2.12—Gene as a DNA base sequence

A gene is a sequence of DNA bases that contains information used to produce a functional polypeptide or RNA. The base sequence is transcribed and, for a protein-coding gene, translated into an amino-acid sequence.

The sequence determines codons, codons determine amino acids and the polypeptide folds into a functional shape. Regulatory sequences and expression level also affect when and where the product is made.

A mutation in a gene can alter one codon, introduce a stop signal or shift the reading frame. The phenotype changes only if the resulting product or its expression changes the relevant pathway.

A gene is not the same as a chromosome, allele or protein: a chromosome contains many genes, an allele is one version of a gene and a protein is a possible product.

Do not infer that every DNA difference changes phenotype. Degeneracy, non-coding changes, regulation and environmental conditions can all weaken a direct gene-to-trait claim.