A dominant allele is a modification of a gene that will produce a certain phenotype even in the existence of other allele. It usually encodes for a functioning protein. A recessive allele is a variety of genetic code that does not produce a phenotype if a dominant allele exist. If a trait tends to be directly acknowledged from parent to child, the trait is a dominant one. If a trait skips generations or pops up out of nowhere, then that is recessive.
Dominant allele is expressed by a capital letter while a recessive allele is expressed by a small letter.