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:
Internal factors
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.
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