Heterosis has a positive effect because in the crossbreds many genes are heterozygous that were homozygous in the parent breeds. Alleles with a negative effect are often recessive. In the crossbreds these negative recessive alleles are ruled out. The amount of heterosis to be expected for a specific characteristic in a cross of two breeds depends of the number of loci involved and the differences between the two breeds in the relevant allele frequencies at these loci.
In the figure below this is illustrated when a trait is determined by the two alleles of one gene. The higher the difference in gene frequency, the higher the amount of heterosis. After crossing the two lines for multi genic traits the amount of heterosis is determined by the average difference in allele frequencies for the total number of genes involved and the dominance effects on each locus. When the difference in allele frequencies is -1 (for all genes the breeds are homozygous for different alleles, than the heterosis is equal to 1 (100%).
