Q.How was it concluded that genes are located on chromosomes?

T.H. Morgan's experiments on the fruit fly, Drosophila melanogaster, provided the definitive experimental proof that genes are located on chromosomes.

His key observations and conclusions were:

• He studied the inheritance of traits like eye color, observing that the gene for white eyes was inherited differently in males and females.
• This pattern, known as sex-linked inheritance, showed that the gene for eye color was located on the X chromosome, and its inheritance directly paralleled that of the X chromosome itself.
• Further experiments with multiple genes on the same chromosome demonstrated linkage, where genes located close together on a chromosome tended to be inherited together.
• The frequency of recombination between linked genes was used to determine their relative positions, or map them, on the chromosome. …

The understanding that genes are the fundamental units of heredity, passed from parents to offspring, was established by Gregor Mendel in the mid-19th century. However, the physical location and nature of these 'factors' (as Mendel called them) remained unknown for decades. It was the work of several scientists, culminating in the meticulous experiments of Thomas Hunt Morgan, that definitively concluded genes reside on chromosomes.

The journey began with the rediscovery of Mendel's laws in 1900. Soon after, in 1902, Walter Sutton and Theodor Boveri independently proposed the Chromosomal Theory of Inheritance. They observed that the behaviour of chromosomes during meiosis (cell division that produces gametes) perfectly paralleled the behaviour of Mendel's factors. They noted that chromosomes occur in pairs, segregate during gamete formation, and then pair up again during fertilisation, much like the inheritance patterns of traits. This theory suggested that chromosomes were indeed the carriers of genetic information, but direct experimental proof was still needed.

This crucial experimental verification came from Thomas Hunt Morgan and his colleagues, who worked extensively with the fruit fly, Drosophila melanogaster. Morgan chose Drosophila for several practical reasons that made it an ideal organism for genetic studies:

• They could be grown on simple synthetic media in the laboratory.
• They complete their life cycle in about two weeks.
• A single mating can produce a large number of offspring.
• There is a clear differentiation between male and female flies.
• They possess many hereditary variations that can be easily observed with low-power microscopes.

Morgan's most famous experiment involved studying the inheritance of eye colour in Drosophila. He observed a spontaneous mutation that resulted in white-eyed flies, while the wild type had red eyes. When he crossed a white-eyed male with a red-eyed female, he made a groundbreaking observation:

• The first filial (F1) generation consisted entirely of red-eyed flies, indicating that red eye colour was dominant over white.
• When he intercrossed the F1 generation, the second filial (F2) generation showed the expected Mendelian 3:1 ratio of red to white eyes. However, all the white-eyed flies in the F2 generation were male. The females were all red-eyed. …

Lay the parallel behaviour of genes and chromosomes side by side as a two-column table (pairing, segregation, independent assortment), then treat Morgan's sex-linked eye-colour cross as …