Can be a number of things, rain fall quota, climate, population, vegetation..any of those.
Crossing over occurs at the four-strand stage when two homologous chromosomes, each consisting of a pair of sister chromatids, are paired. Each crossover involves just two of the four strands and generates two recombinant strands.
Hope this helps!
Answer:
Nucleus
Explanation:
The nucleus stores hereditart material and coordinates activities of the cell