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Key Terms
Allele:
One member of a pair of
genes occupying a
specific spot on a
chromosome that
controls the same trait.
Chromosome:
A structure within the
cell that bears the
genetic material of
an organism.
Codominance:
A condition in which the alleles
of a gene pair in a heterozygote
are fully expressed thereby
resulting in offspring with a
phenotype that is neither
dominant nor recessive.
Example: AB Blood Type
Dominant Allele:
The allele that determines
the phenotype of an
organism, masking the
effect of the recessive
allele when present.
Gamete:
A reproductive
cell that contains
the haploid set of
chromosomes.
Gene:
A segment of DNA
that is responsible
for the phenotype of
an organism.
Genotype:
The entire set of genes
in an organism that
determines the
expression of a
particular phenotype.
Heredity:
The passing of
genetic factors from
parent to offspring.
Heterozygous:
An organism containing two
different alleles (a dominant
and a recessive) for a
particular trait.
Homozygous:
An organism containing
two copies of the same
allele (two dominant or two
recessive) for a particular
trait.
Homozygous Dominant:
An organism that
has two dominant
alleles (RR).
Homozygous Recessive:
An organism that
has two recessive
genes (rr).
Incomplete Dominance:
A kind of dominance occurring in
heterozygotes in which the
dominant allele is only partially
expressed. This usually results
in offspring with an intermediate
phenotype because the
organism has one dominant
allele and one recessive allele.
Genetic Locus:
The location of a
gene on a
chromosome.
Monohybrid Cross:
A crossbreeding experiment
that follows the inheritance of
a single characteristic across
one or more generations. In
other words this is a cross of
one trait from two parents.
Multiple alleles:
The existence of
several known
alleles of a gene.
Phenotype:
The physical appearance
of an organism as a result
of the interaction of its
genotype and the
environment.
Polygenic Trait:
A trait that is controlled by two
or more genes at different loci
on different chromosomes.
Examples: height, skin color,
weight.
Recessive:
An allele whose phenotypic
expression is masked by a
dominant allele. The
recessive trait may be
expressed when the
recessive genes are in a
homozygous condition.
Trait:
A qualitative
characteristic. An
attribute of
phenotype, not of
genotype.