 Many traits are not discrete features ( e.g. purple or white flowers ) but are instead continuous features ( e.g. human height and skin color ) . These complex traits are products of many genes . The influence of these genes is mediated , to varying degrees , by the environment an organism has experienced . The degree to which an organism 's genes contribute to a complex trait is called heritability . Measurement of the heritability of a trait is relative — in a more variable environment , the environment has a bigger influence on the total variation of the trait . For example , human height is a trait with complex causes . It has a heritability of 89 % in the United States . In Nigeria , however , where people experience a more variable access to good nutrition and health care , height has a heritability of only 62 % . 
