The importance of DNA in determining individual traits and transmitting genetic information from one generation to the next was not discovered until about 70 years ago. Of course, even before this discovery, people knew that certain traits could be inherited. In fact, many family names come from hereditary traits, such as Armstrong, Fairfax and Blunt (fair-haired). 

DNA is a molecule composed of two chains that coil around each other. The structure of a DNA molecule is often described as a zipper or a double helix.

The sugar phosphate backbone is an important structural component of DNA. There are four nitrogenous bases found in DNA that are called:

• Guanine.
• Adenine.
• Thymine.
• Cytosine.

The order of these bases in the DNA molecule determines the structure of the gene and the effects its has on the organism as a whole.

For example, if the the bases that regulate growth hormone secretion are of a certain order, an exceptionally large amount of growth hormone is secreted. If the order is different, less growth hormone is secreted. This affects the size of the individual.