T. H. Morgan’s discovery that genes were linked into groups and strung along chromosomes was the second major step in peeling back the mystery of heredity and evolution. Morgan’s discovery formed much of the foundation for later discoveries of how genes and chromosomes do their work as well as the structure of the DNA molecule.
Mendel established that traits (called “genes”) are passed from parents into the next generation. Darwin established the concepts that dictated evolution of species. Still, science had no idea how species evolved or how individual genes were passed to new generations.
Studying a species of fruit flies, Professor T. H. Morgan at Columbia University both proved that Mendel’s theory was correct and established the existence of chromosomes as the carriers for genes.
By 1910, 44-year-old professor T. H. Morgan was the head of the biology department at New York’s Columbia University. All his energy, however, he saved for his research. Morgan refused to accept Mendel’s theories on heredity. Morgan didn’t believe in the existence of genes since no one had physically seen a gene.
Neither did he accept Darwin’s concept of survival of the fittest as the driving force of evolution. Morgan believed that evolution came from random mutations that slowly worked their way into and through a population. Morgan created “The Fly Room” to prove his ideas.
Morgan’s Fly Room laboratory was a small, messy room with the overpowering reek of rotting bananas. Two walls were lined floor to ceiling with rows of corked glass bottles containing tens of thousands of tiny fruit flies. Their constant buzz was difficult to talk over.
He chose to study fruit flies for four reasons. First, they were small (only ¼-inch long). Second, they lived their entire lives on nothing but mashed banana. Third, they created a new generation in less than two weeks. Morgan could study almost 30 generations a year. Finally, they had few genes and so were much easier to study than more complex species.
Morgan searched and waited for a random physical mutation (like eye color) to appear in one of the thousands of fruit flies born each month. He would then carefully track that mutation through subsequent generations to see if spread across the population and proved his theory. It was a mind-numbing effort for Morgan and his assistants. Each month, many thousands of new fruit flies had to be carefully examined under the microscope for mutations.
In September 1910 Morgan found a mutation, a male fruit fly with clear white eyes instead of the normal deep red. The white-eyed male was carefully segregated in his own bottle and mated with a normal red-eyed female.
If the eyes of these hatchlings were white, off-white, or even rose colored (as Morgan believed they would be), this random mutation, that provided no real Darwinian survival benefit or advantage, would have evolved (permanently changed) the species and Morgan’s theory of evolution by mutation would have been confirmed.
It took three days to examine the 1,237 new flies. Every one had normal red eyes. Morgan was crushed. The mutation had disappeared. It hadn’t changed the species at all. Morgan was wrong.
By October 20 the grandchildren of the original white-eyed male were hatched. One-quarter of this generation had white eyes; three-quarters had normal red eyes. 3 to 1: That was Mendel’s ratio for the interaction of a dominant and a recessive characteristic. T. H. Morgan’s own experiment had just proved himself wrong and Mendel’s gene theory right.
Additional mutations occurred frequently over the next two years. By studying these mutations and their effect on many generations of descendents, Morgan and his assistants realized that many of the inherited genes were always grouped together. (They called it “linked.”)
By 1912 the team was able to establish that fruit fly genes were linked into four groups. Knowing that fruit flies had four chromosomes, Morgan suspected that genes must be strung along, and carried by, chromosomes. After 18 months of additional research, Morgan was able to prove this new theory. Chromosomes carried genes, and genes were strung in fixed-order lines (like beads) along chromosomes.
While attempting to disprove Mendel’s work, Morgan both confirmed that Mendel was right and discovered the function of chromosomes and the relationship between chromosomes and genes.
Fruit flies can lay up to 500 eggs at a time, and their entire lifecycle is complete in about a week.