The unit of inheritance. Defined no more precisely than this, the term 'gene' corresponds with Mendel's 'factors' responsible for the manifestation of a particular characteristic. A particular gene may be further defined as being located at a specific point on one of the chromosomes and capable of existing in alternative forms or alleles. Thus there would be a gene for 'seed colour' in peas, with alternative alleles of 'green' and 'yellow'. In a diploid cell, alleles of a gene are paired, one on each of a pair of homologous chromosomes. If contrasting alleles are present in the same organism, e.g. one for 'green' and one for 'yellow', then the phenotype of the organism will depend upon the dominance relationship between the alleles.
However, in addition to determining characteristics, inherited information also mutates and recombines. A gene defined as a unit of function (*
cistron ) may be a piece of *
DNA several hundred nucleotides long, whereas a gene defined as a unit of mutation (muton) may be a single nucleotide. The unit of recombination (recon) lies between these two extremes. In modern usage the term gene is most commonly applied to functional units of DNA. Thus *
structural genes determine protein structure and regulator, promoter, and operator genes control the regulation of protein production. Genes are extremely stable and so spontaneous *
mutation occurs very rarely. However it is the ultimate source of all genetic variation, i.e. the reason why the 'seed colour' gene exists in different forms, such as green and yellow. Once mutation has created alternative alleles, a further source of genetic variation is by genetic *
recombination, i.e. the rearrangement of whole chromosomes or parts of chromosomes during meiosis.
Though the majority of DNA, and hence the genes, is confined to the chromosomes, some DNA is present in such organelles as mitochondria and chloroplasts. The pattern of inheritance shown by cytoplasmic genes differs from that of chromosomal (Mendelian) genes and they are referred to as plasmagenes in order to distinguish them from the latter.
