Explain why scientist have failed to produce vaccine against AIDS?

Explain why gymnosperms fail to produce fruits?

Explain why elements have fractional atomic masses ?

Explain why in case of Bt. insecticide protein produced by the bacterium kill the insect pest but not the cell of bacterium?

Some varieties of wheat are knows as spring wheat while other are called winter wheat. Former variety is sown and planted in spring and it harvested by the end of the same season. However, winter varieties, if planted in spring, fail to flower or produce mature grains within a span of a flowering seasons. explain, why?