Mendelian inheritance

Imagine that each parent hands a child not a smooth blend of paint but two complete decks of trading cards, one inherited from each grandparent. The child keeps one card of each kind, drawn at random, and later passes a randomly chosen card to its own children. The cards stay whole — they never melt together. This is the heart of Mendelian inheritance: traits are carried by discrete factors that are shuffled and dealt, not mixed.

Gregor Mendel, a 19th-century friar working with pea plants, deduced that hereditary factors (what we now call genes) come in pairs, that one of each pair separates into each gamete, and that the factors for different traits are handed out independently. From this he derived two principles — the law of segregation and the law of independent assortment — that explain the simple numerical ratios he saw in his crosses.

Mendelian inheritance describes traits governed by a single gene with clear dominant and recessive alleles. Many real traits are more complicated: several genes contribute, the environment matters, or genes sit close together on a chromosome and are inherited as a set. Mendel's framework is the foundation that all of these richer patterns are measured against, not a rule that every trait obeys.