Final answer:
Mendel did not notice gene linkage because the traits he studied in pea plants were on different chromosomes or sufficiently distant on the same chromosome, resulting in apparent independent assortment. Modern genetic mapping confirms these traits to be effectively unlinked, aligning with Mendel's observations.
Step-by-step explanation:
Gregor Mendel, renowned for his foundational work in genetics, did not notice gene linkage because he worked with pea plants which have seven pairs of chromosomes, choosing seven traits for his experiments.
Mendel might have been lucky to select traits that were either located on different chromosomes or sufficiently distant on the same chromosome, making them statistically unlinked due to frequent recombination or crossover events during meiosis. As a result, the traits Mendel examined appeared to assort independently, thus observing no linkage and supporting his law of independent assortment.
Contemporary studies on pea plant genomes have shown all the traits Mendel investigated to be effectively unlinked, either on separate chromosomes or far apart on the same one. Later research by Thomas Hunt Morgan and his team with Drosophila melanogaster confirmed the Chromosomal Theory of Inheritance, and that genes on the same chromosome can be linked. Morgan's work also demonstrated that linked genes could become unlinked through homologous recombination, a process Mendel's studies did not uncover due to his focus on unlinked traits.