The seeds can not be used in second generation is

The seeds that cannot be used in second generation is hybrid seeds.
Hybrid seed production is predominant in modern agriculture and home gardening. All of the hybrid seeds planted by the farmer will produce similar plants, while the seeds of the next generation from those hybrids will not consistently have the desired characteristics.
Hybrids are chosen to improve the characteristics of the resulting plants, such as better yield, greater uniformity, improved colour and disease resistance.