There are a number of factors which should carefully be thought about when choosing a pollinizer cultivar. Flowering characteristics Compatibility, the amount of viable pollen produced, and time of pollen shed must be considered when selecting a pollinizer. Pollinizers are required in hazelnut orchards because cultivars are self-incompatible. This incompatibility is controlled by a single gene. This single locus has 28 alleles.
Diploid cultivars have two alleles.
In a female flower, both alleles are expressed therefore they are codominant. The pollen may express one or both alleles, therefore are dominant or co-dominant, respectively. If an allele expressed in the pollen is met with the same allele in the female flower, then the cross is incompatible. This cross is incompatible. Whereas, 'Barcelona' flowers pollinated with 'Hall's Giant' is a compatible cross because the alleles expressed in the pollen are S 5 and S 15 .