Inheritance of Color and Pattern
By George P. Johnson
The following article is the first in a series which will appear periodically in the Times. The intent of these articles is to review the fundamental concepts of genetics as they pertain to the Australian Shepherd, answer questions about genetics from the ASCA membership, and inform the membership about discoveries in the field of genetics research and how these discoveries will affect our Aussies. It is my hope that these articles will in some small way encourage the membership to learn more about the genetics of the Australian Shepherd.
The study of genetics is often divided into three specific areas: transmission, population, and molecular. While somewhat artificial, these divisions make the subject easier to discuss and roughly approximate the development of the discipline; therefore, we will follow them in this series. The oldest of the three divisions is transmission genetics which deals with the passing of genetic information from parent to offspring which we see in our dogs’ pedigrees. This area is also called classical or Mendelian genetics after Gregor Mendel, who discovered the basic principles of inheritance. Unfortunately, Mendel worked in relative isolation and his ideas, although published, were not widely seen by others. Several years after his death the principles of inheritance which he discovered were independently re-discovered by others. Although the concept that “Like Begets Like” is an ancient one, Mendel was apparently the first person to accurately record results of experimental crosses and make predictions concerning the outcomes of others. Much of Mendel’s work with pea plants can be directly applied to Australian Shepherds.
In the process of reproduction, dogs pass genetic information contained within sperm and eggs to their offspring. The combination of the sire’s and dam’s information is responsible for the characteristics of the puppies. Examples of such characteristics would be color, pattern, presence of white and copper trim, and such genetic diseases as Collie Eye Anomaly and Progressive Retinal Atrophy. Each parent contributes 39 chromosomes to each offspring, making a total of 78 chromosomes per dog. A chromosome is composed of a linear sequence of units of genetic information; each unit is known as a gene. In the simplest of situations, each gene is responsible for a single characteristic or trait. In the following discussion, the term dog is used to refer to both sexes unless specifically stated otherwise.
Inheritance of Color in The Australian Shepherd:
The trait of flower color that Mendel studied in pea plants is controlled by the activity of a single gene with two variant color forms. The Australian Shepherd parallels this as the base color of Aussies is also controlled by a single gene (at a location known as the B locus) with two variant forms (alleles), black and red. The appearance of the dog is referred to as its phenotype and an Aussie is either phenotypically black or red. Merle versus solid pattern is controlled by a separate gene and will be discussed later in this article. A merle is either a black or a red dog because color and pattern are separate traits. The black color phenotype is due to the presence of at least one copy of the dominant black allele (B) while the red phenotype is due to the presence of two copies of the recessive red allele (b). By convention, dominant alleles are always represented by “UPPER” case letters and recessive alleles are always represented by “lower” case letters. Due to the ability of the black allele to overshadow or “dominate” the red allele, a phenotypically black dog may be a carrier of a hidden red allele; such dogs are described as red factored or red carriers.
While the appearance of a dog is known as its phenotype, the genetic makeup of the dog is known as its genotype. Each dog actually carries two alleles for color – one contributed by its sire, and one contributed by its dam. Because of the dominant nature of the black allele (B), a black dog’s genotype may be either BB or Bb; therefore, it is impossible to tell the genotype of a black dog by looking (B?). On the other hand, due to the recessive nature of the red allele (b), a red dog is always known to have the bb genotype. The solution for the problem of determining whether a dog with the dominant black phenotype is homozygous dominant (BB) or heterozygous (Bb) was solved by Mendel. (Note: the prefix ‘homo’ means same. The prefix ‘hetero’ means different.) The solution is known as a testcross in which a dog with the dominant black phenotype (BB or Bb) is mated to a dog with the recessive red phenotype (bb); the goal is to determine if the black dog carries the recessive red allele (b).
Figure 1 shows the outcome of a testcross involving a homozygous dominant black dog (BB) when mated to a red dog (bb). All pups will be black but red factored (Bb). All the pups are heterozygous or carriers of the red allele. This representation of a mating is known as a Punnett Square and represents the genotypes of the parents and the possible genotypes of the offspring. While each individual dog carries two color alleles, during the process of sperm and egg formation, a separation of the alleles occurs so that each parent contributes only one color allele to each pup. Each egg and sperm that combine to form a pup will contribute one allele for each trait, in this case giving the pup its two copies of the allele for color.
In a situation where we are considering only one trait, known as a monohybrid, the Punnett Square will have four cells. The number of cells of a particular genotype will give the probability or chance of having that genotype in the offspring. In the case of a testcross involving a homozygous dominant (BB) black dog, four out of four cells are heterozygous and all offspring will exhibit the dominant phenotype. The probability of getting a black dog from this mating is 4 out of 4 cells = 1.0 = 100%. A red pup cannot result from this mating.
Figure 2 is the Punnett Square for the testcross of a heterozygous (Bb) or red-carrier black dog. Because the B and b alleles will separate in forming either eggs or sperm, two combinations are possible in the offspring. If a pup receives the B (black) allele from the red-carrier parent it will have the Bb genotype and the black phenotype. But, if the pup receives the b (red) allele from the red-carrier parent it will be matched with the b (red) allele from the red parent, giving the pup the bb genotype and the red phenotype. This is where chance dictates what genotype/phenotype a pup will have. Two of the four cells of the Punnett Square have the heterozygous genotype (Bb) and two cells have the homozygous recessive genotype (bb). There are 2/4 chances for one genotype or the other. On average 50% of the pups will be of one color and 50% will be of the other.
Unfortunately, in any testcross, the appearance of black pups tells you little about the genotype of the individual in question. After the production of seven black pups there is better than a 99% chance that the dog was indeed homozygous dominant. The birth of additional offspring will increase this confidence level but you can never be 100% sure of the dog’s genotype. But, the appearance of a single red pup immediately indicates that the dog is heterozygous for color (Bb) and it is expected that on average, 50% of the pups will be red. One note of caution – while we can determine the probability or chance of an event happening, that does not mean that it will happen with that frequency. It is possible (although unlikely) that a heterozygote would never produce a red pup in test matings.
While every mating is potentially a testcross, in actuality matings are generally not made with the intent of discovering the presence of recessive alleles (except in a situation designed to detect the carrier of a recessive genetic disease). Nevertheless, surprises happen and the occurrence of one or more red puppies in a litter does happen even when there is no indication from the pedigree that any of the immediate ancestors were red or red-factored. In large, randomly breeding populations, the largest number of recessive alleles often occur in heterozygous individuals. And while purebred dogs are not random-breeding, a sizeable number of the recessive red alleles (b) are found in individuals of the dominant phenotype. Because of this, forms of traits controlled by recessive alleles follow a typical pattern: 1) expression of the recessive phenotype may skip generations; 2) all of the offspring of parents of the recessive phenotype (bb) are also of the recessive phenotype (bb); 3) in litters where both parents are of the dominant phenotype but heterozygous (Bb), approximately 1/4 of the offspring will be of the recessive phenotype (bb); and, 4) there will be approximately equal numbers of males and females of each color. The gene for color is not associated with the chromosomes that determine sex.
The opposite situation exists for the forms of traits controlled by dominant alleles: 1) the dominant phenotype is seen in each generation; 2) every offspring with the dominant phenotype has at least one parent with the dominant phenotype;
3) in litters where both parents are of the dominant phenotype (BB or Bb), all or 3/4 of the offspring will be of the dominant phenotype, respectively; and, 4) as with recessive alleles, there will be approximately equal numbers of males and females of each color.
Another example of a trait in Aussies that is controlled by one gene with two alleles, one dominant and one recessive, is ticking (the T locus). Ticks appear as copper or black spots in the white areas of the dog; this is especially noticeable on the feet and muzzle. The presence of ticks is determined by the dominant allele (T) while the absence of ticks is due to the recessive allele (t). Two dogs without ticking (tt) cannot produce pups with ticks because the dogs are each homozygous recessive.
Inheritance of Pattern In the Australian Shepherd
There are two patterns possible in Aussies, merle and solid. This trait is controlled by the gene at the M locus which is an entirely different gene than the one for base color. The gene controlling pattern is even on a different chromosome than the gene for color so that any and all combinations of color and pattern are possible: blue merle, black, red merle, and red.
Pattern is a little more complicated than color in Aussies, because while there are two alleles for pattern (M for merle and m for solid), the heterozygous merle (Mm) is distinguishable from the homozygous merle (MM) and the solid (mm). Homozygous merle Aussies are almost always excessively white, and to complicate the situation even more, may suffer from eye, ear and other problems and should be euthanized. Many breeders avoid this situation by only breeding a merle to a solid thereby eliminating any possibility of a homozygous merle. For genes where the heterozygote is intermediate between and distinguishable from both homozygotes the interaction between the alleles is known as incomplete dominance or lack of dominance. Two of the most common examples of this type of interaction are Roan coloration in cattle and pink coloration in Four-o’ Clock flowers where the heterozygotes (roan and pink, respectively) are intermediate between the red and white homozygotes. Heterozygous merles are considered to be normal and not to have the problems associated with homozygous merles.
Although the interaction among the alleles for pattern is more complicated than those for color, mating a heterozygous merle (Mm) to a solid (mm) is essentially a testcross. Since each pup has a 50/50 chance of inheriting the M or the m allele, one half of the litter is expected to be merle (Mm) and the other half is expected to be solid (mm). If only one parent is a merle the complication of the homozygous merle is avoided; there can be none.
If both color and pattern are considered together, because two genes are being considered, the situation is known as a dihybrid. The Punnett Square would now be composed of sixteen cells, and the probabilities associated with the various genotypes would be measured in sixteenths. Each parent would pass on an allele for color as well as an allele for pattern. The greatest amount of variation possible in the litter would be achieved by mating a red-factored black dog (Bb) to a red dog (bb), one of the dogs being merle (Mm) and the other being solid (mm). It would make no difference which color was solid and which was merle; all color and pattern combinations are possible regardless of parental color/pattern because the genes are not linked on the same chromosome. There would be a 25% chance (4 of 16 cells of the Punnett Square) of producing either a blue merle, red merle, black solid or red solid dog.
Extensions of Mendelian Genetics
The principles of Mendelian or transmission genetics can be applied to more complex situations such as the genes in Aussies responsible for the presence and distribution of white and copper trim. These genes have multiple alleles and present a broader range of outcomes than the two or three associated with color and merling, respectively, presented here. Nevertheless, they can be analyzed in the same way.
The recent decision by ASCA’s Board of Directors to phase-in the DNA fingerprinting of all breeding Aussies is one of the most exciting applications of the principles of transmission genetics. Although the DNA segments known as markers used to fingerprint dogs do not represent Mendelian genes, the variants of the markers act like alleles and are transmitted in Mendelian fashion and can be traced through the pedigree. Therefore, the sire and dam of a pup transmit one of the two alleles that they have at each marker (locus) to each pup, just as they did for the alleles of color and pattern. The alleles present in each pup can be attributed to the sire and dam thereby confirming that they are the parents of record.