A gene often has two alleles. One allele is dominant and the other allele is recessive. When is a recessive allele expressed as a characteristic?

A gene has two alleles, they can be either dominant (A) or recessive (a).So the possible combinations are:AAAaaaDominant alleles are expressed as they are 'stronger' . So in the Aa example the dominant allele 'wins' and is expressed. So the only time a dominant allele isn't expressed is when there is no dominant allele ( such as the case aa)