A dihybrid cross involves two characteristics that are controlled by two separate genes with two or more alleles. The genes are located on different chromosomes and the alleles are therefore not necessarily inherited together. When the same alleles for two or more genes are inherited together (such as the gene for high production of ear wax and the one for strong-smelling sweat), this is known as Autosomal Linkage and is calculated using the Mendellian ratios for a monohybrid cross.

The process for a dihybrid cross is essentially the same as for the monohybrid cross, except that two separate crosses are occuring at the same moment, i.e. a cross occurs with more than one characteristic. The typical example brings us back to Gregor Mendel and his pea plants (see explanation on monohybrid cross), except that we are investigating height (T/t) and also seed shape (R/r).

Once again, the letter T represents the dominant allele (tall) and the letter t represents the recessive allele (dwarf). Similarly, the letter R represents the dominant allele of another gene (round seed) and the letter r represents the recessive allele (wrinkled seed). A homozygous dominant specimen would therefore have a genotype of TTRR, whereas a homozygous recessive plant would be ttrr. Being homozygous, the gametes of each parent would be the same (TR or tr, depending on the specimen) and therefore the cross would be written as follows:

           tr         tr

TR    TtRr     TtRr

TR    TtRr     TtRr

The resulting offspring are all heterozygous in genotype (TtRr) but, since the dominant allele expresses the tall and round-seeded phenotype, all appear identical to the homozygous dominant parent (TTRR).

If these heterozygous offspring were interbred, the result would be somewhat different. Firstly, the gametes will not be identical, since there are four different combinations: the true dominant (TR), the true recessive (tr) and the two heterozygous combinations (Tr and Rt). This cross is calculated like so:

           TR          Tr          tR           tr

TR    TTRR     TTRr     TtRR     TtRr

Tr      TTRr      TTrr     TtRr       Ttrr

tR      TtRR      TtRr     ttRR       ttRr

tr       TtRr        Ttrr      ttRr         ttrr

The resulting offspring are quite a mixture of alleles, but there are clearly two homozygous specimens (TTRR and ttrr) and four identical to the parents (TtRr). The remaining offspring are of various combinations, but the phenotype ratio is 9:3:3:1. There are nine of the dominant combination (tall with round seeds), one true recessive (dwarf with wrinkled seeds) and three of each of the other combinations (tall with wrinkled seeds, dwarf with round seeds).

As with the monohybrid cross, when the heterozygous form (TtRr) is crossed with either homozygous form (TTRR or ttrr), the ratio is 1:1 (or rather, since there are four combinations and phenotypes, the ratio is 1:1:1:1). These are the same combinations as mentioned in the above paragraph, but in equal amounts.

Source: Notes from A level Biology