Linkage is a measure of the degree of association between genes on the same chromosome, and this association is a product of the average number of crossovers that occur between the two genes. Inheritance of alleles from the dihybrid can be directly ascertained through phenotypic observation. For the case of testing linked genes, it is best to test the data in a contingency table, where the data are organized in categories within a table. The chi-square test is then applied between the observed and expected numbers, 19.8 is the value obtained. The df in this case is the number of rows minus 1 x the number of columns minus 1.
Linkage is measured by the frequency of recombinants between loci.
The resulting offspring is a dihybrid, Aa Bb , with red eyes and tan body. The dihybrid is testcrossed to the homozygous recessive line, aa bb , with apricot eyes and black body. We will add the C locus to our original A and B loci. The C allele is dominant for crossveins present in the wings, and c is recessive for crossveinless wings. The resulting offspring is a trihybrid, Aa Bb Cc and this is testcrossed to the homozygous recessive line, aa bb cc .