Gene

A gene is a stretch of DNA that encodes the information needed to build a functional product — most often a protein, but sometimes a functional RNA such as transfer RNA or ribosomal RNA. Genes are the fundamental units of heredity: they are copied and passed from parent to offspring, which is why traits run in families.

Genes do their work through the central dogma of molecular biology. A gene's DNA sequence is transcribed into messenger RNA, and that mRNA is translated into a chain of amino acids that folds into a protein. A typical human gene contains coding segments (exons) separated by non-coding segments (introns) that are spliced out of the RNA, plus regulatory regions such as promoters that control when and where the gene is switched on.

Each gene occupies a specific location on a chromosome, called its locus. Alternate versions of a gene are alleles — humans inherit one allele of each gene from each parent, and the combination (the genotype) shapes the observable trait (the phenotype). Changes in a gene's sequence, called mutations, can alter or eliminate the product and cause genetic disease, which is why understanding gene structure underpins medical genetics.

For students preparing for the USMLE Step 1 exam, genetics fundamentals — gene structure, transcription and translation, alleles, and inheritance patterns — form the foundation for high-yield topics from single-gene disorders to bacterial gene regulation.