In biology, genes are the molecular units of heredity of living organisms. They are used extensively by the scientific community as a name given to some stretches of deoxyribonucleic acids (DNA) and ribonucleic acids (RNA) that code for a polypeptide or for an RNA chain that has a function in the organism.
Living organisms depend on genes, as they specify all proteins and functional RNA chains. Genes hold the information to build and maintain an organism's cells and pass genetic traits to offspring.
All organisms have genes corresponding to various biological traits, some of which are instantly visible, such as:
- eye color or
- number of limbs, and
some of which are not visible, such as:
- blood type,
- increased risk for specific diseases, or
- the thousands of basic biochemical processes that comprise life.
The word gene is derived from the Greek word genesis meaning "birth", or genos meaning "origin" (see pangenesis).
The chemical structure of a four-base fragment of a DNA double helix.
A modern working definition of a gene is
"a locatable region of genomic sequence, corresponding to a unit of inheritance, which is associated with regulatory regions, transcribed regions, and or other functional sequence regions ".
Colloquial usage of the term gene (e.g. "good genes", "hair color gene") may actually refer to an allele:
- a gene is the basic instruction— a sequence of nucleic acids (DNA or, in the case of certain viruses RNA), while
- an allele is one variant of that gene.
Thus, when the mainstream press refers to "having" a "gene" for a specific trait, this is customarily inaccurate.
In most cases, all people would have a gene for the trait in question, although certain people will have a specific allele of that gene, which results in the trait variant.
Further, genes code for proteins, which might result in identifiable traits, but it is the gene, not the trait, which is inherited.
Big genes are a class of genes whose nuclear transcript spans 500 kb (1kb = 1,000 base pairs) or more of chromosomal DNA.
The largest of the big genes is the gene for dystrophin, which spans 2.3 Mb.
Many big genes have modestly sized mRNAs; the exons encoding these RNAs typically encompass about 1% of the total chromosomal gene region in which they occur.