Dominant / Recessive
In Mendelian genetics, an allele is considered either dominant or recessive, and this characteristic controls its expression. A dominant allele will be expressed and its corresponding trait will be exhibited, even if it is paired with a recessive allele. A recessive allele will only be visible when paired with another recessive allele.
In this animation, the brown allele, B, is dominant to the grey allele, b. Thus, cats with the genotypes BB and Bb are brown, while only bb cats are grey. In general, upper-case letters are used to designate dominant alleles, while lower-case letters designate recessive alleles.
The fact that one allele is dominant to another allele is frequently indicated with a '>' sign. So for this example, we could write the relationship between the B and b alleles as B>b.