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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 ½ 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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