Heterozygous

In Mendelian genetics, every gene exists as two alleles — one inherited from each parent. When both alleles are different (e.g., one dominant "A" and one recessive "a"), the plant is heterozygous at that locus. When both alleles match (AA or aa), it is homozygous. Heterozygosity is the genetic engine behind hybrid vigor (heterosis): plants carrying diverse allele combinations from genetically dissimilar parents typically show stronger growth, higher yield, better disease resistance, and more uniform performance than either parent. This is why F1 hybrids — produced by crossing two genetically distant IBL parents — are the gold standard for commercial seed production. However, heterozygosity comes with a cost for breeders: heterozygous plants do not breed true. When two heterozygous F1 plants are crossed (F1 x F1 = F2), Mendelian segregation produces a 1:2:1 ratio of AA:Aa:aa offspring at each locus, meaning F2 seeds exhibit dramatic phenotype variation. To produce stable IBL parent lines for future F1 crosses, breeders must inbreed across multiple generations to gradually shift the population from heterozygous to homozygous at the target loci. This trade-off — hybrid vigor vs. genetic stability — defines the entire breeding cycle: IBLs (homozygous, stable, lower vigor) cross to produce F1s (heterozygous, vigorous, uniform), then F2s (segregating) to discover new pheno expressions, repeat.