Copy of `Modern Horse Reference - Genetics Glossary`

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Modern Horse Reference - Genetics Glossary
Category: Animals and Nature > Horses
Date & country: 11/09/2007, UK
Words: 24


Allele
One 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.

Co-Dominant Alleles
Refers 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.

Dilute or Dilution Genes
A 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.

Dominant
Refers to an allele that will always express itself in the phenotype when it is present. It will 'override' other recessive alleles.

Epistasis
Refers 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 …

Eumelanin
The black pigment contained in the hair shaft.

Genotype
Refers to the exact genes and alleles (genetic code) an individual possesses.

Hermaphrodite
A 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.)

Heterozygous
When each allele of a gene pair (locus) are different from each other, they are known as heterozygous.

Homozygous
When both alleles of a gene pair (locus) are the same, they are known as homozygous.

Lethal Traits or Genes
Genes 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 Linkage
Loci 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 chr…

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.

Mutagenic
An 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

Mutation
A 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.

Partial or Incomplete Dominant
Refers 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.

Phenotype
The physical result you can see (or measure) that a genotype produces.

Pheomelanin
The red pigment contained in the hair shaft.

Polygenic Traits
Traits 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.

Recessive
Refers 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.

Sex-Chromosomes
The 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 Traits
Since 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.

Trait or Characteristic
The physical manifestation of gene actions. A trait could be blue eyes, yellow coat color, and so forth.

Wild-Type Genes
Genes 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.