Essential Genetics Terminology and Definitions

Fundamental Genetics Terminology

• Homologous Chromosomes: The two members of a pair of chromosomes that are physically identical in shape, length, and centromere position. In organisms that reproduce sexually, each parent contributes a member of the pair; the counterpart is called the homolog.
• Gene: The unit of inheritance. Gene pairs are located in corresponding positions on homologous chromosomes, with each member of the pair known as an allele.
• Alleles: Genes located at the same locus on a pair of homologous chromosomes. Alleles interact to determine a particular trait and are usually designated by pairs of letters, where each letter represents a particular character.
• Recessive: Refers to a character that is expressed only in its homozygous state. Recessiveness is generally designated by lowercase letters.
• Segregation: The separation of gene alleles into different gametes; this regularly occurs during meiosis.
• Phenotypic Ratio: The number of genotypic combinations that determine the expression of different alternatives of a character in a cross.
• Test-Cross: A cross conducted between an individual whose genotype is unknown and an individual that is homozygous recessive.
• Hybrid: An organism resulting from a cross between parents who differ in one or more pairs of alleles.
• Locus: The specific position or place of a gene on a chromosome.
• Genome: The complete set of genes found in a set of chromosomes.
• Haploid Set: The set composed of one representative from each pair of homologous chromosomes. This corresponds to exactly half the full or diploid number of chromosomes.
• Genotype: The specific genetic makeup of an organism, representing the type of genes an individual possesses for a particular character.
• Phenotype: The character produced by the expression of genes identified by the genotype and the environment in which an organism develops.
• Homozygous: A condition characterized by the existence of identical alleles. An individual whose alleles for a particular character are equal or indistinguishable is said to be homozygous for that character.
• Heterozygosity: The existence of different alleles for a given character. The term heterozygote is sometimes used as a synonym for hybrid.
• Monohybrid Cross: A cross between parents who differ with respect to a single pair of specific alleles.
• Dominant: Refers to a character that is expressed in both the homozygous and heterozygous states. Dominance is generally designated by capital letters (e.g., A, B, C).