Genetics Vocabulary - Glossary of Terms

COMMON GENETIC TERMS

Allele: a variant form of a gene.

Allele frequency: the fraction of all the alleles of a gene in a population that are of one type.

Carrier: an individual who carries and is capable of passing on a genetic mutation associated with a disease and may or may not display disease symptoms. Carriers are associated with diseases inherited as recessive traits. In order to have the disease, an individual must have inherited mutated alleles from both parents. An individual having one normal allele and one mutated allele does not have the disease. Two carriers may produce children with the disease.

Chromosome: strings of genes (DNA  that  codes  for  proteins) with  non-coding DNA between them. Offspring get half of their chromosomes from their dam and half from their sire.

Crossbreeding: Crossbreeding is the general practice of mating two dogs of different varieties or breeds. See: Cross-breeding - Articles - DogWellNet.com

DNA: Deoxyribo Nucleic Acid: made up of very long chains of chemical 'letters': Adenine (A), Guanine (G), Thymine (T) and Cytosine (C).

DNA sequencing: A laboratory technique used to determine the exact sequence of bases (A, C, G, and T) in a DNA molecule. The DNA base sequence carries the information a cell needs to assemble protein and RNA molecules. DNA sequence information is important to scientists investigating the functions of genes.

Dominant: refers to the relationship between two versions of a gene. Individuals receive two versions of each gene, known as alleles, from each parent. If the alleles of a gene are different, one allele will be expressed; it is the dominant gene. The effect of the other allele, called recessive, is masked.

Estimated Breeding Value (EBV): EBVs are a calculation that, in the case of dogs, generally uses the clinical/phenotype data (such as hip or elbow scheme scores) along with the clinical/phenotype data in a dog's pedigree, to estimate the likely genes/genotype of the dog which may be passed on to their offspring. EBVs can be calculated for wide variety of genetic attributes that cannot be determined through other tools, such as a genetic test. A good-quality EBV is a much better estimation of a dog's risk (or quality) than phenotypic information alone. (See also: Genetic Breeding Value)

Exon: the portion of a gene that codes for amino acids.

Founder effect: is the reduction in genetic variation that results when a small subset of a large population is used to establish a separate breed population (purebred dog breeds).

Gene: the basic physical and functional unit of heredity. Genes, which are made up of DNA, act as instructions to make molecules called proteins. Genes are the basic physical unit of inheritance. Genes are passed from parents to offspring and contain the information needed to specify traits.

Gene dropping: loss of alleles due to genetic drift.

Gene mapping: the process of establishing the locations of genes on the chromosomes.

Genetic Breeding Value (GBV): GBVs use genomic data (as opposed to phenotype data alone in EBVs), normally SNPs, to estimate a breeding value for an individual. Calculating a GBV is one tool that can be used to better estimate risk of inheritance of traits with low-heritability. E.g. complex conditions that are inherited, but may have low-predictability of inheritance due to a combination of multiple genes, environmental factors, etc. GBVs are more common in livestock. (See also: Single Nucleotide Polymorphisms (SNPs) and Estimated Breeding Values.)

Genetic counseling: the professional interaction between a healthcare provider with specialized knowledge of genetics and an individual. The genetic counselor determines whether a condition in the family may be genetic and estimates the chances that another relative may be affected. Genetic counselors also offer and interpret genetic tests that may help to estimate risk of disease.

Genetic heterogeneity: a common phenomenon with both single-gene diseases and complex multi-factorial diseases. It should not be surprising that multiple affected family members may experience different levels of disease severity and outcomes. This effect may be due to other genes influencing the disease phenotype or different mutations in the same gene resulting in similar, but not identical phenotypes.

Genome: an individual's complete set of DNA, including all of its genes. Each genome contains all of the information needed to build and maintain that individual. 

Genotype: the genetic contribution to the phenotype (physical traits).

Heterozygous: Two genes in the same location that are unlike in action (Aa). Dominant traits may be expressed in the heterozygous state.

Homozygous: Two identical genes (a matched pair) in the same location for the same trait (AA).

Incomplete penetrance: if not all individuals tested and shown to have a mutation develop features of the disorder associated with the mutation, then the condition is said to have reduced or incomplete penetrance.

Linkage: the close association of genes or other DNA sequences on the same chromosome. The closer two genes are to each other on the chromosome, the greater the probability that they will be inherited together.

Locus|Loci: the specific physical location of a gene or other DNA sequence on a chromosome.

Marker: A genetic marker is a DNA sequence with a known physical location on a chromosome. Genetic markers can help link an inherited disease or trait with the responsible gene. 

Mendelian Modes of Inheritance:

•  Autosomal Dominant (Each affected dog has an affected parent, occurs in every generation);
•   Autosomal Recessive (Both parents of an affected dog are carriers, not typically seen in every generation);
•  X-linked Dominant (Females more frequently affected, can have affected males and females in same generation);
•  X-linked Recessive (Males more frequently affected, affected males often present in each generation);
•  Mitochondrial (Can affect both males and females, but only passed on by females, can appear in every generation)

ALSO SEE: the HGTD Database Resources article: Acronyms - Mode of Inheritance

Monomorphic genes: have only one common allele (rare alleles with frequencies of less than 0.001% may still occur).

Mutation: a change in the sequence of the base pairs in a DNA molecule.

Non-Mendelian Inheritance: complex inheritance - co-dominance, incomplete dominance, multiple alleles, and environmental influences.

Outcross: Out-crossing, sometimes called out-breeding is the practice of crossing between different dogs of the same breed, with no known common ancestors. The opposite of linebreeding or inbreeding. 

Phenotype: observable traits, such as height, eye color, and blood type.

Polygenic: characteristics are controlled by more than one gene, and each gene may have two or more alleles. The genes may be on the same chromosome or on nonhomologous chromosomes. Many polygenic traits are also influenced by the environment and are called multifactorial.

Population genomics: application of genomic technologies to understand populations. In dogs, population genomics typically refers to applying technology in the quest to understand how genes contribute to health and well-being.

Recessive: In the case of a recessive genetic disorder, an individual must inherit two copies of a mutated allele in order for the disease to be present. Identification of these animals carrying recessive allele mutations linked to disease development and expression is important to a breeding program since the dog appears completely normal but can produce affected offspring.

Short Tandem Repeat (STR): An STR is a place in DNA code where a sequence is repeated. Eg: ATAGATAGATAG is ATAG repeated 3 times. STRs have a fast mutation rate. When they change, it is an increase or decrease in the number of repeats. You may see genetic tests, such as parentage testing, that are based on STR or SNPs (See: Single Nucleotide Polymorphism). These methods are not interchangeable, and do not "read" to each other. 

Single Nucleotide Polymorphism (SNP): Single nucleotide polymorphisms, called SNPs (“snips”), are the most common type of genetic variation in animals (including people). Each SNP represents a difference in a single DNA building block, called a nucleotide. Many genetic tests, and other genetic tools, are based on SNPs - the places along the genome where variations/differences can occur. 

Trait: is a specific characteristic of a dog. Traits can be determined by genes or the environment, or more commonly by interactions between them. The genetic contribution to a trait is called the genotype. The outward expression of the genotype is called the phenotype.

Variable expressivity: refers to the range of signs and symptoms that can occur in different dogs with the same genetic condition.