Question

how do we know that alleles actually separate during gamete formation?
zvgotes will not reach maturity if the alleles do not separate.
scientists are unsure whether the alleles separate.
if the alleles did not separate, recessive alleles would never reappear.

Explanation:

Mendel's experiments showed that when alleles do not separate (violating the law of segregation), recessive traits would not reappear in the expected ratios in later generations. For example, in a monohybrid cross, if alleles of the F1 generation (heterozygous) did not separate during gamete formation, all offspring in the F2 generation would show the dominant trait. But in reality, recessive traits reappear, indicating allele separation.

Answer:

If the alleles did not separate, recessive alleles would never reappear.