What determines whether an allele is dominant or recessive

A dominant allele in the genotype is always expressed in the phenotype. For example, if brown eye color is dominant in a species, an animal that inherits even a single brown allele will always have brown eyes. If it's recessive, he'll only have brown eyes if he inherits a brown allele from each parent.

Explanation:

the wires in the toaster turning red and the toast crisping and turning brown