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.