I think the answer to this is incomplete dominance. Incomplete dominance means that the dominant gene hasn't completely taken over the recessive gene.
For example, lets say a red flower is dominant over a white flower. Following the rule of incomplete dominance, these two colors then begin to mix. Red + White = Pink
This is somewhat similar to co-dominance, however the two genes will not mix together in co-dominance. Instead, some parts will be from a dominant gene and some parts will be from a recessive gene. An example of this would be a black and white spotted cow with a black father and a white mother.
