The fruit fly, Drosophila melanogaster, has a long history in genetics. For over a century, this small insect has been studied, providing insights into how traits are passed from one generation to the next. The unique characteristics of its chromosomes, the structures carrying genetic information as DNA, have made the fruit fly an effective model for biological research.
The Standard Chromosome Set in Fruit Flies
The genome of Drosophila melanogaster is organized into a small number of chromosomes, which simplifies genetic analysis. A typical fruit fly cell contains four pairs of chromosomes for a total of eight. This count includes three pairs of autosomes (non-sex chromosomes) and one pair of sex chromosomes that determine gender.
Fruit flies use an XX and XY system for sex determination, but the mechanism differs from humans. In flies, sex is determined by the ratio of X chromosomes to the sets of autosomes (the X:A ratio). A fly with a 2X:2A ratio develops as a female, while a fly with a 1X:2A ratio develops as a male.
Unlike in mammals, the Y chromosome in Drosophila does not determine maleness. Instead, it contains genes involved in processes like sperm formation in adult males. Therefore, a fly with an XO chromosomal arrangement (one X and no Y) is a sterile male, whereas in humans, an XO individual is female.
Polytene Chromosomes: The Giant Advantage
Certain tissues in the fruit fly larva, most notably the salivary glands, possess polytene chromosomes. These are giant chromosomes formed through endoreduplication, where the DNA repeatedly replicates without cell division. The resulting structure is a thick chromosome containing over a thousand identical DNA strands aligned in perfect register.
The large size of polytene chromosomes makes them visible with a standard light microscope, an advantage for early geneticists. When stained, these chromosomes display a distinct and reproducible pattern of light and dark bands. Each band corresponds to a specific genetic location, creating a visible, physical map of the fly’s genes.
These bands also provide direct visual evidence of gene activity. Regions of the chromosome that are actively being transcribed into RNA swell into larger structures called “puffs.” Observing these puffs allows researchers to see which genes are turned on at different stages of development or in response to environmental stimuli.
Balancer Chromosomes as Genetic Tools
Geneticists studying Drosophila have developed tools to manipulate its chromosomes, one of the most powerful being the balancer chromosome. These engineered chromosomes maintain genetic stocks, particularly those carrying recessive mutations that would be lethal or cause sterility if a fly had two copies. Balancers prevent these scientifically interesting mutations from being lost from a population.
The function of a balancer chromosome relies on two properties. First, they contain multiple chromosomal inversions—long segments of the chromosome that have been flipped end-to-end. These inversions prevent meiotic recombination, or “crossing over,” with the homologous chromosome, ensuring the desired mutant allele is passed down as a complete block.
To make them easy to track, balancers also carry dominant marker genes that produce obvious, visible traits, such as curly wings. This allows researchers to identify which flies in a cross have inherited the balancer chromosome. Combined with their own recessive lethal mutations that prevent a fly from surviving with two copies of the balancer, these features create a stable system for managing genetic lines.
Foundational Role in Genetic Research
The study of fruit fly chromosomes provided physical evidence for the Chromosomal Theory of Inheritance. In the early 20th century, Thomas Hunt Morgan and his students conducted experiments that linked a specific gene to a specific chromosome for the first time. Their work began with the discovery of a single male fly with white eyes, a stark contrast to the normal red-eyed population.
Through a series of crosses, Morgan’s group demonstrated that the trait for white eyes was inherited along with the X chromosome, establishing the concept of sex-linked inheritance. Further work by Alfred Sturtevant used recombination frequencies between linked genes to create the first genetic maps, showing that genes are arranged in a linear order on chromosomes.
These foundational discoveries, made possible by the fly’s simple chromosomal makeup, established Drosophila melanogaster as a model organism. Even with advanced DNA sequencing, the ability to observe polytene chromosomes and use tools like balancers makes the fruit fly a valuable resource. It remains central to research on the genetic basis of human development and disease.