Final answer:
Mendel discovered the principles of inheritance before it was clear that 'genes' are the fundamental units of heredity carried on chromosomes, paving the way for the chromosomal theory of inheritance.
Step-by-step explanation:
Gregor Mendel discovered the principles of inheritance before it was clear what 'genes' are. Mendel's pioneering work in the field of genetics involved his experiments with pea plants, establishing many of the fundamental principles of heredity. However, at the time of his research, the concept of genes as the fundamental unit of heredity on chromosomes within the nucleus of a cell was not yet established. It wasn't until the turn of the 20th century, post-Mendel, that chromosomes were visualized and theorized to be the carriers of genetic information, leading to the eventual identification of genes as specific segments of DNA responsible for inherited traits. Mendel's laws, the law of segregation and the law of independent assortment, later became justified through the chromosomal theory of inheritance.