Genetics Terms
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Glossary of Genetics Terms |
| by C. Williams | |
Locus(plural= "loci") Refers to a specific location on a chromosome that affects a certain trait or characteristic. Each individual possesses two alleles at any particular locus (with the exception of the Sex or "XY" chromosome pair.) There may be one, two or more different possible or existing alleles for a particular locus, but a specific individual will possess any/only two.
AlleleOne half of a "gene pair". Each "gene" consists of two alleles. An allele is a segment of "genetic instructions" that affect a trait or characteristic.
Trait or CharacteristicThe physical manifestation of gene actions. A trait could be blue eyes, yellow coat color, and so forth.
GenotypeRefers to the exact genes and alleles (genetic code) an individual possesses.
PhenotypeThe physical result you can see (or measure) that a genotype produces.
HomozygousWhen both alleles of a gene pair (locus) are the same, they are known as homozygous.
HeterozygousWhen each allele of a gene pair (locus) are different from each other, they are known as heterozygous.
DominantRefers to an allele that will always express itself in the phenotype when it is present. It will "override" other recessive alleles.
RecessiveRefers to an allele that can only express itself in the phenotype when a dominant allele is not present along with it. Recessive alleles can only express themselves when they are homozygous... that is, when both alleles are recessive, and there is no dominant allele to override them.
Partial or Incomplete DominantRefers to an allele that has a different or more intense phenotypic action in its homozygous state, than in it's heterozygous state. When the genotype is heterozygous for this locus, only part of the trait is visible in the phenotype. When homozygous for both alleles... the effect can be said to be "doubled"-- the two alleles together cause more visible effect. A good example of Partially Dominant alleles is at the Cream (C) Locus, that is responsible for Palomino and Buckskin Colors.
Co-Dominant AllelesRefers to the case when two different alleles are present, both express themselves in the phenotype. This concept is best exemplified with bloodtypes, in which the heterozygote will retain characteristics of both of the bloodtype alleles simultaneously.
EpistasisRefers to the condition where the genotype at one locus has a direct effect on the expression (phenotype) of the gene pair/alleles at a different locus. Typically... the instructions at one gene locus masks or overrides the instructions at another. The locus that masks another is called the Epistatic locus. The locus that is masked, or overridden, is referred to as the Hypostatic locus.
The fundamental determination of the basic coat colors Black, Bay and Chestnut, is due to Epistasis occurring at two loci, A and E.
F1 or First GenerationThe first generation of offspring from two parents, each of which are homozygous for different alleles at the same locus, in order to begin to establish the type of gene action of the trait in question.
F1 offspring are often referred to as "hybrids", and are heterozygotes. The "F" refers to Foundation-- the original parents are the foundation stock.
F2 or Second GenerationThe offspring of two F1 or First Generation parents. These offspring often clearly define the type of gene action of the locus being studied, and may consist of homozygous and heterozygous individuals.
EumelaninThe black pigment contained in the hair shaft.
PheomelaninThe red pigment contained in the hair shaft.
Dilute or Dilution GenesA dilute gene is one that causes the hair (and/or skin) pigment to be diluted... that is, contains less pigment. The loci for Cream, Champagne and Dun are examples of Dilution genes. A "Dilute Color" refers to a phenotypic color produced by a dilute gene.
Wild-Type GenesGenes believed to be present in a species prior to domestication are referred to as "Wild-Type". Most known wild-type genes are believed to be dominant.
Lethal Traits or GenesGenes that are lethal (cause the animal's death either prior to birth or soon after) are usually Partially Dominant. Many have no detrimental effect in their heterozygous form, but are lethal in their homozygous forms. Classic Roan and Lethal White/Frame Overo are known lethal genes in the horse.
Linked Traits or LinkageLoci that are very close together on the same chromosome are often "linked". That is, the genetic material "crosses over" in groups and blocks of amino acids (rather than completely randomly) and the linked alleles are very likely to be transferred to the offspring together as a "team". The closer the loci are to each other on the chromosome, the less likely the "team" will be broken up. The farther away the loci are... the more likely the "team" will be broken up. Loci that are on different chromosomes are not considered "linked".
Sex-ChromosomesThe chromosomes that determine whether an animal will be male or female are known as the Sex Chromosomes or the X and Y Chromosomes. Females have a XX Genotype, while males have a XY Genotype. The female X chromosome contains more genetic material than the male Y chromosome, unlike the other chromosome pairs. (All other chromosomes are known as Autosomes.)
Sex-Linked TraitsSince the female X chromosome is larger and contains more genetic material than the male Y counterpart, it is possible for the female chromosome to contain loci for which there is no male counterpart. For those loci, the animal receives only one allele if it is male, and will express that allele, even if it is recessive. Hemophilia is a known Sex-Linked genetic condition.
Polygenic TraitsTraits or phenotypic character that is the result of the interaction of many genes. Coat color is a polygenic trait in the horse... many different genes interact to create the result you see.
HermaphroditeA condition in which an animal possesses more than two sex chromosomes. Sometimes the individual displays characteristics of both genders. (This occurrence is rare to non-existent in horses.)
MutationA mutation is basically a "typo" that can sometimes occur when cells replicate themselves. Some factors such as certain chemicals, as well as radiation, can cause more mutations than usual to occur.
MutagenicAn environmental factor that causes genes to mutate more frequently. Radiation is a known mutagenic. Some chemicals are known to have mutagenic effects. HYPP is a documented modern-day mutation that occurred in the horse.
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