roote-prokop-2013-g3-drosophila-genetics-training-all

A. Prokop - A rough guide to Drosophila mating schemes 30
Appendix 2. A trihybrid cross
Example of a trihybrid cross between heterozygous parents (P, top) involving recessive alleles on X, 2 nd and
3 rd chromosomes (separated by semicolons). Homologous alleles are separated by a horizontal line; maternal
alleles are shown in black, paternal ones in blue. Mutant alleles are w (white; white eyes), vg (vestigial;
reduced wings), e (ebony; dark body colour); phenotypes are indicated by fly diagrams (compare Fig. 9). In
the first offspring/filial generation (F1) each chromosome has undergone independent assortment of alleles
(demarcated by curly brackets) and each of the four possible outcomes per chromosome can be combined
with any of the outcomes of the other two chromosomes resulting in 4 x 4 x 4 = 64 combinations. The Punnett
square at the bottom systematically lists all possible combinations (different phenotype classes are colourcoded
and display a 18:18:6:6:6:6:2:2 distribution; symbols are explained at the bottom). Red and blue
stippled boxes show the same examples of two possible offspring in both the curly bracket scheme and the
Punnett square. Note that the Punnett square reflects the numerical outcome of this cross in its full
complexity, whereas the curly bracket strategy only qualitatively reflects potential combinations and is easier
to interpret for the purpose of mating scheme design (Box 5).