Heredity factors influence what an organism develops into because of genetic influences, whereas the environment plays a role in determining what the organism becomes. One example of this is height, which is partially determined by the person's genes, but is also determined by dietary differences.
The hereditary aspect of an organism's makeup depends on the genetics that pass down from its parents' DNA. For example, identical twins both bear the same external features, which they inherit from their parents' genes. In contrast, environmental influences are factors in an organism's environment that causes it to develop. One example of this is phenylketonuria, which is a genetic defect that once resulted in learning difficulties. Scientists found that reducing the amount of phenylalanine in the diets of children with phenylketonuria prevented learning difficulties. This demonstrates how an environmental influence determines the organism's outcomes.
Heredity and environmental factors often interact with each other. Another example is Siamese cats, which are genetically coded to have dark fur, but develop darker fur when they live in environments that are colder than their body temperature. A further example is the rubella virus crossing the placenta and infecting a fetus. With this virus present in the fetus's environment, it is likely to develop congenital defects.