Qualitative and Quantitative Characters, Population Genetics, and Evolution

These flashcards provide definitions for key vocabulary terms regarding qualitative and quantitative genetics, population genetics models, and the mechanisms of evolution and speciation.

Qualitative characters

Characters that show distinct classes, are little affected by the environment, and are governed by $1$ or few genes with large distinct effects (oligogene).

Quantitative characters

Characters that show continuous distribution, are generally influenced by the environment, and are controlled by several genes with small and cumulative effect.

Pleiotropy

The phenomenon of a single major gene affecting more than $1$ character.

Penetrance

The ability of a gene to express itself in the individual carrying it in the appropriate genotype.

Expressivity

The ability of a gene to express itself in the uniformity in all individuals that carry it.

Threshold characters

Characters whose development depends upon a specific environment.

Modifying genes

Genes that do not seem to have any effect of their own but they increase or decrease the expression of other major genes.

Gene interaction

When genes show various relationships with each other in producing the character.

Linkage

When genes tend to be inherited together and do not show independent segregation.

Phenotype

The totality of the effect of genotype, environment, and the interaction between genotype $\times$ environment.

Additive component

The genotypic variance component arising from differences between the $2$ homozygotes for a gene.

Dominance component

The genotypic variance component due to the deviation of the heterozygote from the averages of the $2$ homozygotes; also referred to as intraallelic interaction.

Interaction or epistatic component

The component resulting from interaction between $2$ or more genes; also referred to as interallelic interaction.

Population

A community of potentiality interbreeding individuals which share a common gene pool.

Gene pool

The sum total of genes in the reproductive gametes of a population.

Gene frequency

The proportion of different alleles of a gene in a population.

Genotypic frequency

The proportion of the genotype or individuals in the population.

Hardy-Weinberg Law ($1908$)

States that in a large population, both the gene frequencies and the genotypic frequencies are constant from generation to generation in the absence of mutation, migration, and selection.

Panmixis

Conditions of random mating in a large population where all genotypes are equally variable.

Mutation

Genetic alterations that can affect gene frequencies.

Selection

Mechanisms responsible for modifying the reproductive success of a genotype.

Migration

Transfer of genetic information among populations by the movement of individuals or groups from one population to another.

Random genetic drift

A nondirectional force which arises from variable sampling of the gene pool each generation.

Meiotic drive

Mechanisms associated with chromosomal structure changes in heterozygous form that affect genetic composition via inequalities in chromosomal transmission during meiosis.

Inbreeding

Mating between relatives that has little effect on overall gene frequencies but increases the frequencies of homozygotes.

Heterosis

Superiority of the hybrid vigor, where the offspring is superior to both parents.

Species

One or more groups of interbreeding or potentially interbreeding organisms that are reproductively isolated in nature from all other organisms.

Speciation

The process of splitting a genetically homogeneous population into $2$ or more populations that undergo genetic differentiation and eventual reproductive isolation.

Phyletic evolution or anagenesis

A process over a long period of time where species $A$ becomes transformed into species $B$.

Cladogenesis

The process where $1$ species gives rise to $2$ or more species over a long period of time, or rarely, in a generation or two.

Geographic or allopatric speciation

A process where physical isolation is the first step, followed by isolated populations undergoing independent genetic changes and divergence to produce $2$ distinct species.

Sympatric speciation

Speciation where the first step is genetic divergence and a shift to a new subenvironment or niche, with reproductive isolation achieved later.

Statispatric speciation

Speciation where reproductive isolation precedes the development of genetic variability.

Reproductive isolating mechanisms

Biological and behavioral properties of organisms that prevent or reduce interbreeding.

Prezygotic Mechanisms

Mechanisms that prevent fertilization and zygote formation, such as geographic, seasonal, behavioral, mechanical, or physiological isolation.

Postzygotic Mechanisms

Mechanisms where fertilization takes place and hybrid zygotes are formed, but they are nonviable, weak, or sterile.

Developmental hybrid sterility

A postzygotic mechanism where hybrids are sterile because gonads develop abnormally or meiosis breaks down before completion.

Segregation hybrid sterility

A postzygotic mechanism where hybrids are sterile due to abnormal segregation of whole chromosomes, segments, or gene combinations to the gametes.

$F_2$ breakdown

A condition where $F_1$ hybrids are normal and fertile, but the $F_2$ contains many weak or sterile individuals.