The inheritance of two characteristics

Dihybrid inheritanceis the inheritance of two characteristics, each controlled by a different gene at a different locus. In one experiment Mendel studied dihybrid inheritance by crossing plants from two pure-breeding strains: one tall with purple flowers, the other dwarf with white flowers. All the offspring in the F1 generation were tall with purple flowers, these being the dominant characteristics. The F1 generation were self-crossed, producing the following phenotypes and ratios in the F₂ generation:

§ 9 tall purple-flowered

§ 3 tall white-flowered

§ 3 dwarf purple-flowered

§ 1 dwarf white-flowered.

Mendel observed that two phenotypes resembled one or other of the parents, and two phenotypes had combined the characteristics of both parents. He also observed that the ratio of tall plants to dwarf plants was 3:1, and that the ratio of purple-flowered plants to white-flowered plants was 3:1. This was the same ratio that occurred in the monohybrid crosses. He concluded from these results that the two pairs of characteristics behave quite independently of each other. This led him to formulate his law of independent assortment, which states that any one of a pair of characteristics may combine with any one of another pair.

Interpreting the results of a dihybrid cross

Mendel's results can be explained in terms of alleles and the behaviour of chromosomes during meiosis. Notice that the two alleles for one gene are always written together (for example, TtPp,not TPtp).This makes it easier to interpret the crosses. The pure-breeding adult plants, being diploid, have two alleles for each gene. The genes for height and flower colour are carried on separate chromosomes. During gamete formation, meiosis occurs, producing gametes containing one allele for each gene. In the F1 generation, the only possible genotype is TtPp.When these plants are self-crossed, there are four possible combinations of alleles in both the female and male gametes: TP, Tp, tP,and tp.Assuming fertilisation is random, any male gamete can fuse with any female gamete, so there are 16 possible combinations for the offspring, as shown in the Punnett square. These combinations can produce four different phenotypes from nine genotypes.

The only genotype that can be worked out simply by looking at the plants is that of the dwarf white-flowered plants. Genotypes of the other plants can be established by test crosses.