Genetics and alleles ( dominant and recessive )?

it's because there are many kind of interaction, such as polygenic (where more than 2 genes determined 1 phenotype). polygenic is the reason for many variation of skin colour. the other interaction are epistasis, and pleiotropy. the dominance itself its divided unto complete dominance and incomplete dominance. you can search more in Biology - Campbell, Reece, Mitchell or other genetic books.