How do alleles affect phenotype?

Dominant and recessive alleles are ways biologists determine the expression of a gene.

Alleles are represented by alphabetic letters: either homozygous dominant (BB), Heterozygous dominant (Bb), or homozygous recessive (bb). Dominance simply means that it will represent itself physically (the definition of "phenotype") over another allele. A recessive allele will only be seen (phenotype, again) if it is not being competed against with a dominant allele, for example: "bb".

Just because a trait is recessive does not mean that it is less common. there are many traits that are recessive that are very common in certain populations, because there may be fewer dominant alleles present to compete against.

The combination of alleles (called the genotype), determines the phenotype, or the physical representation of the trait - what we see, in many cases.