Canine Genetics and Breeding Strategies: The Basics
Part One: A Genetic Roadmap
By Arliss Paddock
Dogs have been the companions of humans for more than 12,000 years, and over this time humans have developed Canis familiaris into the most diverse animal species on the planet. From the Chihuahua to the Irish Wolfhound, dogs exhibit an amazing size range and a staggering variety in physical features such as ears, tail, head shape, skeletal structure, and coat type, as well as in behavioral characteristics. Worldwide there are now more than 350 distinct dog breeds suited to a wide array of functions, situations, climates, and terrain.
How did this come about? Evidence suggests that as early as 3,000 to 4,000 years ago, several distinct types of domesticated dog had already emerged, descended from the gray wolf. Long before the science of genetics was understood as it is today, humans selectively bred dogs with certain desirable traits—and refrained from breeding those with undesirable traits.
Mary Bloom ©AKC
Although the original functions of some breeds are diminished or nonexistent now, breeds often take on new, important roles for which their original traits make them particularly suited. The unique character of each breed is preserved today thanks to dedicated, knowledgeable breeders. These breeders are the caretakers of each breed’s quality and legacy, and they ensure that the breed will continue to benefit humans in various ways and be appreciated and enjoyed in the future.
If not for these devoted caretakers, the breeds as we know them would deteriorate into indistinguishable types generally lacking in consistency and valued characteristics and rife with undesirable traits. Today’s dedicated breeders serve to preserve our rich, wonderful tapestry of dog breeds for future generations.
To stay on course, ideally the breeder joins forces with and learns from other serious fanciers and students of the breed via the breed’s national organization and regularly participates in recognized venues where stock is assessed and confirmed to be worthy of breeding. The breeder also makes use of recent scientific advances that are excellent tools in ensuring breed health and quality.
To achieve the best results in breeding, however, a breeder must have at least a basic understanding of how traits are transmitted genetically.
A species subdivided into breeds
In seeking this understanding, one has to understand dogs first as a species, then as genetic individuals, according to Jerold S. Bell, DVM, a veterinary practitioner and director of the Clinical Veterinary Genetics course at the Tufts Cummings School of Veterinary Medicine.
Bell, who is on the AKC Canine Health and Welfare Advisory Panel and lectures to all-breed and specialty clubs on canine-genetics topics, offers the following perspective on the interrelation of all dogs as a species:
“The species Canis familiaris includes all breeds of domestic dogs. Although one can argue that there is little similarity between a Chihuahua and a Saint Bernard, or that established breeds are separate entities among themselves, they all are genetically the same species. While a mating within a breed may be considered outbred, it still must be viewed as part of the whole genetic picture: a mating within an isolated, closely related, interbred population.” (From “Getting What You Want From Your Breeding Program,” September 1992 AKC Gazette.)
The history of the dog’s emergence as a species and the subsequent development of numerous types and breeds is long and interesting. Writes noted genetic researcher Dr. Elaine Ostrander, founder of the project to map the canine genome, “The domestic dog has long fascinated evolutionary biologists and geneticists because of the extreme phenotypic diversity exhibited by the species and the short time frame over which this diversity has evolved. Molecular genetic evidence suggests that dogs are indeed the oldest domesticated species, and their origin may have even well preceded their first appearance in the archeological record about 15,000 years ago.”
(Ostrander, Elaine A., and Wayne, Robert K., “The canine genome,” Genome Research, 2005. 15:1706–1716.)
Of the development of many new breeds since the 1800s, Ostrander writes:
“The explosion of dog breeds over the past two centuries represents perhaps one of the greatest genetic experiments ever conducted by humans. Distilled from the genome of the wild wolf are animals that differ by more than 40-fold in size with the ability to herd, guard, hunt, and guide.”
Bell notes that each breed was developed by close breeding among a specific group of canine ancestors, resulting in a relatively homogeneous genetic base that makes it possible for the breed to “breed true.” He explains further, “It is the variable [genetic components], like those that control color, size, and angulation, that produce variations within a breed.” It is through understanding aspects of genetic inheritance in individual dogs that breeders can effectively control traits such as these.
Glossary of genetic terms
A good starting place in learning how traits are passed along through breeding is to have a basic familiarity with some important terms and concepts.
• Genes—A gene is the fundamental unit of heredity. Each individual dog is what he is because of the approximately 19,000 genes in his makeup.
Genes are composed of DNA, a molecule that takes the shape of a double helix, like a spiral ladder. Each rung of the ladder consists of two paired chemicals called bases.
There are four types of bases, each symbolized by the first letter of its name: A, C, T, and G. Different sequences of these base pairs form coded messages. Each gene consists of a particular sequence of many bases, and these unique combinations determine each gene’s specific function, much as letters join together to form words. Together the combined genes act like a massive program influencing every detail of that individual’s appearance and behavior.
(For many traits, environmental influences also play a part—for example, an individual’s body shape is determined by factors such as nutrition and exercise as well as by genetic encoding.)
• Chromosomes—Genes are strung together in a precise array on much larger structures: long strands called chromosomes, which reside in the nucleus of every cell in the body. Every chromosome is paired with another, corresponding chromosome. In every pair, one chromosome comes from the mother, one from the father.
A chromosome can be seen as a sort of package that keeps a large amount of genetic information organized. Each chromosome holds hundreds of specific genes—some coding for good traits, some coding for undesirable ones.
Different species have different numbers of chromosomes. A mosquito has six chromosomes, a pea plant has 14, a sunflower 34, and a human being 46. A dog has 78 chromosomes, or 39 pairs.
Except for egg or sperm cells, every cell of an organism’s body has within its nucleus the identical, complete set of that individual’s chromosome pairs.
• Sex chromosomes—Both of the chromosomes in every pair look alike, except in a male. There is one pair of chromosomes that determines the gender of the individual. These are called the sex chromosomes. In a female, this pair consists of two X chromosomes; in a male, it consists of an X and a Y. All other chromosomes are referred to as autosomes.
• Gametes—All animals that reproduce sexually produce special cells called gametes. The gametes produced by females are eggs, or ova; those produced by males are sperm.
Unlike all other cells in the body, which each contain that individual’s complete set of chromosome pairs, and two copies of each gene, gametes contain only one strand of each chromosome, and only one copy of each gene.
In reproduction, the two gametes fuse together to become an embryo. With the creation of the embryo, the lone chromosome strands inherited from each parent join together into corresponding pairs.
This new genetic pairing within the embryo serves as the template for the newly formed individual. From that point on, for that individual’s entire lifetime, every cell in his body will be encoded with that particular genetic combination that was created when he began as a single-celled embryo.
• Locus—Each gene is always to be found at a specific location on a specific chromosome. This location is called its locus.
• Alleles—Each gene controls a specific trait; alleles are all the various possible forms of that particular gene. For example, for one of the genes controlling coat color in Labrador Retrievers, the possible alleles are B, for black color, and b, for chocolate color.
Alleles are usually represented by symbols, with upper-case letters indicating dominant alleles and lower-case letters indicating recessive alleles.
Some traits are controlled by the interaction of only one pair of alleles, while other traits are controlled by the interaction of a series of alleles.
• Phenotype—Phenotype refers to an individual’s manifested or observable trait or traits. In terms of physical traits, phenotype is what you see.
• Genotype—Genotype is an individual’s genetic makeup.
• Dominant—A dominant allele is one that yields its corresponding phenotype when it is paired with any allele for that trait. The dominant allele can be thought of as overriding any other allele.
In the example of the gene that controls black versus chocolate coat color in Labs, the allele for black, B, is dominant over the allele for chocolate, b. Thus if one puppy in a Lab litter has a gene pair for coat color consisting of the alleles Bb, her coat will be black. Another puppy whose gene pair consists of the alleles BB will also be black.
• Recessive—Recessive alleles result in their corresponding phenotype only when paired with another copy of that same allele. In terms of our Lab litter, another puppy’s coat color will be chocolate only if his gene pair for coat color consists of the alleles bb.
• Incomplete dominance—Incomplete dominance, which sometimes occurs, is a form of intermediate inheritance. In this case, one allele for a specific trait is not completely dominant over the other allele, and a combined phenotype results.
• Homozygous—An individual is said to be homozygous for a certain trait when having two identical alleles for that gene. With the Lab litter, for example, any puppies having the allele combination of either BB or bb would be homozygous for coat color.
When an individual is homozygous for a certain trait, it indicates that each parent contributed the same allele for that trait.
• Heterozygous—An individual is heterozygous for a certain trait when having nonidentical alleles for a specific gene. The Lab puppy with the allele pair Bb would be heterozygous for coat color.
When an individual is heterozygous for a certain trait, it indicates that the parents each contributed a different allele for that trait.
It is important to understand that although for a given gene there may be more than two possible alleles or variant forms in the population, a normal individual can never have more than two alleles at any locus—only one allele inherited from each parent.
For example, although in the Pointer population there may exist a wide range of possible alleles for a gene for coat color, any individual Pointer cannot have more than two of the coat-color alleles in his particular genetic makeup. Of the two alleles that his sire possessed for that gene, the sire contributed only one of them to each of his progeny; likewise for the dam. Each parent has only one chance to influence each genetic trait in each offspring by contributing one gene at each locus. Thus every pup has exactly half his genetic heritage coming from his sire, and half from his dam.
• Genome—The total complement of genes in an organism or cell is known as its genome. The canine genome is similar in size to that of humans and other mammals, containing approximately 2.5 billion DNA base pairs.
[In Part Two, we will next look at pedigree analysis and the genetic aspects of certain strategies in dog breeding.]
Arliss Paddock breeds and shows English Cocker Spaniels and is former managing editor of the AKC Gazette.