Sutton and Boveri in 1902 observed that chromosomes from two parents come together in the zygote as a result of the fusion of two gametes and again separate out during meiosis at the time of formation of gametes. Chromosomes are filamentous bodies present in the nucleus and seen only during cell division. Gametes have half (n) number of chromosomes or are haploid and zygote is diploid or has (2n) or double the number of chromosomes when compared to chromosome number in the gametes.
The observations proved that there is a remarkable similarity between the behaviour of Mendelian factors or genes during inheritance and that of chromosomes during meiosis. This led Sutton and Boveri to propose chromosomal theory of inheritance.
1. The somatic or body cells of an organism, which are derived by the repeated division of zygote have two identical sets of chromosomes i.e. they are diploid. Out of these, one set of chromosomes is received from the mother (maternal chromosomes) and one set from the father (paternal chromosomes). Two chromosomes of one type (carrying genes controlling the same set of characters) constitute a homologous pair. Humans have 23 pairs of chromosomes.
2. The chromosomes of homologous pair separate out during meiosis at the time of gamete formation.
3. The behaviour of chromosomes during meiosis indicates that Mendelian factors or genes are located linearly on the chromosomes. With progress in molecular biology it is now known that a chromosome is made of a molecule of DNA and specific sets of segments of DNA are the genes.