La réponse courte
If DNA is an enormous instruction manual, a gene is one specific recipe inside it: the instructions for making one particular protein. Humans have roughly 20,000 protein-coding genes — a surprisingly small number, and about the same as a microscopic worm. What makes us complex is not how many recipes we have but how elaborately their use is controlled.
A gene is a sequence of DNA that specifies a functional product, typically a protein but sometimes a functional RNA. Protein-coding genes comprise exons, which are retained in the mature transcript, and introns, which are spliced out, together with regulatory elements including promoters and enhancers. Humans have roughly 19,000–20,000 protein-coding genes.
Genes come in pairs
You inherit one copy of most genes from each parent, so you carry two versions — alleles — of almost every gene. Sometimes one working copy is enough and a fault in the other causes no problem: that is recessive inheritance. Sometimes one faulty copy is enough to cause disease: that is dominant. Which pattern applies determines what a treatment has to achieve.
Why gene editing cares
Almost everything on this site follows from a small number of possibilities: a gene may be broken and its protein missing, or it may be making something actively harmful, or it may simply be produced in the wrong amount. Those three situations call for completely different interventions — restore, remove, or adjust — and which one applies is the first question about any disease.
Sources
- National Human Genome Research Institute
Talking Glossary of Genomic Terms ↗ - MedlinePlus Genetics, U.S. National Library of Medicine
Help Me Understand Genetics ↗