Punnett squares.

Step 1 of 5

Filling the square

Each sex cell gets one of its parent’s two copies, and the two copies are equally likely. That is Mendel’s law of segregation, and meiosis is the reason for it: in the first division the two chromosomes of each pair go to different cells.

A Punnett square lays out every combination: one parent’s alleles go down the side, the other’s across the top, and each box holds one allele from its row and one from its column. Each of the four boxes is as likely as the others, because each allele comes with chance one half, and $\frac12 \times \frac12 = \frac14$$\frac12 \times \frac12 =$$\frac14$⁠.

Cross the parents is fixed at Parents: fill the square in your head, or on paper.

Step 2 of 5

From genotypes to colours

Once the square is full, read it twice. Once by genotype: how many boxes are PP, how many Pp and how many pp. And once by phenotype: every box with at least one P has a purple flower.

Step 3 of 5

The test cross

A purple plant can be PP or Pp, and from outside you cannot tell. Breeders need to know: for a pure variety, they must be sure it hides no allele. Mendel’s answer: cross it with a white plant, pp, and look at the offspring.

Seed parent is fixed at Pp, the plant you want to test.

Step 4 of 5

All carriers

Now both parents are open. Try a few combinations, and see how the square and the line under it change.

Step 5 of 5

Purple, but which?

When every offspring is purple, the colour does not tell the whole story. The square does.