Linkage occurs when the genes for two different characteristics are found on the same chromosome. This means that they do not independently assort during metaphase one of meiosis and so pass into gametes together; at fertilisation they then pass into the offspring and are inherited together. This reduces the possible allele combinations and so reduces genetic variation in the population. The majority of F2 organisms inherit the same combinations of characteristics as the F1 generations, as the only potential variation comes about as a result of crossing over events (which are comparatively rare). 

Linkage is the close association of genes or other DNA sequences on the same chromosome. The closer two genes are to each other on the chromosome, the greater the probability that they will be inherited together.