A gene is an abstraction used in the description and quantification of heredity. It is a discrete unit of inheritance, identified by enumerating how specific traits of organisms are passed on from parents to children. One may speak of, for example, the "gene for blue eyes" being passed from a mother to her children.
Genes are said to be expressed when their trait is exhibited in the organism that contains them (for example, when the child does in fact have blue eyes). Individuals may have more than one kind of gene that compete or combine for traits; for example, a brown-eyed father and blue-eyed mother may each pass on different genes for eye color to the same child. Which trait is expressed is determined by the nature of the genes; sometimes one is said to dominate another (that is, a child with a mixed set will always show the dominant trait), and sometimes they combine to form a mixed trait.
In modern usage since the discovery of DNA as the mechanism of heredity, the term is often used to refer to a specific sequence of DNA at a specific locus on a chromosome that codes for a particular protein, whether or not that protein results in a single identifiable heritable trait. Active genes generally are delimited by a "start" and a "stop" codon (a short DNA sequence which is recognized by the cell as a point either to start or to stop translation of the gene), and contains any number of codons in between that code for amino acids according to the genetic code. Different genes in the population that occupy the same chromosomal locus (and therefore compete or combine for similar traits) are called alleles.
Much of the chromosomal DNA in many organisms does not code for proteins, and is of no apparent function (sometimes referred to as "junk DNA"). In many cases, this probably represents genes which have become inactive due to DNA rearrangement or mutation at the delimiting codon sites. An interesting field of exploration is the attempt to "re-activate" such "lost" genes, producing such things as real hen's teeth, for instance. Other junk DNA is thought to serve structural roles in the chromosome, such as the regions of heterochromatin near the chromosome's centromere or the telomeres at the ends. Some junk DNA may serve a regulatory role, providing binding sites for the many signal proteins that affect gene transcription. Finally, as Richard Dawkins points out in The Selfish Gene, it is quite possible that much DNA exists literally for no reason other than to propagate itself, even at the expense of the host organism.
Typical numbers of genes in an organism:
|organism||# of genes||base pairs|