DICTIONARY OF GENETICS
COMPARATIVE GENOMICS CENTRE

Mail Address: Comparative Genomics Centre,
Molecular Sciences Bldg 21, James Cook University,
Townsville, 4811, Queensland, Australia
Telephone: 61-7-4781 6265 Fax:  61-7-4781 6078



TERM DEFINITION
 
Allele One of a series (of at least 2) forms of a gene, differing in genetic sequence and resulting in observable differences in heritable character (phenotype).
 
Anticipation The tendency for a genetic disorder to become more severe, show earlier onset, or both in succeeding generations. The usual cause is an unstable tandem nucleotide repeat.
 
Association Study A test for linkage which measures the relative frequency with which a particular polymorphism occurs together with (i.e. is associated with) the trait of interest in a population.
 
Complex Trait A trait resulting from the interplay of multiple factors, each of which has a relatively small effect.
 
Concordance Phenotypic similarity. Twins are said to be concordant for a given trait if both express it.
 
Dominant Descriptor for an allele that fully manifests its phenotype when present in an individual who has inherited different alleles at the locus of homologous chromosomes (i.e. in heterozygosity).
 
Gene A hereditary unit that occupies a position in the genome (locus) and conveys a phenotype
 
Heritability The proportion of risk of developing a trait that is atributable to genetic variation, within a specific range of environmental conditions (h2).
 
Heterozygosity A state of having one or more pairs of dissimilar alleles at the corresponding loci of homologous chromosomes.
 
Homolog A disparate gene, so similar in genetic sequenece that it is assumed to have originated from a single gene in a ancestral species. May be a paralog or an ortholog
 
Homozygosity A state of having identical alleles at the corresponding loci of homologous chromosomes.
 
Imprinting A (relatively unusual) process by which the phenotype conveyed by an allele is affected by its parental origin. 
 
Liability The total risk of developing a trait, incorporating all sources of variation, including those external to the organism.
 
Linkage A greater association of two alleles at two different loci than is expected by random assortment. Genes are linked because they exist on the same genomic segment (e.g. chromosome).
 
Loci Pleural of locus
 
Locus The position that a gene occupies within a segment of genome, such as a chromosome (pleural: loci)
 
Ortholog A gene from a disparate species, so similar in genetic sequenece that it is assumed to have originated from a single gene in a common ancestral species.
 
Paralog A disparate gene within a common species, so similar in genetic sequenece that it is assumed to have originated from a single gene in a ancestral species.
 
Penetrance The proportion of individuals of a specific genotype that express the associated phenotype under defined environmental conditions.
 
Phenocopy An alteration of phenotype, resembling that produced by a specific gene, induced by an environmental modification.
 
Phenotype Observable properties of the hertitable character of an organism.
 
Qualitative Trait A categorical (discontinuous) hertitable character of an organism (e.g. sick, albino, colour blind).
 
Qantitative Trait A metric (continuous and measurable) hertitable character of an organism (e.g. height, weight, blood pressure, milk production). 
 
Recessive Descriptor for an allele that only manifests its phenotype when present in an individual who has inherited that allele at that locus of homologous chromosomes (i.e. in homozygosity). 
 
Recombination Occurrence of progeny with combinations of genes other than those that occurred in the parents due to independent assortment or crossing over.
 
Segregation Separation into different gametes (or progeny) of each of a pair of alleles possessed by a diploid parent at a locus. 
 
Susceptibility The innate (genetic) tendency to develop a trait.
 
Trait An observable property of the hertitable character of an organism (a phenoype).
 
Transmission/ 
disequilibrium Test
A test for linkage which compares the number of times an allele of interest is transmitted from heterozygous parents to selected offspring with the number of times some other alleles are transmitted from these same heterozygous parents to selected offspring.


OTHER GENETICS LINKS OTHER LINKS: Comparative Genomics Centre, James Cook University, Key words: Autoimmune diabetes, Type 1 diabetes mellitus, childhood diabetes, lupus, systemic lupus erythematosus, haemolytic anaemia, hemolytic anemia, Coombs' test, antinuclear antibodies, renal failure, glomerulonephritis, gastritis, type A gastritis, pernicious anemia.