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Glossary of Genetics Terms


Allele: Gene variations which can occur at each location (locus) of the chromosome. There is one gene variation (allele) at each location (locus). A different gene variation (allele) can occur at the same location (locus) on the second chromosome of the pair.
Back Cross: Mating of dominant phenotype with a homozygous recessive. This type of mating will determine the genotype of the dominant parent: if dominant parent is AA then AA x aa will result in 100% Aa. If dominant parent is Aa then Aa x aa will result in 50% Aa and 50% aa.
Chromosome: A DNA -containing linear body of the cell nuclei of plants and animals, responsible for the determination and transmission of hereditary characteristics. Chromosomes occur in pairs - each parent contributes of the pair to the offspring.
Chromosome Crossover: When two genes are closely linked but are not alleles, then both genes would end up on the same chromosome instead of separate chromosomes. This happening would be rare for genes that are closely linked, and the resulting chromosome with both genes would be fairly stable.
Co-dominance: Expression of heterozygous phenotype is a blend of traits seen in homozygous states. *Defined for pairs of alleles. The situation in which an animal heterozygous for two alleles (A1 and A2 at the A locus) expresses both of the phenotypes observed in the two corresponding homozygotes. Thus, the heterozygote (A1/A2) and both homozygotes (A1/A1 and A2/A2) are all distinguishable from each other and A1 and A2 would be considered to be "codominant."
Co-dominant: Also called intermediate inheritance. When different alleles at the same locus produce a blending of characteristics in the phenotype.
Consanguineous Mating: A mating is consanguineous when the mating couple is related to one another through a common ancestor (e.g., "by blood"). A consanguineous mating is one which occurs between related individuals (e.g., cousins, siblings).
Crossing Over: 1)The breaking during meiosis of one maternal and one paternal chromosome, the exchange of corresponding sections of DNA, and the rejoining of the chromosomes. This process can result in an exchange of alleles between chromosomes.2) A process in which homologous chromosomes exchange parts normally reciprocally but sometimes unequally. The exchange of corresponding chromosome parts between homologues by breakage and reunion of DNA molecules normally during prophase I of meiosis but also occasionally during mitosis.
Cytogenetic line: A group of identical, homozygous, pure-breeding diploid or polyploid organisms, distinguished from others of the same species by some unique genotype.
Dihybrid Cross: Mating individuals that are heterozygous (hybrid) at two loci.
Diploid: A cell with two complete sets of chromosomes. Dominant: When different genetic variations (alleles) occur at the same location (locus) on the chromosome and one of the variations (alleles) is not expressed when the other is present, the trait which is expressed is considered dominant.
Dose: Commonly used to designate the gene variation (allele) transmitted by a parent. A "double dose" of a characteristic, such as eye color, means each parent passed one gene of the color on to the offspring.
Epistasis: The masking of the phenotypic effect of alleles at one gene by alleles of another gene. A situation in which the phenotypic expression of a genotype at one locus depends upon the genotype at another locus.
F1 Generation: The progeny resulting from parental crosses.
F2 Generation: The progeny resulting from random crossing of individuals from the F1 generation.
F3 Generation: The progeny resulting from random crossing of individuals from the F2 generation.
Gamete: A germ cell having a haploid chromosome complement. Gametes from parents of opposite sexes fuse to form zygotes. A specialized haploid cell that fuses with a gamete from the opposite sex or mating type to form a diploid zygote; in mammals, an egg or a sperm.
Genes: Genes contain the genetic material which allows characteristics to be transmitted to the next generation.
Genotypes: The specific allelic composition of a cell, either of the entire cell or more commonly for a certain gene or a set of genes. The genes that an organism possesses.
Haploid: A cell with one complete set of chromosomes. Sperm and egg contain a haploid set of chromosomes.
Homozygote: An individual having a homozygous gene pair. A diploid or a polyploid with identical alleles at a locus.
Homozynous: A trait which is determined by two identical gene variations (alleles) at the same location (locus) on the chromosome. This results in a true strain which consistently produces that trait.
Heterozynous: A trait which is determined by two different gene variations (alleles) at the same location (locus) on the chromosome. This results in a hybrid which does not consistently pass that trait to offspring, although a percentage of the offspring will have that trait.
Hybrid: When used in the context of genetics it refers to an offspring resulting from a cross of two genetically varied parents, i.e., two different alleles. A hybrid will not breed true for all characteristics. The breeder can expect that a percentage of the hybrid Angelfish offspring to be true breeding.
Inbreeding: Mating of closely related parents, i.e. brother to sister, mother to son, etc.
Incomplete dominance: The situation in which both alleles of a heterozygote influence the phenotype. The phenotype is usually intermediate between the two homozygous phenotypes. The situation in which a heterozygote shows a phenotype somewhere (but not exactly half-way) intermediate between the corresponding homozygote phenotypes. (Exact intermediacy is no dominance.)
Incomplete Penetrance: A dominant gene that is not always expressed.
Intermediate Inheritance: When different genetic variations (alleles) occur at the same location on the chromosome (locus) and the traits expressed by the variations are blended. Each trait is an incomplete dominant.
Line-breeding: The process whereby relatives are bred to one another in a breeding process.
Locus or Loci: A given gene occupies a specific place on a specific chromosome. The location of the gene is its locus (plural, loci: Latin, meaning place). A specific point along a chromosome is a locus, whether there is a gene there or not, and you can talk about a gene without knowing where its locus is. Much of molecular genetics involves finding the locus for your gene of interest, because then you can isolate the gene, sequence it, and figure out what the protein is and what it does.
Monohybrid Cross: Mating individuals that are heterozygous (hybrid) at one loci.
Mutation: An abrupt permanent change in the genetic material. The mutation is a sudden change in the genetic material as opposed to a variation over generations of gradual change. A mutation rarely breeds true, but can be a quite valuable individual used for breeding if the mutation is attractive.
Outcrossing: Mating of parents that are several generations apart.
Phenotype: The total characteristics of an individual (i.e. it's appearance) resulting from interaction between its genotype (qv) and its environment. 1)The form taken by some character (or group of characters) in a specific individual.2)The detectable outward manifestations of a specific genotype.3)The observable attributes of an organism.
Progeny: Predicted offspring types from particular crosses.
Progeny testing: Breeding of offspring to determine their genotypes and that of their parents.
Punnet square: 1) A table listing all male gamete types across the top and all the female gametes down the side. Subsequent combination of the gametic types give the genotype of each of the possible offspring types. 2)A diagrammatic representation of a particular cross used to predict the progeny of the cross. A grid used as a graphic representation of the progeny zygotes resulting from different gamete fusions in a specific cross.
Recessive: When different gene variations (alleles) occur at the same location (locus) on the chromosome and one of the variations (alleles) is not expressed when the other is present, the trait which is not expressed is considered recessive.
Strain: A group of organisms of the same species, having distinctive characteristics, but not usually considered a separate breed or variety.
Wild-type: The genotype or phenotype that is found in nature or in the standard laboratory stock for a given organism. The phenotype of a particular organism when first seen in nature.
Zygote: The unique diploid cell formed by the fusion of two haploid cells (often an egg and a sperm) that will divide mitotically to create a differentiated diploid


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