Genes are encoded with different traits. Each of these traits carry different functions to the new offspring that is developing. As an offspring is developing, different factors can effect the development of genes. Sometimes, internal or external factors can trigger a gene to mutate. For example if a woman who is pregnant decides to smoke or drink (during the whole pregnancy), this will effect the development of the fetus or even cause a miscarriage. Babies will be born with what is called fetal alcohol syndrome.
There are genes that are dormant and something externally triggers their development. One example of this is cancer. People carry this gene, but it could stay dormant forever. So, genes can be affected in many ways both externally and internally.
The genes you inherit from your parents at the moment of conception will largely determine what traits you will express. However, there are both intenal and external factors that will alter gene expression. And some genes control the expression of other genes. It can get a bit complicated. Let me give you a few examples:
Age can affect the expression of some traits like hair color and skin's elasticity. As one ages, the hair pigment genes begin to shut down so there is not as much pigment produced and one sees hair color begin to fade and eventually grey. Skin elasticity is the same - skin looses it's elasticity - as cells slow down in their reproduction - and wrinkles begin to appear. Another example of internal factors would be sex. Different sexes express the same genotype in different ways (like baldness).
External environmental facors
Another set of factors will also affect the expression of certain traits. If you go out in the sun your skin produces more pigment to prevent the UV rays from damaging your skin, so, the genes that control skin color will be "enhanced". Cold temperatures can influence fur color in many animals - especially those that live in the artic regions. Fur color in these animals changes to white in cold temperatures but will be colored with pigment in normal temperatures. Himalayan rabbit fur does this in all temperatures - ears, tail, paws and nose are black because they are cooler than normal body temperature, but the rest of the body is white.
So, there are many factors that influence the expession of our genes.
The genes that an organism inherits can affect gene function. For example they can have a recessive gene for something that might affect them. Also external factors such as the environment an organism makes and the food it eats can affect gene function.
What characteristics of an organism can affect gene function? Essentially, genes are the "set point" for different characterisitics. For example, a person may have the genetic set up for a certain height (say 6 feet) but their childhood nutrition is such that they do not achieve it. Additionally, a person may carry a recessive gene for a disorder that would not ordinarily express (say, lung cancer), but because of environmental exposures (smoking, fire fighting) they more easily contract the disease.
An organism's age, and the organism's fertilization