An organism that is heterozygous for two particular genes, as in AaBb. Dihybrids are most conveniently produced from crossing parents that are homozygous for the genes in question, e.g. AABB x aabb or AAbb x aaBB. A * test cross normally yields four kinds of offspring: A_B_ (both dominant characters expressed); A_bb, and aaB_ (only one dominant character expressed); and aabb (no dominant characters expressed). If the genes A/a and B/b are not linked, equal numbers of all kinds of offspring will be produced. If they are linked, two classes, the parentals, will be more common, and two classes, the recombinants (cross overs), will be rarer. Selfing a dihybrid normally yields a 9:3:3:1 ratio (see dihybrid ratio) in the case of independent assortment but a much more complex result in the case of linkage. See also monohybrid , trihybrid. |
