Among the results obtained from the numerous hybridization experiments of the late 18 th and the beginning 19 th century was the discovery of self-incompatibility. Self-incompatibility means that the pollen of a plant is unable to develop a pollen tube at the stigma of the same plant, while pollen of another plant of the same species does fertilize the plant. Self-incompatibility is a barrier against inbreeding and the homozygosis caused by it. It was possible to search for the genetic basis of self-incompatibility after the Mendelian laws of heredity had been rediscovered.
Incompatibility occurs whenever the two plants to be crossed carry the same alleles.
Since pollen is haploid is just one of the SI alleles expressed at its surface, while the diploid stigma surface bears the products of two alleles. Ferilization occurs only, if the allele product of the pollen and those of the stigma are unlike. Such bifactorial systems were later on found in numerous monocot and dicot families. All this aggravates a genetic analysis since it is impossible to identify all components of the combination matrix individually. The situation becomes completely confused in case of polyploidy.