DNA Structure and Function
DNA provides the set of coded instructions required by every organism for specifying its traits. The DNA molecule also provides for a reliable way for parents to pass their genetic code from one generation to the next.   Heredity refers to this passage of these instructions from one generation to another.  

DNA is a double stranded molecule which has the shape of a twisted ladder.   This shape is called an alpha helix.    The sides of this twisted ladder are composed of alternating phosphate and deoxyribose sugar units, while the rungs of the ladder are composed of pairs of nitrogenous bases.   These bases are called adenine (A), thymine (T), guanine (G), and cytosine (C).   These bases exist in pairs on the rungs of the ladder with A always pairing with T and G pairing with C.   This principle is sometimes called complementary base pairing.   (The saying G CAT provides a means of remembering this idea.)

Structure of the DNA molecule

Gene-Chromosome Model
Hereditary information is contained in genes, which are composed of DNA,  located in the chromosomes of each cell.   Chromosomes are found in the nucleus of each cell.

The Gene Chromosome Model

Each gene carries a separate piece of information. An inherited trait of an individual can be determined by one genes, but is usually determined by the interaction of many different genes. 
A single gene can influence more than one trait.  A human cell contains many thousands of different genes coding for many different traits.  Changes in the sequence of the DNA molecule and therefore the gene are called mutations.  A mutation may change the manner in which a trait is expressed by an organism.