The chromosomal theory of inheritance states that chromosomes are the agents responsible for passing genetic information from one generation to the next. First put forward by Walter Sutton in 1902, the theory showed how the behavior of chromosomes during meiosis provided a mechanism for the sorting and passing on of genetic material that fit with Mendel's law of heredity.
A chromosome is a long, thread-like structure found in the nucleus of cells. It is primarily made up of a single coiled piece of DNA, which is the molecule that encodes the genetic instructions used for building and maintaining all of the structures of an organism. The Genetics Society of America explains that at the time of Sutton's theory, scientists already understood that traits were passed from parents to offspring according to certain patterns or rules.
Gregor Mendel's experiments with pea plants established many of the rules of heredity and they were widely accepted. At this time, however, it was not known what the mechanism of heredity could be - the function of DNA was unknown, according to the Genetics Society of America. The breakthrough came when scientists recognized that chromosomes organized themselves into pairs during meiosis; those pairs split during meiotic division in such a way that each daughter cell received only one from each pair. If the chromosomes themselves carried the genetic information, then this behavior explained Mendel's laws of inheritance.