Genes are units of heredity composed of specific lengths of DNA that determine the characteristics an organism displays. Specific genes are at specific loci on specific chromosomes. The phenotype displayed by an organism is the result of the effect of the environment on the ability of the genes to express themselves.

Diploid organisms have two genes for each characteristic. The alternativeforms of genes for a characteristic are called alleles. There may be many different alleles for a particular characteristic. Organisms with two identical alleles are homozygous for a characteristic; those with differentalleles are heterozygous. Some alleles are dominant over other alleles that are said to be recessive.

Sometimes two alleles both express themselves, and often a gene has more than one recognizable effect on the phenotype of the organism. Some characteristics may be determined by several different pairsof alleles.

The successful operation of a living cell depends on its ability to accuratelyreproduce genes and control chemical reactions. DNA replication results in an exact doubling of the genetic material. The process virtually guarantees that identical strands of DNA will be passed on to the next generation of cells.

The enzymes are responsible for the efficient control of a cell's metabolism. However, the production of protein molecules is under the control of the nucleic acids, the primary control molecules of the cell. The structure of the nucleic acids DNA and RNA determine the structure ofthe proteins, whereas the structure of the proteins determines their function in the cell's life cycle. Protein synthesis involves the decoding of the DNA into specific protein molecules and the use of the intermediatemolecules,mRNA and tRNA, at the ribosome. Errors in any of the codons of these molecules may produce observable changes in the cell's functioning and lead to cell death.

Methods of manipulating DNA have led to the controlled transferof genes from one kind of organism to another. This has made it possible for bacteria to produce a number of human gene products.