The ABO blood group system is an example of multiple alleles because there are three different alleles (A, B, and O) that determine a person's blood type.

Alleles are the different forms of a gene that occur at a specific locus on a chromosome.

An individual inherits two alleles for each gene, one from each parent.

The phenotype is the expression of a gene in an individual.

The genotype is the genetic makeup of an individual.

An individual who has two different alleles for a gene is heterozygous.

An individual who has two identical alleles for a gene is homozygous.

Brown eyes are an example of a dominant allele because they are expressed in individuals who are either homozygous or heterozygous for the brown eye allele.

Blue eyes are an example of a recessive allele because they are only expressed in individuals who are homozygous for the blue eye allele.

The dominant allele is the allele that is expressed in an individual who is heterozygous for a gene.

The recessive allele is the allele that is not expressed in an individual who is heterozygous for a gene.

An incomplete dominant allele is an allele that is expressed in an individual who is heterozygous for a gene, but does not completely mask the expression of the other allele.

A codominant allele is an allele that is expressed in an individual who is heterozygous for a gene, and is expressed equally with the other allele.

Inheritance is the process by which alleles are passed from parents to offspring.

Genetics is the study of inheritance.