Alleles are the different forms of a gene in a chromosome. Copies of all alleles are symmetrically inherited from both parents by genetic segregation. In diploid organisms the offspring inherit two copies of each chromosome with two alleles making up the individual's genotype1.
Alleles can be dominant, recessive, co-dominant, or incomplete-dominant depending on how they express on the phenotype of the organisms. Mendel's peas experiments are classic examples of dominance/recessive allele interaction, where dominant traits are expressed in 1/4 of the offspring, while recessive expresses in 1/4 of offspring carrying both recessive alleles.2 Exceptions to Mendelian genetics are co-dominance and incomplete-dominance. Co-dominance shares partial expression of both inherited alleles, while incomplete-dominance result in a blend of both alleles.3
2. Mendel's paper in English: Gregor Mendel (1865): http://www.mendelweb.org/Mendel.html