Codominance occurs when:
a) both of the alleles in a heterozygote are expressed phenotypically in an individual.
b) expression of two different alleles alternates from one generation to the next.
c) a heterozygote expresses an intermediate phenotype.
d) offspring exhibit several different phenotypic expressions of a single trait.

Crinoids, also known as sea lilies, appear to be sea plants but are not plants at all. crinoids are an echinoderm species which uses anchoring structures called holdfasts to attach themselves to the sea floor. crinoids were most abundant in pennsylvania during the mississippian period, which ranged from 375 to 320 million years ago. fossils of the crinoids shows that during most of this time pennsylvania was that provided favorable conditions for crinoid growth. a) glacial terrain b) a dry and sandy plain c) mostly mountain ranges d) covered by warm, shallow seas