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Merle coat color
Are you wondering if your dog is a carrier for merle coat color, why its coat is not merled despite the merle mutation present, and why caution is important when mating dogs with a merle mutation? You can find answers to these and some other questions regarding the inheritance of merle coat color below.   The merle coat color is characterized by irregularly shaped patches of diluted pigment and patches of solid pigmented coat color and it occurs in many dog breeds. Short part of DNA (i.e. SINE insertion) is responsible for the merle coat as it is inserted into the gene involved in coat pigmentation and consequently affects the development of this coat color. In addition to coat color, the mutation can also affect the color of eyes, nose, pads, and skin. Because of the dark eye pigment discoloration such dogs have blue eyes. Due to random discoloration of the pigment neither, one or both eyes can be blue, or even partially blue. The color of the nose and pads can be black or mottled pink.     Merle coat color is inherited in autosomal, incompletely dominant manner, which means that only one allele associated with the merle coat color is enough to express it. Dogs that inherit two copies of the merle allele have been found to be at higher risk for hearing and vision problems. The mutation is therefore also important in terms of health problems. It is hypothesized that a combination of sufficiently long merle alleles cause the death of pigment cells in the skin, retina, and inner ear, resulting in a predominantly white coat color, hearing and vision problems.   The mechanism of merle coat color formation In a special organelle (eumelanosome) of skin pigment cells a dark pigment (eumelanin) is formed and stored. The skeletal structure in these organelles has an important role in pigment cells and black coloration. The gene associated with the merle coat color is responsible for the proper functioning of that skeletal structure. Dogs with a change in this gene have an altered skeletal structure in the organelles of the pigment cells, which interferes with the formation of black pigment and consequently reduced pigmentation of the coat. The skeleton of pigment cell organelles is not required for the formation of a yellow-red pigment (pheomelanin), thus the merle color is expressed only in dogs with a dark coat color (black, chocolate, blue and isabela). Dogs that are have only yellow-red pigment do not show merle coloration, so we call them hidden merles. These dogs can transfer merle coat color to their offspring. Merle coat color is also difficult to see in sable dogs because they have the dark pigment only on the hair tips.   Inheritance peculiarities The speciality of the mutation responsible for merle coat color is that dogs can have offspring with different lengths of alleles. Usually, each dog has two alleles for an inherited trait - one allele from the mother and one allele from the father. In merle coat color a dog can have more than two alleles that differ in length, and such dogs are called merle mosaics. Different cells of their body have variants of alleles of different lengths and this mosaicism occurs because the gene responsible for merle is very susceptible to mutations. Mosaicism may be present in a small proportion of cells that are not present in the analysed sample, making it difficult to always detect it reliably with a genetic test. Inheritance of the additional allele and its effect on appearance (phenotype) cannot be predicted, as the distribution of mosaic cells throughout the body is different and not all cells are included in the test sample.   Due to the complexity of the mutation responsible for the merle coat color the genetic test for merle coat color is also very complex. You can read more details about the merle coat color and genetic test on our website.
Effects of selection on genetic health of purebred dogs
Natural selection directs the emergence of new species and the eradication of inherited traits that reduce the animal’s chances of survival. Artificial selection (the formation of dog breeds) is directed by people and their breeding decisions that are not always in line with the principles of natural selection (better survival). Purebred dogs are genetically isolated populations within a species with specific traits developed through artificial selection. From the perspective of genetic diseases artificial selection can reduce, maintain, or increase the frequency of disease genes. Most breeds originated from a limited number of animals and consequently limited genetic material. The effect of the formation of geographically separated subpopulations within the breed, the overuse of popular males, and line breeding further limit the heterogeneity of genetic material, leading to an increased risk of developing genetic diseases.     Unwanted genetic conditions or diseases are the result of planned or unplanned breeding errors. Excessive selection for certain body traits has led to conditions that cause breed specific health problems (brachiocephalic syndrome, excessive amount of skin). The transfer of disease genes can be random or linked to the selection of desired traits. An example of a genetic disease linked to the selection of a specific desired trait is hyperuricosuria in Dalmatians. A recessive disease mutation is responsible for abnormal urine metabolism. Due to the linked inheritance of this mutation with the desired dotted pattern of coat, it has become fixed in Dalmatians (present in all dogs). With the project of planned mating with a healthy pointer and further planned breeding, we already have quite a few registered Dalmatians with a characteristic spotted coat and without a disease mutation (1).   In the past, breeding plans were limited only to traits that were expressed in animals. In the recessive mode of inheritance, there is a problem of elimination of carrier animals that do not develop disease symptoms but pass the causative mutation on to their offspring. In such cases diseased animals can appear in each generation. Late onset diseases which develop clinical signs at an old age are also a problem, as these animals cannot be excluded from breeding in time. Progress in the field of genetics in recent years has enabled the development of genetic tests, which are of great help to today's breeder. Based on the result of a genetic test mating can be planned with the aim of disease eradication. Without direct selection against predispositions to genetic diseases, no improvement in breed health is expected.   A genetic test is an extremely powerful tool that can be very helpful in making informed breeding decisions. For its proper use, several factors need to be considered: the presence of a specific disease in a breed, frequency of occurrence, penetrance (proportion of animals with a mutation that develop the disease), mode of inheritance and type of mutation (loss of function / increased risk). It is necessary to be aware that the genetic test gives us information about a specific disease and not about the general health of the dog, therefore the result of a genetic test must be placed in a broader context when making breeding decisions (general health of the dog, behavioural characteristics, dog function).   The fact is that advances in genetics and the development of various genetic tests have a major impact on modern dog breeding. If in the past breeding strategies were formed based on visible traits, today it is possible to plan and predict consequences of breeding with the help of genetic tests and to avoid undesirable traits in offspring.   Genetic homogeneity is not necessarily a problem as it is the result of species and breed formation, but it can be a problem if it leads to homozygous alleles on disease genes. Appropriate selection focused on health improvement ensures the long-term existence of dog breeds. Healthy and heterogeneous genetic material of purebred dogs is responsibility of breeders. Modern genetic tools give a wealth of information that can be used to pursuit breeding goals focused on health and functionality of dogs.   Reference: (1) Schaible, Robert H. (1981). "A Dalmatian Study: The Genetic Correction of Health Problems". The AKC Gazette.