There has been much debate on the influence of genetics and the environment on different traits, but the solution is usually an unequivocal "it depends." Factors that go into determining exactly where the balance stands include how strongly the trait is tied to genetics, the number and degree of environmental influences and the interactions between the genes and the environment. Averages can be found for a population, but the relative influence can change over time and from individual to individual.
How Genes Influence Trait Expression
Individuals get one copy of a gene from each parent, but the interactions between the genes influence how the trait is expressed. Dominant genes are expressed all the time, but recessive genes are only expressed when you get two copies. Genes also interact in patterns such as partial dominance or co-dominance, the trait is expressed as a mix between two genes. Those are the possibilities for one gene. Most traits are influenced by many genes, there are many different ways for those genes to influence how the trait is expressed.
How the Environment Influences Trait Expression
The environment in which an organism lives can be thought of as an infinite series of variables that can influence how that organism expresses its genome. Drugs, chemicals, temperature and light are just a few of the variables that can control how traits are expressed. For example, if you grew up on a planet with half the gravity of Earth, you would probably be much taller than you are right now. Closer to home, identical twins could have very different skin tones if one spent more time in the sun.
How Genes and the Environment Interact
While the influence of genetics and the environment is usually proposed as a dichotomy, the reality is that genetics and the environment often interact to produce traits. Environmental factors turn genes on or off or change the function of the encoded proteins and enzymes. In Himalayan rabbits, there is a gene that codes for dark colored hair, but it only turns on at cools temperatures. Normally the dark hair only shows up on cool parts of the body, but if the rabbit lives in a hot climate, it won’t have any dark hair.
The Concept of Heritability
Heritability is a way that you can assign a number to the relative influence of genes and the environment for any one trait. It is calculated by dividing the variance in the genes for a trait by the total variance in the population. The possible influence of genetics ranges from 0 to 100 percent, so the value for heritability runs from 0 to 1. Heritability is often used in breeding livestock to determine how likely they are to pass on traits such as rapid weight gain or milk production. The caveat when using heritability is that the value can vary over time and between populations.