Glaucoma is a leading cause of blindness. Studies have linked a common form of glaucoma to three genes and more than 20 genetic loci. Which best describes glaucoma?
a recessive trait that requires multiple recessive alleles to be expressed
a polygenic trait that is influenced by the alleles of several genes
a codominant trait that is expressed more often when more dominant alleles are present
a dominant trait that depends on the presence of only one of many dominant alleles