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Alleles - The different forms of a gene. For
example, X versus x. Alleles occupy the same location on a
chromosome.
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Autosomal - Relating to a chromosome that is not a
sex chromosome.
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Co-dominant Alleles - A term used to refer to a
situation where both alleles affect the phenotype of the heterozygote.
A common example of co-dominant alleles is blood type: The three
possible genotypes for blood type are AA, BB, OO. These genotypes
correspond to phenotypes of blood type A, B, and O respectively. Since
O is recessive, genotypes AO and BO also correspond to phenotypes A and B,
respectively. However, the genotype AB corresponds to blood type AB in
which both the A and B alleles are contributors to the phenotype of the
heterozygote. Since both alleles contribute to the phenotype, we call
this relationshp "co-dominant."
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Complete Linkage - The state in which two loci are so
close together that alleles of these loci are virtually never separated by
crossing over (in other terms, the observed crossover frequency between the
loci is equal to zero).
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Dioecious - A term used to describe organisms that
only produce one type of gamete. (Humans, for example).
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Dominant Trait - A trait that is expressed over
another trait. In other words, one trait dominates another for
expression.
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Drosophila melanogaster - the fruit fly, an
organism that is commonly used in genetic research.
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Epistasis - A gene is epistatic when its presence
suppresses the effect of a gene at another locus. Sometimes called
inhibiting genes.
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F1 Generation - The first filial generation,
resulting from a cross between true breeding plants. Homozygous for
the trait of interest.
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F2 Generation - The second filial generation.
The offspring resulting from a cross of F1 Generation plants.
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Genotype - An organism's genetic constitution as it
relates to the expression of a phenotype. A genotype may be
heterozygous (Xx) or homozygous (XX or xx).
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Hemizygous - When one part of the genome, in a
normally diploid species, is present in only one copy. Results in a
recessive phenotype.
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Heterozygous - Different alleles for a trait.
For Example Xx.
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Homologous Chromosomes - A pair of chromosomes
in a diploid species that have the same genetic content, overall. One
member of the pair is inherited from each parent.
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Homozygous - The two alleles are the same - either
Homozygous dominant (XX) or homozygous recessive (Xx).
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Hybrid - A Heterozygote that is usually the offspring
of a two homozygous individuals with differing traits.
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Incomplete Dominance - Incomplete dominance results
when both alleles have a say in the resulting phenotype. Therefore,
the resulting phenotype is nearly an intermediate between the phenotypes
that would have resulted from the homozygous genotype. A good example
of incomplete dominance is the snap dragon: Snap dragons with the
genotype RR have red blossoms, while rr has white blossoms. The
genotype Rr results in pink blossoms, a result of incomplete
dominance.
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Lethal Alleles - Mutated genes that may result in
death of the individual.
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Linkage - Linkage refers to genes that are inherited
together on the same chromosome. Linkage has to do with the location
of the markers (i.e., genes) on the chromosome - the closer the markers are,
the more likely they are to be inherited together.
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Mendel's Law of Independent Assortment Alleles -
Mendel's Second Law. Allele pairs of different genes assort
independently at meiosis.
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Mendel's Law of Segregation - Mendel's First
Law. Alleles segregate from one another during meiosis.
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Monoecious - Referring to an organism that produces
both male and female gametes.
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Monohybrid Cross - A cross between two individuals
identically heterozygous at one gene pair for example, Aa x Aa.
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Mutation - Change in the DNA sequence of a gene to a
new form. Generally occurs with a recessive allele, although this is
not always the case.
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Non-linkage - The state in which two loci are far
enough from each other to have a crossover frequency of at least
1.
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Partial Linkage - The state in which two loci are far
enough from each other to have a crossover frequency of more than zero but
less than 1.
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Phenotype - The physical manifestation of a
genotype. In other words, Red blossoms are the phenotype (visual
result) of the genotype RR.
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Pleiotropic - A term that refers to a single gene
that determines more than one phenotype.
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Recessive Trait - An allele that is not expressed in
the heterozygous condition.
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Reciprocal Cross - A cross that alternates the use
of both male and female gametes for two different traits.
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Sex Chromosomes - Chromosomes that determine the
gender of the individual.
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Test Cross - A cross of an organism with an unknown
genotype with a homozygous recessive organism (referred to as the
tester). The phenotype of the offspring will indicate whether
unknown organism was heterozygous or homozygous.
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True-Breeding - Homozygous for the true-breeding
trait.
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Wild-type Allele - The phenotype of an organism when
it is first seen in nature. In other words, the natural, non-mutant
allele of an organism.
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