““The gene responsible for creating white eyes (called white eyed), for instance, was inescapably linked to maleness: no matter how Morgan crossed his flies, only males were born with white eyes. Similarly, the gene for sable color was linked with the gene that specified the shape of a wing. For Morgan, this genetic linkage could only mean one thing: genes had to be physically linked to each other. In flies, the gene for sable color was never (or rarely) inherited independently from the gene for miniature wings because they were both carried on the same chromosome. If two beads are on the same string, then they are always tied together, no matter how one attempts to mix and match strings. For two genes on the same chromosome, the same principle applied: there was no simple way to separate the forked-bristle gene from the coat-color gene. The inseparability of features had a material basis: the chromosome was a "string" along which certain genes were permanently strung.””
