Allele
This is a different form of gene that is represented by letter. There can be dominant or recessive allele. Dominant allele are expressed even when paired with different allele. This is usually represented by capital letter. On the other hand, recessive is an allele that is hidden or masked if paired with another allele. Recessive allele are represented by lower case letter. These allele are usually expressed by phenotype or the physical expression of trait.