, and tail tip. This pattern is seen in many breeds but has been dubbed Irish White due to its prevalence in the herding breeds of Ireland and the U.K.
Piebald (parti, S-locus spotting, random white)
Piebald is a gene denoted by Sp. It is considered an incomplete dominant with two copies of Sp yielding more white than one copy. This gene is usually associated with Parti color dogs, such as Cocker Spaniels and Poodles. There are other genes that haven’t been located yet that can produce similar patterns to piebald.
White Face (white head, bald face, baldy)
White face is the name of a set of markings thought to be caused by an incomplete dominant mode of inheritance. A study to determine the gene responsible for white face and to develop an accurate genetic marker test is currently under way. White face ranges from a wide blaze to a fully white head.
The genetics behind split face are not known, but it is a common pattern in border collies.
Ticking is the name of the small specks of color that occur in the white patches of some dogs. This pattern is most commonly found in working line border collies. When ticked dogs are born, they appear solid and white like non-ticked dogs. As they age, the dogs gradually develop more ticking. The tick marks can range from pin pricks to the size of a quarter. This is the gene responsible for Dalmatian spotting and is why the puppies in 101 Dalmatians were born white. This gene is often associated with Australian Cattle Dogs, German Short Haired Pointers, English Setters, and Blue Tick Hounds.