gene
Meaning of gene: Few traits trace back to a single gene, which is why most inherited risk is spread thinly across a great many of them.
Definition of gene
A gene is a stretch of DNA that carries the instructions for making a product the cell needs, usually a protein and sometimes a functional RNA. Genes sit at fixed positions along chromosomes, and the full set an organism carries is its genome. Most human genes are inherited in two copies, one from each parent.
How does a gene become a protein?
The gene is first copied into messenger RNA in the nucleus. That copy is edited, with non-coding stretches called introns removed and the remaining pieces joined, which allows one gene to yield several different messages and so several related proteins. The finished message passes out into the cytoplasm, where ribosomes read it three letters at a time, each triplet specifying one amino acid, and build the chain. The chain then folds into the shape that determines what it does. Only part of the genome codes for protein at all; much of the rest regulates when and where genes are read, or is copied into RNA that works as RNA rather than being translated. Which genes are active differs from tissue to tissue, so a liver cell and a nerve cell carry identical genes and use different ones.
What happens when the sequence changes?
| Term | What it means |
|---|---|
| Variant | Any difference from the reference sequence; the neutral term now preferred |
| Benign variant | A difference known not to cause disease; most variants are of this kind |
| Pathogenic variant | A difference known to cause or strongly predispose to disease |
| Variant of uncertain significance | A difference whose effect is not yet known |
| Mutation | Older usage, still common, implying a change from the normal sequence |
A mutation may alter the shape of the protein, stop it being made, or change only how much is produced. It may be inherited or arise during life, as the acquired changes in tumor cells do. Effects also depend on the other copy: a recessive condition needs both copies altered, while a dominant one needs only one.
Where are genes found?
Almost all of them lie on the chromosomes in the nucleus, packaged with proteins so that a length of DNA far longer than the cell fits inside it. Each gene has a fixed address on a particular chromosome. A small separate set sits inside the mitochondria and is inherited from the mother alone, which is why conditions caused by those genes follow an unusual pattern of inheritance. The reading of any gene is controlled by switches that may lie some distance away along the DNA, and this control, known as gene expression, is why carrying a gene and being affected by it are different things.
Used in a sentence
Genetic testing was performed to look for a mutation in a specific gene associated with the condition.
Good to know
A gene refers to a specific segment of DNA with a defined function, which is distinct from a chromosome, the far larger structure made up of many genes along with other DNA.
Word origin
Where “gene” comes from
From Greek genos, meaning birth, race or descent, the root from which the word gene was coined to describe the unit of heredity.