Statement (B) is correct because, in polygenic inheritance, the effect of each allele is additive, meaning multiple alleles collectively influence the trait. Statement (C) is also correct; a gene is termed pleiotropic when it has multiple phenotypic effects. Statement (A) is incorrect because incomplete dominance involves a blending of traits, where neither allele is fully dominant. Statement (D) is incorrect since mutation can alter both genotype and phenotype, not necessarily affecting only the phenotype.