Biology - DNA & Gens

Allele:
An allele is one member of a pair or series of different forms of a gene.
Gene and the relationship between gene and DNA:
A gene is the basic unit of heredity in a living organism. The field of -genetics predates modern molecular biology, but it is now known that all living things depend on DNA to pass on their traits to offspring. Loosely speaking, a gene is a segment of genomic information that, taken as a whole, specifies a trait. The colloquial usage of the term gene often refers to the scientific concept of an allele.
DNA:
DNA is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms and some viruses.

chromosome and the relationship between DNA
A chromosome is an organized structure of DNA and protein that is found in cells. A chromosome is a single piece of DNA that contains many genes, regulatory elements and other nucleotide sequences.

Allel - Different forms of a gene (ex. dominant & recessive gen)
Chromosome - consist of chromatin, found in the nucleus of a eukaryotic cell and most visible during mitosis and meiosis. Threadlike, gene carrying structure.
Gene - a discrete unit of hereditary information consisting of a specific nucleotide sequence in DNA. Most are located in its chromosomal DNA of a eukaryote. (ex. TT, Tt, tt)
DNA - the genetic material that organisms inherit from their parents. A double stranded helical macromolecule consisted of nucleotide monomers with deoxyribose sugar and the nitrogenous bases adenine, cytosine, guanine, and thymine.

Hope I helped. =]