Three generations: P, F1, F2
A monohybrid cross follows a single gene. We start with the P generation (the parents): a true-breeding purple plant (PP) crossed with a true-breeding white plant (pp). Their children are the F1 generation. Each F1 plant gets one P from one parent and one p from the other, so every F1 is Pp — heterozygous, and therefore purple. This is the disappearing-white mystery from Guide 1, now in notation.
Now let the F1 plants self-pollinate. Their children are the F2 generation — the grandchildren of the original parents. This is where white reappears, and where the 3:1 ratio lives. To see why, we need one more rule and one diagram.
The law of segregation
Mendel's first law, the law of segregation, says this: the two alleles of a gene separate when an organism makes its gametes (egg or pollen), so each gamete carries only one allele, picked at random. A Pp plant therefore makes two kinds of gametes in equal numbers — half carry P, half carry p. Each offspring is then built by combining one gamete from each parent.
The Punnett square and the ratios
A Punnett square is a simple grid that combines every possible gamete from one parent with every possible gamete from the other. For the F1 self-cross (Pp × Pp), each parent offers P or p, so the grid has four boxes — and those four equally likely combinations are exactly what produce the ratio.
Cross: Pp x Pp (F1 self-pollination)
pollen P pollen p
+---------------+---------------+
egg P | PP | Pp |
| purple | purple |
+---------------+---------------+
egg p | Pp | pp |
| purple | white |
+---------------+---------------+
Genotypes: 1 PP : 2 Pp : 1 pp
Phenotypes: 3 purple : 1 white -> 3 : 1Read the four boxes and the pattern jumps out. The genotype ratio is 1 PP : 2 Pp : 1 pp. But because PP and Pp both look purple, the phenotype ratio collapses to 3 purple : 1 white. That is the famous 3:1. Mendel didn't guess it — he counted 929 F2 plants in one experiment and found 705 purple to 224 white, almost exactly 3:1. The Punnett square explains why white had to come back, and why it comes back at just that rate.