Suppose only one gene with 2 alleles determines a particular trait but the alleles are codominant. how mant different phenotypes could exist for this trait

The answer is one trait. In Codominant alleles, neither allele is dominant over the other. Both alleles are expressed equally hence no recessive or dominant phenotypes. An example of that of fur color in rabbits where one  parent with black fur color and the other with white fur color will produce offspring of black and white fur color. This should not be confused with incomplete dominance  whre the two alleles blend in phenotype.Example of incomplete dominance is a parent with straight hair in humans and another with curled hair will produce wavy hair offsprings.

his research revealed two genetically different groups of prokaryotes, which justified splitting kingdom monera into two kingdoms. the genetic and cellular differences were so great, that these two kingdoms were eventually placed into different domains in the current three-domain system.

d. the weight of the book