Often, when an individual has two different alleles that determine its certain trait, the effect of the dominant allele will be expressed, while the effect of the recessive allele will be hidden. For example, this is how we inherit eye color.
However, it isn't always like this. Sometimes,the effects of both (different) alleles are expressed. This phenomenon is called codominance, and the scenario you were given is an example of it.
Seafloor spreading is a process that occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge.
