The increase in the level of production traits due to crossing is called hybrid vigour or heterosis. The term hybrid vigour points at the vigour, which is a real factor that can bee seen by the naked eye, in comparison to pure breeding. The other word ‘heterosis’ is derived from heterozygosity and points toward the direction form where the effect arises. The direction of the effect of crossing is opposite the effect of inbreeding.
When dominance deviation occurs a positive gene provides most effect in the heterozygotes. The dominance deviation occurs, as shown in chapter 6, when the heterozygote’s values for a quantitative character deviates from the mean value of the two homozygotes. In cases with complete dominance the heterozygote has the same value as the dominant homozygote.
The increase in heterozygosity is the under laying cause of hybrid vigour. The increase in hetrozygosity gives a higher mean value, which is proportional to the increase in the production traits.