P generation

The P generation — short for parental — is the starting line of a breeding experiment: the original two parents whose cross kicks off a series of generations that geneticists then follow. The 'P' marks where the family tree being studied begins.

In Mendel's design the P generation is usually two true-breeding parents that differ in the trait of interest, such as a pure tall plant crossed with a pure short one. Their children form the first filial generation (F1), the grandchildren the F2, and so on, with each generation given its own label so crosses can be tracked precisely.

P is a bookkeeping label, not a fixed biological category: it simply names whichever parents you choose as the origin of a particular pedigree. The same individual could be the P generation in one experiment and an F2 offspring in another, depending on where the observer starts counting.