A recipe with one wrong word
A gene is a stretch of DNA that spells out the instructions for building one working molecule, usually a protein. Think of it as a recipe. Most of the time the recipe is copied faithfully from parent to child. But sometimes one letter is swapped, dropped, or repeated — a mutation — and the recipe now produces a protein that is missing, misshapen, or made in the wrong amount.
A single-gene disorder (also called a Mendelian disorder) is a genetic disorder in which a fault in just one gene is enough to cause the disease. That is a strong claim: it means the gene's allele — its particular spelling — is the main driver, not a tangle of hundreds of genes plus lifestyle. This is what makes these diseases so instructive: their inheritance follows clean, predictable rules.
Two copies of every recipe
You carry two copies of almost every gene: one from each parent. The two copies sit at the same locus on a pair of homologous chromosomes. If both copies are the same spelling you are homozygous; if they differ you are heterozygous. Having a spare copy matters enormously — for many diseases, one good copy is enough to stay healthy.
Many disease alleles cause loss of function: the protein simply stops working. A few cause gain of function, where the altered protein does something new and harmful. Which of these it is helps explain why a disorder behaves as recessive or dominant — the theme of the next guide.
From genotype to phenotype
Your genotype is the pair of alleles you carry; your phenotype is what shows up — symptoms, blood tests, or simply nothing at all. The link between them is not always one-to-one. The same genotype can produce a strong or mild phenotype in different people, and we will meet that wrinkle (penetrance and expressivity) later. For now, hold the clean version: one faulty gene, a missing or broken protein, a recognizable disease.