What happens during metaphase 1 that leads to genetic variation?

In metaphase I, the tetrads line themselves up at the metaphase plate and homologous pairs orient themselves randomly. This variation is due to independent assortment and crossing-over during meiosis, and random union of gametes during fertilization.

Explanation:

answer by yourhope:

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in which structure in the nucleus are ribosomes made?

b) nucleolus!

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in 1859, charles darwin set out his theory of evolution by natural selection as an explanation for adaptation and speciation. he defined natural selection as the "principle by which each slight variation [of a trait], if useful, is preserved".