Why do mature germ cells of one organism carry different combinations of genes?

The most important achievement of sexual reproduction is the ability to produce offspring with different combinations of genes, which allows the offspring to survive in changing environmental conditions.
The mature germ cells will eventually merge into a zygote and give rise to a new organism. For this organism to have the viability and adaptability to new environmental conditions, it is important that the germ cells that form the zygote have a different set of chromosomes.

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