Bateson and Punnett performed a dihybrid cross with true breeding varieties of sweet pea (Lathyrus sativus) and instead of 9:3:3:1 ratio in F2 generation they got the ratio 7:1:1:7. It means that the characters controlled by the two genes chosen for the experiment do not follow the principle of independent assortment as postulated by Mendel. Instead they tend to be inherited together or are linked together. Thus genes present on the same chromosome tend to be inherited together and are said to be linked. This phenomenon is called linkage.
All the genes present on the same pair of chromosomes and with a tendency to be inherited together forms a linkage group.
In the above experiment some recombinant type of individuals were also produced. They are produced by another phenomenon called crossing over.
Crossing over is the physical exchange of parts of the non sister chromatids of the chromosomes of a homologous pair.
Crossing over occurs during prophase I at meiosis I of the time of gamete formation. The point where crossing over occurs is called chiasma. Linked genes get separated from each other by crossing over.
Because of linkage and crossing over a heterozygous individual can produce four types of gametes.