Dihybrid Cross and the Law of Independent Assortment

A dihybrid cross is a genetic cross that follows the simultaneous inheritance of two DIFFERENT genes at once, each controlling a separate, independently observable character, rather than the single character followed in a monohybrid cross. Mendel's own dihybrid cross, exactly as recorded in his 1865 paper, crossed a pea plant that was true-breeding for both round AND yellow seeds (genotype RRYY) with a pea plant that was true-breeding for both wrinkled AND green seeds (genotype rryy); since round seed shape (R) is dominant over wrinkled (r), and yellow seed colour (Y) is dominant over green (y), every single plant of the resulting F1 generation was doubly heterozygous, genotype RrYy, and showed the doubly dominant phenotype — round, yellow seeds — exactly as the Law of Dominance (§4.3) would predict for each character considered separately.

The truly informative result, however, appeared only in the F2 generation, obtained by self-pollinating these RrYy F1 plants. Rather than a simple two-class outcome, the F2 generation segregated into FOUR distinct phenotypic classes, appearing in the closely reproducible ratio of 9 round-yellow seeds : 3 round-green seeds : 3 wrinkled-yellow seeds : 1 wrinkled-green seed — the famous 9:3:3:1 dihybrid ratio that, alongside the 3:1 monohybrid ratio, has become the very signature of classical Mendelian genetics. What made this F2 result so significant to Mendel was the appearance of two entirely NEW combinations of characters — round-green seeds, and wrinkled-yellow seeds — that had been present in NEITHER of the two original parental plants (the round parent had been yellow-seeded, and the wrinkled parent had been green-seeded). The appearance of these new, "recombinant" combinations, in a precisely predictable numerical proportion, demonstrated that the alleles governing seed shape had segregated into the gametes of the F1 plants entirely independently of how the alleles governing seed colour had segregated — the two gene pairs had not stayed "loyally" grouped together in the same combination in which they originally entered the cross, but had instead assorted freely and randomly with respect to one another. …