In human skin colour which is a polygenic trait, all dominant and all recessive alleles show

Human skin colour is a classic example for polygenic trait the phenotype reflects the contribution of each allele, i.e. the effect of each allele is additive. To understand this better let us assume that three genes, i.e. A, B and C control skin colour in human with  the dominant forms A, B and C responsible for dark skin colour and the recessive forms a, b and c for light skin colour. The genotype with all the dominant alleles (AABBCC) will have the darkest skin colour and that with all the recessive alleles (aabbcc) will have the lightest skin colour.