single trait cross double trait cross codominant polygenic none of the above .
polygenic Poly many, genic genes. When many (2+) genes contribute in an additive manner, it is a polygenic trait. Skin pigmentation is a good example. The more 'dark' alleles one has for the genes controlling melanin, the darker the skin. In codominance, two (or more) genes are all expressed, but not in an additive manner. Roan cattle have both red and white hair. Both the red and the white genes are expressed, but not additively.
