Good Dog genetics glossary

By Dr. Mikel Delgado, PhD

At Good Dog, there’s a lot of talk about genetics and sometimes it can become overwhelming. We’ve created this reference so that you can have a solid overview of terms that come up in our health articles, webinars and podcasts. If you have suggestions for terms we should add, please let us know.

Additive

Many traits (such as eye color, degree of hip dysplasia) are caused by multiple genes. When multiple genes “contribute” to a phenotype they are considered “additive” when the combined effect is equal to the sum of the individual effects of each allele.

Affected

When an individual has a heritable disease.

Allele

An alternative form of a gene. For example, there are two alleles for the dilute locus which can lighten coats: D (dominant full color) and d (recessive dilute). 

Allele frequency

How frequently an allele occurs in a population. For example, a 2019 study found that the tailless allele (causing a dog to be born with a shortened or absent tail) was detected in 13% of tested Australian Shepherds.

Artificial selection

Intentional breeding for specific traits. This requires variation in a trait (e.g., some dogs have pointed ears, some have rounded ears) and that the trait is inherited. Animals with that trait are selectively bred in the hopes that their offspring will also have that trait.

Autosomal

Describing all chromosomes, except for the sex-linked ones (X and Y) in mammals.

Backcrossing

The mating of a hybrid with one of its parent breeds, reducing the level of heterozygosity in a breeding pool.

Candidate gene

A gene that has been associated with a disease or trait. Candidate genes are sometimes chosen for further testing/assessment due to an association with a disease or trait in another species.

Carrier

An individual carrying one copy of a recessive allele. They do not express the phenotype/trait of the recessive allele but may pass on the allele to their offspring.

Chromosome

A structure in the nucleus of cells made of DNA that carries genetic information.

Codominance

When both versions of an allele are expressed in a heterozygous individual, e.g., the cause of white spotting in some dogs. In humans, a commonly used example of codominance is the A-B blood type in humans, which is caused by inheriting one A allele, and one B allele.

COI

The Coefficient of Inbreeding indicates the level of inbreeding in a pedigree and is a measure of the probability of receiving two identical copies of an allele (be homozygous) due to inheritance. 

Complex trait

Traits that are impacted by more than just simple inheritance (Mendelian laws). Complex traits are common, and are believed to be impacted by multiple genes, as well as by the environment in addition to genetic factors.

Complete penetrance

The relationship between possessing a given allele and expressing an associated trait. When penetrance is complete, all individuals with that specific genotype will express the trait.

Congenital

A trait that is present from birth. Can be, but is not necessarily, inherited.

Crossbreed

Dogs with parents who are from different breeds.

Deletion

A mutation in which part of the chromosome is omitted.

DNA

DNA stands for deoxyribonucleic acid, which is the hereditary material contained in our cells (usually in the nucleus). The building blocks of DNA are the chemical bases: adenine (A), guanine (G), cytosine (C), and thymine, which are the “instructions” for building proteins.

Diploid

A cell with two copies of each chromosome. One copy is inherited from each parent.

Dominance

When one allele masks or hides the effect of another. The trait is expressed when at least one dominant allele is present. 

Epigenetics

The study of how the environment can impact genetic expression and how your body interprets your DNA sequence, without altering the DNA sequence.

Epistatic

When the effect of one genetic mutation is dependent on the effects of another genetic mutation.

Estimated Breeding Value (EBV)

A statistical method that allows the calculation of which individuals in a breeding pool are more or less likely to lead to a trait. EBVs depend on information about a dog’s pedigree. The more information about the pedigree, the better ability to predict traits in a dog’s offspring.

Expressivity

The degree to which a phenotype of a genotype is expressed. For example, a trait that causes polydactyly (extra toes) in cats will always lead to extra toes, but the number of extra toes will vary.

Fixation

When one allele increases in frequency and another is lost from a population; 100% of a population becomes homozygous for a trait.

Founder

A small group who interbreed/are interbred. This interbreeding increases the chance of a mutation occurring and becoming common in a population.

Frequency

How common an allele is in a given population.

Gene

The basic unit of heredity, a sequence of proteins that contains our DNA.

Gene expression

How a genotype influences the phenotype -- how a gene leads to a certain appearance or trait.

Genetic association

When a genotype and a phenotype co-occur more than would be expected by chance.

Genetic bottleneck

When a gene pool is restricted by a rapid and significant reduction in the size of a population. 

Genetic counseling

Advice about the risk of genetic disorders, and in the case of dog breeding, with the aim of reducing the production of dogs with heritable disorders or who are carriers.

Genetic diversity

The variety of genes and genotypes within a population.

Genetic drift

The changes in frequency of an allele over time due to chance (not due to selective breeding).

Genetic marker

A sequence of DNA that is in a known location on a chromosome, which can be used to identify a species, individual, or a gene that is associated with a disease.

Genetic predisposition

An increased risk of developing a trait or condition due to one’s genetic makeup.

Genetic testing

DNA testing, that looks for mutations in your genetic material. Some tests look at the entire genome, and other tests only inspect particular genes. In dog genetic tests, the laboratory is identifying variants in the genome that are associated with breed, traits or medical conditions.

Genome

The complete genetic sequence of an individual.

Germ cells

Sperm and eggs, also known as gametes; each contains half of the chromosomes needed by an individual. In sexually reproducing species, the germ cells combine to create the genetic information of their offspring.

Germ-line mutation

A heritable change to the DNA sequence.

Haploid

A cell with a single set of chromosomes (such as germ cells).

Haplotype

A group of alleles that tend to be inherited together.

Heredity

The passing on of traits from parents to their offspring.

Heterozygous

Having two different forms of a gene (alleles) at a loci.

Homologous chromosomes

Matched pairs of chromosomes, one inherited from each parent.

Homozygous

Having two identical forms of a gene (alleles) at a loci.

Hybrid

The offspring of two parents from different species or breeds.

Hybrid vigor (heterosis)

An increase in fertility or other biological enhancements which are sometimes observed in hybrid individuals.

Inbreeding

When related individuals breed, including dogs with greater shared ancestry than the species average.

Incomplete penetrance

A trait that is not expressed in all individuals with an associated allele.

Linkage

Genes that are close to one another (physically) on the chromosome. Linked genes are often inherited together.

Loci/locus

A specific location on a chromosome.

Marker

A segment of DNA that has a known location on a chromosome, typically used for identification purposes.

Mendelian 

Related to traits that follow patterns of single-gene inheritance, as described by Austrian monk and scientist Gregor Mendel in the 1800s. Mendel described three laws of inheritance:

• Law of segregation: each germ cell carries one allele for a gene
• Law of dominance: some alleles are dominant and some are recessive
• Law of independent assortment: different traits are inherited independently

Mode of inheritance

How a genetic trait is inherited, e.g., sex-linked, autosomal dominant or recessive.

Natural selection

The process of evolution (adaptation and change of individuals) where some traits:

• Are heritable
• Vary in form
• Lead to different rates of survival and reproduction

Over time, advantageous traits become more common in the population.

Outcrossing

The introduction of novel genetic material into a breeding population.

Pedigree

The recorded ancestry of an individual.

Penetrance

The proportion of individuals that possess a trait when they have a dominant allele. 

Phenotype

Expressed traits, including physical features, appearance, and behavior.

Pleiotropy

When one gene can influence multiple traits.

Ploidy

How many sets of chromosomes a cell contains.

Polygenic trait

One trait that is influenced by multiple genes.

Polymorphic

A gene with multiple possible alleles.

Prevalence

The frequency of an inherited condition within a population.

Purebred

Producing offspring with the known characteristics, typically the same traits as the parents.

Recessive

A trait that can be masked by other traits. Typically expressed when an individual inherits two copies of a recessive allele (is homozygous).

Selection

In the case of breeding dogs, choosing which individuals will be mated, usually to increase the likelihood of the presence or absence of certain traits.

Sex linkage

A gene on a sex chromosome, usually on the X chromosome. Inheritance patterns can be influenced by the fact that males have one X and one Y chromosome, whereas females have two X chromosomes. The Y chromosome has few associated genes.

Silent mutation

Changes to an individual’s DNA that do not result in an observable change in phenotype.

Trait

A measurable characteristic of an organism.

True breeding

When a parent always passes on certain traits to their offspring.

Variant

A region of the genome that can differ.

Wild type

The ancestral or typical version of a gene in a population or species.

X-linked

A gene that is only present on the X chromosome. Typically all males (X|Y) that inherit an allele related to an X-linked trait will express that trait (whether it is recessive or dominant). Females (X|X) will only express an X-linked recessive trait if they inherit two copies of the associated allele.

Zygosity

Whether the alleles at a loci are the same or different.