Gene - a sequence of bases on DNA that codes for a protein.
Allele - A different version of a gene.
Genotype - The alleles an organism has.
Phenotype - Characteristics the alleles produce.
Codominant - Alleles that are both expressed in the phenotype.
Locus - Fixed position of a gene on a chromosome. Alleles are found at the same locus.
Both alleles are expressed in the phenotype. eg. sickle cells.