Each parent contributes exactly half its genes to its offspring.
This law becomes evident from the second generation (F2) onward: If both parents belong to the first (F1) generation they have both the same set of genes Rr and show the dominant colour. Four combinations of one half of the mother's genes with one half of the father's genes are possible:
Three combinations contain the dominant R allele and produce purple flowers; the fourth combination contains only recessive r alleles, which allows the white colour to appear again in the F2 generation.
Images: public domain