Drosophila Chromosome: Number, Types, and Genetic Role
Examine the chromosome set of *Drosophila* and see how its distinct structure became a key tool for mapping genes and defining the principles of heredity.
For over a century, the common fruit fly, Drosophila melanogaster, has been a subject of intense scientific study. It rose to prominence in biological research due to its manageable size, low-cost maintenance, and a life cycle that completes in under two weeks, allowing for rapid generational experiments. Its straightforward genetics provided a powerful system for exploring heredity, leading to foundational discoveries about how traits are passed to the next generation.
The Standard Chromosome Complement
The genetic blueprint of Drosophila melanogaster is contained within eight chromosomes, organized into four distinct pairs. This set, or karyotype, includes three pairs of autosomes (numbered 2, 3, and 4) and one pair of sex chromosomes (X and Y). Autosomes are the chromosomes that do not determine the sex of the fly.
Under a microscope, these chromosomes have distinguishable shapes and sizes. Chromosomes 2 and 3 are large and V-shaped (metacentric), with the centromere near the middle. The X chromosome is rod-shaped, while the Y chromosome is J-shaped. Chromosome 4 is extremely small and dot-like, containing relatively few genes.
Female flies possess two X chromosomes (XX), while male flies have one X and one Y chromosome (XY). This simple and visually distinct karyotype allowed early geneticists to link observable traits to specific chromosomes. The clear differences between the chromosome pairs provided a physical framework for understanding inheritance patterns.
Giant Polytene Chromosomes
While most fruit fly cells contain the standard eight chromosomes, specialized tissues like the larval salivary glands feature a much larger version known as polytene chromosomes. They are so large they can be seen in detail with a standard light microscope. Their immense size results from a modification of the standard chromosome set, not a different type of chromosome.
These giant structures form through a process called endoreduplication. Unlike a normal cell cycle where DNA is copied once before division, these cells undergo repeated rounds of DNA replication without dividing. This process results in hundreds of identical DNA strands—sometimes up to 1024—fused together in perfect alignment.
This alignment creates a distinct and reproducible banding pattern of dark, dense bands alternating with lighter interbands. Scientists discovered these bands serve as a physical map, with specific bands corresponding to the locations of individual genes. This feature provided a powerful tool for geneticists.
During development, specific bands may decondense and swell, forming structures known as “chromosome puffs.” These puffs are regions where the DNA has uncoiled, indicating high gene activity through transcription. By observing which bands are puffed, researchers can see which genes are active in a specific tissue at a specific time, linking chromosome structure directly to gene function.
The Sex Determination Mechanism
Sex determination in Drosophila differs significantly from the process in humans. While the Y chromosome is the primary trigger for male development in many mammals, a fly’s sex is determined by the balance between its number of X chromosomes and its sets of autosomes.
This system is known as the X:A ratio, which compares the number of X chromosomes to the number of autosomal sets. A normal fly has two sets of autosomes. A fly with two X chromosomes (XX) has an X:A ratio of 2X:2A, or 1.0, which signals the development of a female.
Conversely, a male fly (XY) has an X:A ratio of 1X:2A, or 0.5, which initiates male development. This mechanism explains why a fly with an XXY genotype is a fertile female (ratio = 1.0), unlike in humans where an XXY individual is male. While the Y chromosome does not determine sex in flies, it contains genes required for male fertility.
Role in Mapping the Genome
Drosophila chromosomes were central to confirming the chromosome theory of inheritance, which states that genes are physically located on chromosomes. The work of Thomas Hunt Morgan’s research group in the early 20th century provided the first solid evidence linking a specific trait to a specific chromosome.
Morgan’s breakthrough came in 1910 when he discovered a male fruit fly with white eyes, unlike the normal red-eyed population. When this white-eyed male was crossed with a red-eyed female, all offspring had red eyes, suggesting the white-eye trait was recessive. However, when these offspring were interbred, the white-eyed trait reappeared almost exclusively in males.
This unusual, sex-linked inheritance pattern led Morgan to hypothesize that the gene for eye color was on the X chromosome. Since males have only one X chromosome, they display the trait if they inherit that single copy. Females, with two X chromosomes, would need the recessive version on both to show the trait, which was a rarer occurrence.
Morgan’s work with the white-eyed fly demonstrated that genes were physical parts of chromosomes. This discovery laid the groundwork for his student, Alfred Sturtevant, who realized that genes on the same chromosome are often inherited together, a concept known as genetic linkage.
Sturtevant reasoned that the frequency of recombination—the process where paired chromosomes exchange DNA—could measure the distance between genes. By analyzing how often linked genes were separated during this process, he created the first genetic map. This map showed the linear arrangement of genes along a chromosome.