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Genetic Locus (Plural Loci)

The specific location of a gene or piece of DNA sequence on a chromosome.
When mutations modify the sequence at a locus they generate new alleles-variants of a particular gene or DNA region. Alleles are mutually exclusive alternative states for a genetic locus.


Population Genetics

The study of the distribution of alleles within populations and the mechanisms that can cause allele frequencies to change over time.



A mathematical statement that has been proven based on previously established theorems and axioms. Theorems use deductive reasoning and show that a statement necessarily follows from a series of statements or hypotheses.
--> not the same as a theory


Fixed Allele

An allele becomes fixed in a population when all of the alternative alleles have disappeared. No genetic variation exists at a fixed locus within a population, because all individuals are genetically identical at that locus.


Genetic Drift

The process in which allele frequencies drift randomly away from their starting value. Genetic drift is a mechanism of evolution because it causes the allelic composition of a population to change from generation to generation. Alleles are lost due to genetic drift much more rapidly in small populations than in large populations.


Genetic Bottleneck

Events in which the number of individuals in a population is reduced drastically. Even if this dip in numbers is temporary, it can have lasting effects on the genetic variation of a population.
Alleles can be lost during genetic bottlenecks, and the relative frequencies of the remaining alleles can shift dramatically in just a few generations.


Founder Effect

A type of genetic drift describing the loss of allelic variation that accompanies founding of a new population from a very small number of individuals (a small sample of a much larger source population). This effect can cause the new population to differ considerably from the source population.


Inbreeding coefficient (F)

The probability that the two alleles at any locus in an individual will be identical by descent.



The success of an organism at surviving and reproducing, and thus contributing offspring to future generations


Relative Fitness

(of a genotype) The success of the genotype at producing new individuals (its fitness) standardized by the success of other genotypes in the population (for example, divided by the average fitness of the population)


Average excess of fitness

(of an allele) The difference between the average fitness of individuals bearing the allele and the average fitness of the population as a whole.
-Can be used to predict how the frequency of the allele will change from one generation to the next



The condition when a mutation in a single gene affects the expression of many different phenotypic traits.
-Considered to be antagonistic if a mutation with beneficial effects for one trait also causes detrimental effects on other traits


Negative Selection

Selection that decreases the frequency of alleles within a population.
-Occurs whenever the average excess for fitness of an allele is less than zero


Positive selection

Selection that increases the frequency of alleles within a population.
-Occurs whenever the average excess for fitness of an allele is greater than zero



Occurs when the effects of an allele at one genetic locus are modified by alkleles at one or more other loci


Additive Allele

An allele that yields twice the phenotypic effect when two copies are present at a given locus than when only a single copy is present. Additive alleles are not influenced by the presence of other alleles (eg: there is no dominance)