Linkage was discovered in Drosophila by Thomas Hunt Morgan. Morgan conducted groundbreaking experiments with fruit flies (Drosophila melanogaster) in the early 20th century, which led to the discovery of genetic linkage.

**Morgan's Experiments:**
Morgan used Drosophila as his model organism due to its short generation time, large number of offspring, and easily observable traits. He performed a series of crosses between different strains of fruit flies to study the inheritance patterns of various traits.

**Finding Unexpected Inheritance Patterns:**
During his experiments, Morgan observed that certain traits did not segregate independently as predicted by Mendel's laws of inheritance. Instead, he noticed that certain traits were often inherited together more frequently than would be expected by chance.

**Explanation of Linkage:**
Morgan concluded that these traits were located on the same chromosome and did not assort independently. He termed this phenomenon as "linkage." Linkage occurs when two or more genes are located close to each other on the same chromosome, resulting in their non-random segregation during gamete formation.

**Crossing Over and Recombination:**
Morgan further investigated the phenomenon of linkage and discovered that occasional recombination events could separate the linked genes. He observed that during the process of meiosis, homologous chromosomes could exchange genetic material through a process called crossing over.

**Genetic Mapping:**
Based on his observations of crossing over, Morgan developed a technique called genetic mapping. By analyzing the frequency of crossing over between different linked genes, he was able to map their relative positions along the chromosome.

**Significance of Morgan's Discovery:**
Morgan's discovery of genetic linkage and his subsequent work on genetic mapping provided strong evidence for the chromosomal theory of inheritance. It demonstrated that genes are physically located on chromosomes and that their inheritance patterns can be influenced by their proximity to each other.

**Conclusion:**
In conclusion, Thomas Hunt Morgan discovered genetic linkage in Drosophila through his extensive experiments and observations. His work laid the foundation for our understanding of the physical basis of inheritance and the role of chromosomes in transmitting genetic traits.