What is Cytogenetics?
Before the advent of modern technology, the American biologists Thomas Hunt Morgan, G.W. Beadle, H. Sturtevant, Barbara McClintock, and others contributed greatly to our knowledge of genetics. They studied the microscopic appearance of chromosomes with respect to phenotypic traits, a discipline called cytogenetics.
Morgan chose to study the fruit fly, Drosophila, for several reasons: it has a short life cycle, it produces hundreds of offspring from a single mating, it can easily be raised in the laboratory, and it has only four pairs of chromosomes. The fruit fly is also well suited for cytogenetic studies because the salivary glands contain polytene chromosomes, in which the DNA has replicated many times without cytokinesis; as a result, the genes form condensed bands of different thicknesses called chromomeres that are visible at low magnification. The patterns of the bands, their thickness, spacing, and sharpness, can be used to determine if changes in chromosome structure has occurred.