Artificial selection (of dogs) – refers to genetic selection with human intervention.  This is the selection of the genetics of the next generation of dogs by humans purposefully choosing the parents.  Selection criteria can include physical characteristics as well as performance and can be influenced by novelty.  See: Natural selection (of dogs).

Base pairs – the information units of DNA, consisting of the biochemical letters A, C, G, T.

Breed club – refers to an organized club for purebred dogs that defines a dog breed’s characteristics (usually physical), and maintains a stud or pedigree book for the breed.

Breed structure – the historical (and thus genetic) relationship between modern purebred dog breeds.

Chromosome – a long molecule of DNA that acts as a scaffolding for gene sequences.  The dog has 78 chromosomes in its genome, representing 39 pairs of chromosomes.  Chromosomes come in pairs (one maternal copy, one paternal copy) because our genetic system is duplicated (doubled).  This forms the basis of our heredity.

Clade – a grouping of (dog) breeds based on DNA similarities, and reflecting breed derivations.

Closed stud book – the written and official record of the genealogy of particular animals (dogs) within a particular breed.  “Closed” refers to the fact that in order to register a dog in the stud book, both parents of the animal must already be registered in the stud book.  See: Breed club and Artificial selection (of dogs).

Crossbred dog – a dog that is the result of a mating between two purebred parents of different breeds.  Crossbred animals often do not necessarily resemble their parents, and their physical attributes do not breed true.  Crossbred animals have increased genetic variation and less problems with simple genetic diseases compared to purebred animals.  See: Purebred dog.

Dominant – a genetic trait that is dominant results in an identifiable phenotype even when only a single copy for the trait is present.

DNA – a molecule found within cells that contains biological information and that codes for biological function.  The information within DNA is passed from one generation to the next.

Epigenetics – “over” or “above” genetics.  Epigenetics results in phenotypes that are not coded for by the primary DNA sequence.  Epigenetics involves the influence and interaction of the environment on the functioning of the genome.  See: Nature versus Nurture, and “Missing heritability” problem.

Gene – a fundamental unit of biological information found within DNA and usually coding for a protein molecule.  There are about 20,000 coding genes found within the dog genome.  “Gene” is a contraction of “genesis” or origin.  We have two copies of each gene (one maternal copy, one paternal copy) because our genetic system is duplicated (doubled).  This forms the basis of our heredity.

Genetic bottleneck – a reduction in genetic variation found within a population of animals.  For the dog, an ancient genetic bottleneck was associated with the first domestication events, while more recent genetic bottlenecks have been associated with the advent of purebred dog breeds and closed stud books.

Genetic variation – a measure of the genetic differences at the DNA level found between individuals within a population.  Natural selection tends to increase genetic variation.  Artificial selection, as is practised in purebred dog breeds, tends to decrease genetic variation.

Genome – the sum total of DNA that is required to define an individual or a species.

Genotype – refers to a specific DNA sequence that codes for a particular phenotype (see:  Phenotype).  Genotypes in the form of DNA can be passed on to the next generation.

Heterozygous – “hetero-” refers to dissimilar, “zygote” refers to the early embryo. A genetic trait (DNA sequence) that is heterozygous refers to one in which its two copies are dissimilar.  Increased heterozygosity translates as decreased inbreeding and increased genetic variation.  See: Homozygous.

Homozygous – homo refers to “the same”, zygote refers to the early embryo.  A genetic trait that is homozygous refers to one in which its two copies are the same.  Increased homozygosity translates as increased inbreeding and reduced genetic variation. See: Heterozygous.

Landrace – original or historical type of animal that developed physical and behavioral traits suited to its particular environment.

Locus (singular), loci (plural) – a locus is a genetic address for a particular gene or mutation.  A particular locus (gene, mutation) can give rise to a particular phenotype or visible trait.  A locus consists of two copies of the gene in question, one that is maternally derived on the other that is paternally derived.  A locus can be homozygote (both copies identical) or heterozygote (the two copies are different).

Mendelian genetics – also called “classical” genetics or “simple” genetics.  Simple or Mendelian genetics involves the study of phenotypes resulting from DNA variations at only one site or locus on a chromosome, i.e. genetics at its most simple.  When phenotypes involve the input of multiple genes, the genetics is said to be complex or quantitative.

“Missing heritability” problem – the observation that, in humans, simple genetics (i.e. single genetic variations) do not account for much of the heritability (susceptibility) to common medical diseases and to behavioral traits.  It is now felt that common medical diseases and behavioral traits are the cumulative results of multiple genetic variations (thousands and even hundreds of thousands) within the genome, which individually contribute only a very small effect to the trait in question.  A further complication of estimates of heritability is the epigenetic contribution of the environment on the functioning of the genome.  See: Nature versus Nurture.

Natural selection (of dogs) – genetic selection via the environment.  Selection of the genetics of the next generation of dogs by the survival and reproductive success of the parents.  See: Artificial selection (of dogs).

Nature versus Nurture – the Nature versus Nurture debate is a historical debate as to the source of trait differences between individuals, with particular reference to behavioral traits.  Nature refers to the heritable (i.e. genetic) contribution to a given trait.  Nurture refers to the environmental contribution to a given trait, which is acquired.  The field of epigenetics states that the environment (nurture) can influence the functioning of an animal’s genome (nature).  A conclusion that can be made is that with respect to behavioral traits, the distinction between behavior that is inherited and behavior that is acquired is not black and white but consists of multiple shades of grey.

Phenotype – a characteristic or trait that is visible and is derived from a genetic basis, an environmental basis, or a combination of the two.  “Pheno-” refers to “phenomenon” or “that which is seen”.

Pit bull – a type of dog that is usually medium in size, short-haired, stocky (muscular) in build, with a square head.  Purebred breeds of the pit bull-type include the Bull Terrier (and Miniature Bull Terrier), the Staffordshire Bull Terrier, the American Staffordshire Terrier, the American Pit Bull Terrier, and sometimes the American Bulldog.  There is no breed of dog that is a “pit bull”; there are several breeds of dogs that belong to the pit bull- type of dog, with some controversy among experts as to which breeds these are.

Protodog – the original ancestor of the dog when it diverged from the common ancestor of the modern wolf and modern dog.  This would have been an animal that resembled the ancestral wolf physically but had learned to live in proximity to man by a change in behavior.

Purebred dog – a dog with certain defined physical characteristics (phenotype) adhering to a breed description and registered in an official pedigree or stud book, i.e. whose genealogy is documented.  The parents of a purebred dog were themselves purebred animals.  Purebred dogs tend to breed true to their breed descriptions, have reduced genetic variation (due to increased inbreeding), and more simple recessive genetic diseases than crossbred dogs.  See: Crossbred dog.

Recessive – a genetic trait that is recessive refers to the fact that two copies of the gene or mutation responsible for the trait must be present in order to have a visible phenotype.  If only one copy of a recessive trait is present, the animal is a “carrier” for that trait, but there is no visible phenotype.

Sasha – the name of the female Boxer dog whose genome was sequenced and made public in 2005.

Simple genetics – see Mendelian genetics.

SNP Simple Nucleotide Polymorphism.  This is the most simple form of genetic variation, where one version of a gene contains one nucleotide, and a second version of the same gene contains a different nucleotide at the corresponding place.  With the advent of sequencing at the genome level, SNPs have been relatively easy to identify and have been useful for studying phenotypes based on simple genetics.  SNPs have been less useful for studying and understanding complex phenotypes such as complex diseases and behavioral traits.

Traits – identifiable characteristics (or phenotypes), such as physical or behavioral characteristics that define an animal.  Genetic traits refer to genotypes as in the genes and/or mutations that code for physical traits.  See: Phenotype and Genotype.