Genes code for proteins. That way they determine biological structures and functions. Besides genes, the environment has the great impact on the structure of an organism. Alleles and genes will give a trait, but the environment can affect it. For example, alleles that are responsible for height will give a person some average height. But if the person begins to train basketball at the early age (during growth), the person will overcome the average height. This is the effect of the environment and not genes. The other example is that if a person lives in a sunny region, the person will be tan.
