This figure demonstrates a test cross between flies differing in two characters: body color (b ) and wing size (v ).
The female is heterozygous at both genes and her phenotype is wild type, so she has a gray body and normal wings (B bV v ). The male is homozygous recessive and expresses the mutant phenotypes for both characteristics, black body and vestigial wings (bbvv ). When T.H. Morgan scored this particular experiment and classified the offspring according to phenotype, he found that the parental phenotypes were disproportionately represented among offspring. If the two characters were on different chromosomes and assorted independently, Morgan would have expected to see a ratio of recombinant phenotypes to parental phenotypes of 1:1:1:1. Yet Morgan's observation of disproportionate offspring led him to conclude that the genes for body color and wing size in Drosophila were usually transmitted together from parents to offspring because they were located on the same chromosome. Therefore, the black body color gene and the vestigial wing gene are linked. This means these genes are located close to one another on the same chromosome.