Final answer:
Thomas Morgan demonstrated that each chromosome is a group of linked genes, which tend to be inherited together. However, recombination can lead to independent assortment of these genes if they are far enough apart on the chromosome. Morgan's and Sturtevant's work expanded our understanding of genetic inheritance beyond Mendel's laws.
Step-by-step explanation:
Thomas Morgan discovered that each chromosome is actually a group of linked genes. It is the chromosomes that assort independently, not individual genes. Alleles of different genes tend to be inherited together when those genes are located on the same chromosome.
The Chromosomal Theory of Inheritance, proposed by Sutton and Boveri in the early 1900s and supported through Thomas Hunt Morgan's research with Drosophila melanogaster, confirmed that chromosomes are the vehicles of genetic heredity. Despite chromosomes sorting independently during meiosis, linked genes (genes close together on the same chromosome) tend to be inherited together, which can disrupt the expected outcomes from Mendel's law of independent assortment. However, recombination during meiosis can cause these linked genes to be inherited independently if they are far enough apart on the chromosome.
Sturtevant later further developed this theory by devising a method to estimate the distance between linked genes on a chromosome based on recombination frequency, which introduced the idea that genetic material can be mapped and that genes have a specific linear order on chromosomes.