Term for the existence of different characteristic variants (alleles) at the same gene locations of homologous chromosomes. A characteristic under consideration occurs twice in a double set of chromosomes and often in two different variants in the grapevine. In relation to all grapevine varieties, however, a certain characteristic can exist in up to 30 different variants, whereby in a grapevine plant only two of the possible alleles are realized at one gene locus at any one time. In sexual reproduction, only one single set of chromosomes is passed on per parent. In the vine this is realized by germination of a fertilized seed of the berry to a plant. Which of the two homologous chromosomes of a parent is passed on is decided by chance with a 50:50 chance. Together with half the chromosome set of the second parent, the embryo again has a complete but newly mixed chromosome set with only half of the maternal alleles, but enriched by the new paternal alleles. This is also called mixed, split, uneven or unequal inheritance of the fertilised egg.
In the course of the life history of a vegetatively propagated variety, further mutations occur, so that the cloned variety individuals in their allelic combinations increasingly differ from the original type and from each other (variety clones, mutants, variants, varieties). These deviations are also inherited by the offspring. This phenomenon of polymorphism of alleles within a sexually reproducing population with many different possible alleles per gene locus is particularly pronounced in the vine. Therefore, in contrast to homozygous plants (e.g. cereals), there is an inability to produce offspring of the same variety from seedlings. Heterozygosity means that the seedling from sexual reproduction not only differs greatly in genotype from the original vine (mother vine), but is also phenotypically different. This is used in the new breeding of grape varieties where other characteristics are desired in the offspring. This positive phenomenon is called heterosis effect. The production of genetically identical clones (duplicates) for reproduction, as practised in vine nurseries, can only be achieved by vegetative propagation. See also under DNA.