Glossary
Welcome to the glossary
Allele
One of the variant forms of a gene, differing from other forms in its DNA sequence.
Amino Acid
One of the building blocks of proteins. There are twenty primary amino acids that can be included in a protein, as specified by the genetic code. Secondary amino acids exist as metabolic intermediates that are not incorporated into proteins.
Autosome
One of the chromosomes that is not a sex chromosome.
Base
One of the building blocks that is part of a nucleic acid. In DNA, the bases are adenine (A), cytosine (C), guanine (G), and thymine (T). In RNA, thymine is replaced by uracil (U).
Base substitution
A kind of mutation in which a base in one position in a DNA molecule is replaced by a different base.
See also: Deletion, Insertion, Conservative substitution, Nonconservative substitution, Nonsynonymous substitution, Synonymous substitution
Carrier
An individual that is heterozygous for a recessive allele.
Chromosome
One of the separate portions of the genome that is visible during cell division, consisting of a single molecule of DNA complexed with proteins.
Codominant
One of a series of terms applied to the phenotypic effect of a particular allele in reference to another allele (usually the standard wild-type allele). An allele "a" is said to be codominant with respect to the allele "A" if the A/a heterozygote fully expresses both of the phenotypes associated with the A/A and a/a homozygotes. An example of codominance is the ABO blood type antigens in humans, where AA individuals are type A, BB individuals are type B, and AB individuals are type AB.
See also: Dominant, Recessive, Semidominant
Codon
A group of three adjacent bases in a molecule of DNA or RNA that has the capacity to specify a particular amino acid (or a "stop" signal) using the genetic code.
Complex trait
A trait that is influenced by multiple genetic and environmental factors. In horses, racing performance is a complex trait.
See also: Mendelian trait
Conservative Substitution
A kind of nonsynonymous substitution that changes a codon from the wild-type codon to a different one that encodes a different amino acid, but one which is chemically similar to the amino acid being replaced.
See also: Nonconservative substitution, Synonymous substitution
Deletion
The removal of one or more bases from a DNA sequence.
See also: Base substitution, Insertion
DNA Sequencing
Experimental determination of the specific order of the bases (A, C, G, and T) in a DNA molecule.
Dominant
One of a series of terms applied to the phenotypic effect of a particular allele in reference to another allele (usually the standard wild-type allele) with respect to a given trait. An allele "A" is said to be dominant with respect to the allele "a" if the A/A homozygote and the A/a heterozygote are phenotypically identical and different from the a/a homozygote. For example, in humans, the wild-type allele of the tyrosinase gene is dominant to the variant allele that causes albinism.
See also: Codominant, Semidominant, Recessive
Enzyme
A protein that serves as a biological catalyst, accelerating the rate of a chemical reaction.
See also: Structural protein
Frameshift mutation
A kind of mutation resulting from the deletion or insertion of a number of bases that is not an integral multiple of three. This results in the codons downstream of the mutation being read as a sequence of the wrong amino acids, frequently resulting in a stop codon being reached.
See also: Nonsense mutation
Gamete
A specialized cell that functions in sexual reproduction; in mammals, sperm cells and egg cells.
Genetic code
The relationship of the 64 possible three-letter codons in nucleic acids to the twenty primary amino acids. The genetic code consists of 61 codons that specify primary amino acids (most with synonyms), 3 codons that specify the end of a protein, and a single codon that specifies both a particular amino acid (methionine) and specifies the beginning of a protein.
Genetic test
A method for determining the genotype of an individual using molecular methods, for example, PCR.
Genetics
The field of biology concerned with heredity and variation.
Genotype
The genetic makeup of an individual, consisting of a description of the alleles present for one or more genes.
Genome
All of the genetic material (DNA) present in a single cell of an individual.
Genomics
The comprehensive study of complete sets of genes and their interactions rather than the study of individual genes and traits.
Heritability
The proportion of observed variation in a particular trait that can be attributed to inherited genetic factors, as opposed to environmental factors.
Independent events
In probability, two events, A and B, are independent events if the occurrence of A does not affect the probability of the occurrence of B. For example, coin tosses are independent events; if two fair coins are tossed at the same time, the outcome of heads on one coin does not affect the probability of heads on the other coin.
See also: Mutually exclusive events
See also: Introduction to Probability
Insertion
The addition of one or more bases of DNA within a given DNA sequence.
See also: Base substitution, Deletion
Mendelian trait
A trait that is controlled by a single genetic factor (alleles of a single gene), with no contribution to the trait by environmental factors. In horses, Glycogen branching enzyme deficiency (GBED) is a Mendelian trait.
See also: Complex trait
Metabolic intermediate
Molecules that are precursors or breakdown products of biologically significant molecules (e.g. primary amino acids).
Monomer
A molecule that can react chemically with another molecule of the same type to form a larger molecule of the same type to form a larger molecule, such as a dimer, trimer, tetramer, or polymer. For example, amino acids are the subunits of proteins.
Mutation
A change in the DNA sequence of part of the genome that differs from wild type.
Mututally exclusive events
In probability, two events, A and B, are mutually exclusive if the occurrence of A excludes the occurrence of B. For example, the outcome of heads or tails on a single toss of a single coin are mutually exclusive events; if a fair coin is tossed, the outcome of heads excludes the outcome of tails. The outcome will be one or the other. There can be more than two mutually exclusive outcomes in any given trial; for example, when drawing a single card from a deck of 52 cards, drawing the Ace of Spades or drawing the Queen of Hearts are mutually exclusive events.
See also: Independent events
See also: Introduction to Probability
mRNA
Messenger RNA, the RNA copy of a gene that is translated into a protein.
Nonsense mutation
A kind of base substitution that causes a codon normally encoding an amino acid to be replaced by a stop codon, resulting in the synthesis of a protein that is shorter than the wild-type protein.
See also: Frameshift mutation
Nonconservative substitution
A kind of nonsynonymous substitution that changes a codon from the wild-type codon to a different one that encodes a different amino acid, which is not chemically similar to the amino acid being replaced.
See also: Conservative substitution, Synonymous substitution
Nonsynonymous substitution
A kind of base substitution that changes a codon from the wild-type codon to a different one that encodes a different amino acid.
See also: Conservative substitution, Nonconservative substitution, Nonsense mutation, Synonymous substitution
Nucleic acid
DNA or RNA, two closely related compounds that are polymers of units consisting of a sugar molecule (ribose or deoxyribose), a phosphate group, and a base (A, C, G, T or U).
Penetrance
The proportion of individuals with a particular variant that eventually develop the condition associated with that variant. In the case of variants with complete penetrance, all of the individuals with the variant will develop the condition. In the case of variants with incomplete penetrance, only a portion of the individuals with the variant will develop the condition. In horses, GBE1-T34X, the variant associated with Glycogen branching enzyme deficiency (GBED), is recessive with complete penetrance, while GYS1-R309H, the variant associated with Polysaccharide Storage Myopathy type 1 (PSSM1) is semidominant with incomplete penetrance.
Phenotype
A description of the observable state of an individual with respect to one or more inherited traits. Often, individuals with different genotypes display the same phenotype.
PCR
Polymerase Chain Reaction, a method of amplifying a specific portion of the genome for a genetic test.
Polymer
A molecule that has a structure consisting of similar units bonded together. For example, a protein is a polymer of amino acids.
Polymorphism
The presence of variant alleles in a population.
Predisposition
An inherited tendency to develop a particular trait, possibly in response to environmental factors.
Probability
A field of mathematics that deals with calculating the likelihood of occurrence of a specific event. The likelihood of an event is expressed as a number between zero (the event will never occur) and one (the event is certain). For example, the probability of an outcome of heads on the toss of a fair coin is ½ or 0.5. The probability of an event can also be expressed as a percentage (e.g., an outcome of heads on the toss of a fair coin is 50% likely) or as odds (e.g., the odds of heads on the toss of a fair coin is 1:1).
See also: Introduction to Probability
Product Rule
In probability, the rule that allows the calculation of the probability of the simultaneous occurrence of two independent events. For example, the probability of the outcome of two heads when two fair coins are tossed at the same time is the product of the probability of each outcome, in this case, ½ x ½, or ¼.
See also: Sum Rule
See also: Introduction to Probability
Protein
A polymer of amino acids in a specific order. The order is determined by the DNA sequence of the gene that encodes the protein.
Recessive
One of a series of terms applied to the phenotypic effect of a particular allele in reference to another allele (usually the standard wild-type allele) with respect to a given trait. An allele "a" is said to be recessive with respect to the allele "A" if the A/A homozygote and the A/a heterozygote are phenotypically identical and different from the a/a homozygote. For example, in humans, the variant allele of the tyrosinase gene that causes albinism is recessive to the wild-type allele.
See also: Codominant, Dominant, Semidominant
Replication
The process of producing an exact copy of a DNA molecule.
Semidominant
One of a series of terms applied to the phenotypic effect of a particular allele in reference to another allele (usually the standard wild-type allele) with respect to a given trait. An allele "A" is said to be semidominant with respect to the allele "a" if the A/A homozygote has a mutant phenotype, the A/a heterozygote has a less severe phenotype, while the a/a homozygote is wild type. An example is the GYS1-R309H allele of glycogen synthase associated with PSSM1 in horses, which has increased enzyme activity. Homozygotes (P1/P1) have higher levels of enzyme activity than heterozygotes (n/P1), which in turn have higher activity than homozygous normal horses (n/n).
See also: Codominant, Dominant, Recessive
Sex chromosome
One of the chromosomes associated with sex determination; in mammals, the X and Y chromosomes.
See also: Autosome
Stop codon
A codon that signals that protein synthesis directed by an mRNA should cease at that position. Three of the sixty-four codons are stop codons.
See also: Genetic code
Structural Protein
A protein that is part of structural components of the body, such as bone, cartilage, or connective tissue. Collagen is an example of a structural protein.
See also: Enzyme
SNP
Single-nucleotide polymorphism, a difference between two DNA sequences at a single position; the substitution of one base for another.
Sum Rule
In probability, the rule that allows the calculation of the probability of the occurrence at least one of two or more mutually exclusive events. For example, the probability of the outcome of two heads or two tails when two fair coins are tossed at the same time is ¼ + ¼, or ½.
See also: Product Rule
See also: Introduction to Probability
Synonymous substitution
A kind of base substitution that changes a codon from the wild-type codon to a different one that encodes the same amino acid.
See also: Conservative substitution, Nonconservative substitution, Nonsynonymous substitution
Synthesis
The production of a substance from simpler materials after a chemical reaction.
Trait
A specific inherited characteristic that exhibits variation between individuals. The phenotype of an individual with respect to an inherited characteristic will consist of one or more traits.
Variant
An allele or characteristic trait that is uncommon in a population, as distinct from wild type.
Whole Genome Sequencing
The determination of the complete DNA sequence of the entire genome of an organism.
Wild type
The allele or characteristic trait that prevails among individuals, as distinct from a variant or mutant type.
