GENERAL DEFINITIONS
• a GENE controls 1 characteristic or trait
• ALLELES are alternative forms of a gene. For example: The
GENE is feather colour and the ALLELE is either Blue "a" or Green
"A". Say we are studying height. Alleles determine alternative types
(tall, short) for the gene (height) that we are studying.
• GENOTYPE is the inherited material received from both
parents (e.g.) AA, Aa, or aa
• PHENOTYPE is the physical appearance of the individual
(e.g.): Green or Blue
• AUTOSOMAL refers to genes on the non-sex chromosomes (most
genes)
• SEX - LINKED refers to genes on the sex chromosomes (X
& Y in humans/mammals and Z & W in Birds)
• DOMINANT refers to an allele that is expressed regardless
of the other alleles present for the trait being studied. (e.g.) AA & Aa
both give green birds, "A" is dominant.
• RECESSIVE refers to the allele that is expressed only when
it is the only type of allele present in an organism for the trait being
studied (e.g.) "aa" = Blue birds, "a" is recessive.
• HETEROZYGOTES contain 2 different alleles for the trait
being studied. The phenotype of the heterozygote indicates which allele is
dominant (e.g.) Aa is a heterozygote. Since that bird is green it indicates
that "A" allele (responsible for green colour) is dominant.
• HOMOZYGOTES contain identical alleles for the trait being
studied. The line may be either homozygous dominant (AA) or homozygous
recessive (aa) depending on which allele is present. Either is a true breeding
line.
• PUNNETT SQUARE is a method used to determine all of the
possible genotypes of the offspring and the probability that a given genotype
will occur when the genotypes of the two parents are known. This is done in
chart format. See Single gene and Double gene mutations for examples.
• POLYMORPHISM means many forms. It is the existence in a
population of 2 or more common phenotypic variations for a trait. The different
variants are called morphs. (e.g.) several different feather colours for the
feather colour gene.
DEFINITIONS RELATING TO BREEDING PRACTICES
• PURE LINE is a strain that breeds true for that trait
being studied (e.g.) Blue X Blue will always give you Blue chicks
• PARENTAL GENERATION (P) is the generation from which the
first cross in a series is taken. Usually, but not always, the parents are true
breeding.
• FIRST FILIAL GENERATION (F1) consists of progeny/offspring
from the parental generation. (e.g.) If you are the parental generation then
your kids are the first filial generation.
• SECOND FILIAL GENERATION (F2) consists of progeny from the
first filial generation (e.g.) Your grand kids.
• TEST CROSS is a cross of a homozygous recessive organism
(aa) with an organism of dominant appearance (AA or Aa). It results in a 1:1
ratio if an organism with the dominant appearance is heterozygous (Aa) and a
1:0 ratio if the organism with the dominant appearance is homozygous (AA)
• RECIPROCAL CROSS involves a pair of crosses that switch
the trait being studied with the sex of the parent carrying that trait. (e.g.)
Cross #1 Blue (male) X Green (female). Cross # 2 or the reciprocal cross to # 1
would be Blue (female) X Green (Male).
TERMINOLOGY USED IN PEDIGREE ANALYSIS
• PROPOSITUS is the individual that first called attention
to the family being studied (e.g.) the first blue mutant that came from green
parents.
• CARRIER can refer to a heterozygous individual for either
an autosomal or sex-linked trait. For autosomal it can be either sex, just needs
to have a dominant copy of the gene "A" and a recessive copy of the
gene "a". In this sense it is equivalent to a Split. For Sex-linked,
only female humans/mammals and male birds can be carriers because they are the
only sex which possesses 2 chromosomes which can hold the gene of interest. A
carrier in this case must also have a dominant copy of the gene "A"
and a recessive copy of the gene "a".
POPULATION GENETICS DEFINITIONS
• Adaptation = A trait that increases the survivability of an
individual or its ability to reproduce when compared to individuals that do not
possess that trait
• Adaptive Radiation = Radiation of a group of organisms into
populations adapted to exploit different ecological niches
• Adaptive Trait = A trait that increases the fitness of an
individual
• Allopatric Speciation = Speciation that occurs when
populations become geographically isolated due to genetic drift and when
selection pressures differ between the two populations
• Assortative Mating = A mating pattern that occurs when
individuals tend to mate with other individuals of the same genotype and
phenotype
• Bottleneck = A large scale but short term decrease in the
population size followed by an increase in the population size. Can cause
speciation events
• Convergent Evolution = Similarities between species that are
the result of similar, but evolutionarily independent responses to common
environmental factors. E.g. The wing of a bird and the wing of a butterfly
• Evolution = Descent with modification = a change in the
characteristics of a population over time = changes in the allele frequency of
a population over time
• Fitness = The degree to which an individual contributes
genes to the next generation
• Founder Effect = The establishment of a new population by a
small number of individuals. It can cause speciation events
• Frequency = The proportion of a genotype, phenotype, gamete,
or allele in a population. E.g. 6/10 have brown hair = a frequency of 0.6
• Gene Pool = All of the copies of all of the alleles in a
population that could be contributed by members of the present generation to
members of the next generation
• Genetic Drift = A change in the allele frequency of a
population resulting from sampling error in taking gametes from the gene pool
to make zygotes and from chance variation in the survival/reproductive success
of individuals
• Hardy-Weinberg Equilibrium = An ideal population in which
the allele and genotype frequencies do not change from one generation to the next
generation due to a lack of selection, mutation, migration, and genetic drift
and due to the occurrence of random mating
• Heritability = The fraction of the total phenotypic
variation in a population that is caused by genetic differences between
individuals
• Homology = Similarities between species that results from
the inheritance of traits from a common ancestor
• Homoplasy = Similarities in the traits found in different
species that is due to convergent evolution, parallelism, or reversal. It is
not due to common descent
• Hybrid Zone = A geographic zone where different
populations/species interbreed
• Inbreeding = Mating between relatives
• Inbreeding Depression = A decrease in the fitness of an
individual or a population due to inbreeding. It is often the result of a
decrease in heterozygosity of an increase in the homozygosity (both are due to
inbreeding)
• Inclusive Fitness = An individual's total fitness = indirect
fitness (fitness due to the reproduction by relatives made possible by that
individual) + direct fitness (fitness due to the individual's own reproduction)
• Macroevolution = Large scale evolutionary change = evolution
of the differences between populations that would justify their placement into
different genera (or higher level taxa)
• Microevolution = Changes in the gene frequencies and trait
distributions that occur within species and populations
• Migration = The movement of alleles from one population to
another population due to the movement of individuals or gametes
• Natural Selection = Specific phenotypes confer increased
survivability or reproductive success to the individuals that possess them
• Negative Selection = Selection against deleterious mutations
• Outbreeding = Mating between unrelated individuals
• Polymorphism = The existence of more than one allele or
variant in a population
• Population = A group of individuals capable of interbreeding
plus all of their offspring
• Positive Selection = Selection for advantageous mutations
• Preadaptation = A trait that changes due to natural
selection and takes on a new function
• Relative Fitness = The fitness of an individual, phenotype,
or genotype compared to other individuals in the population
• Species = Groups of populations that are capable of
interbreeding and are evolutionarily independent from other populations
• Sympatric Speciation = A speciation event involving species
living in the same geographic area
• Synapomorphy = A shared derived trait
• Transitional Form = A species exhibiting traits that are
common to both the ancestral and derived groups
PHYLOGENETICS DEFINITIONS
• Bootstrapping = A term commonly used in phylogenetic
reconstruction = A technique used for estimating the strength of evidence for
the existence of a particular node in a phylogenetic tree. Values range between
0% and 100% with 100% being the strongest level of support
• Branch = A branch in a phylogenetic tree. See diagram
• Clade = A group of species descended from a common ancestor
= a monophyletic group
• Evolution = Descent with modification = a change in the
characteristics of a population over time = changes in the allele frequency of
a population over time
• Extant = Living today
• Extinct = Not living today
• Monophyletic Group = A population of a group of species
descended from a common ancestor
• Node = Branching point in a phylogenetic tree. See diagram
• Outgroup = In phylogenetic analysis, a group that diverged
prior to the rest of the taxa
• Paraphyletic Group = A group of species that includes the
common ancestor and some, but not all of that common ancestor's descendants
• Phylogeny = The evolutionary history of a group
• Psuedogene = DNA sequences that are homologous and resemble
functioning genes, but are not transcribed
• Sister Species = Species that diverged from the same node on
a phylogenetic tree
• Species = Groups of populations that are capable of
interbreeding and are evolutionarily independent from other populations
• Taxon = Any named group of organisms
• Tip = The end of a branch on a phylogenetic tree. See
diagram
A = Node
B = Branch
C = Tip
D = Monophyletic Group
E = Paraphyletic Group
F = Outgroup
G = Sister Taxa/Species
REFERENCES
• Freeman, S. and Herron, J. C. 2001. Evolutionary Analysis
Second Edition. Prentice-Hall, Inc. New Jersey.
• Griffiths, A. J. F., Miller, J. H., Suzuki, D. T., Lewontin, R. C., and Gelbart, W. M. 1996. An Introduction to Genetic Analysis Sixth Edition. W.H. Freeman and company. New York.