Thomas Hunt Morgan used fruit flies to study genetics for several reasons. First, fruit flies have a short life cycle that lasts an average of just 30 days. This means genetic traits can be studied over dozens of generations in the course of a year. This rapid reproductive cycle is impossible to match in any mammalian model, even mice or rats. Second, fruit flies produce huge numbers of offspring.
A female fruit fly can lay up to 500 eggs in a single batch. Since fruit flies can lay several batches of eggs in a lifetime, this amounts to 2,000 or more offspring, far more than any female mammal could produce. A third consideration is cost. Fruit fly colonies are extremely inexpensive to maintain. At minimum, they require a food source, such as overripe fruit, and an adjustable thermostat. Interestingly, fruit flies live longer at cold temperatures, but breed more often at temperatures above 86 degrees Fahrenheit.
Additional advantages of using fruit flies became evident as time passed. Fruit flies contain four pairs of giant chromosomes, known as polytene chromosomes, in their salivary glands. These chromosomes are large enough to be seen under a light microscope, the type commonly used at the time. This observation provided solid evidence that genes, the physical units of heredity, are located on chromosomes.
Morgan and his students also discovered that many genes coding for traits such as eye color, wing shape, and bristle length, were inherited together in most fruit fly offspring. Occasionally, these linkage groups, as Morgan called them, were disrupted as fly chromosomes mixed and matched parts during meiosis. Scientists refer to these chromosomal crossover events as recombination. This process is a major source of genetic variation in all species that reproduce sexually.