Just because an individual is carrying an allele (one form of a factor) doesn't mean that it will be expressed. Certain characteristics which are said to expressed over the other are called dominant. The characteristics that are masked, or are only expressed in the homozygous condition are said to be recessive.

Another pattern of heredity is where when two parents are crossed, the offspring that result have characteristics that are intermediate to those of the parents. An example of this would be the crossing of a plant that has red flowers with one that has white flowers, and all the offspring of that cross have pink flowers. In this case, neither trait is dominant over the other, so we call it incomplete dominance.

Still yet another possibility is both genes being expressed in equal fashion in the offspring. A good example of this is blood types. A person with "type A" blood, could have AA or A_ as a genotype. A person with "type B" blood could have BB or B_ as their phenotype. Ever hear of anybody having "type AB blood"? Their genotype is AB, with both factors equally expressed in their phenotype.