Why does a single cross hybrid give better yield compared to double cross hybrid?

Question

Why does a single cross hybrid give better yield compared to double cross hybrid?

Answer 1

Hybrid lines yield better than either parent due to hybrid vigour, a phenomenon not yet thoroughly understood.  Single cross hybrids are generated by crossing 2 inbred (genetically pure - homozygous) parent-lines. So, the hybrid population obtained is genetically heterozygous, but theres no genetic variation between siblings.  Double-cross hybrids are obtained by crossing two different hybrid plants. While these seeds might still have hybrid vigour, there is genetic variation between siblings in the double cross hybrid population.  This is why the double cross hybrids do not perform as well as single cross hybrids. Earlier, use of less vigorous female parents in the production of single cross hybrid seeds led to low hybrid seed yields and higher seed production costs.  The double cross hybrid method was developed to overcome these problems while retaining hybrid vigour. However, due to very good selection processes over the years, parental lines with good seed yields have been identified for generating single cross hybrid seeds.