Which process is crucial for sexual reproduction as it reduces the chromosome number in gametes?

Meiosis is the process crucial for sexual reproduction because it reduces the chromosome number by half in gametes, which are the sperm and egg cells. This reduction is essential because it ensures that when fertilization occurs, the resulting zygote has the correct diploid number of chromosomes, maintaining the species' chromosome count across generations.

During meiosis, a single diploid cell undergoes two rounds of division, ultimately producing four haploid cells. Each of these haploid cells contains only one set of chromosomes, which is vital for sexual reproduction. When the gametes from two parents (one from each parent) fuse during fertilization, their haploid numbers combine to restore the diploid state in the new organism.

In contrast, mitosis is not involved in sexual reproduction; it produces identical daughter cells for growth and repair. Fertilization, although vital for combining genetic material, does not alter the chromosome number itself. Binary fission is a method of asexual reproduction observed in prokaryotic organisms, which does not apply to the context of gametes or sexual reproduction.