In the "dna isolation and amplification" lab we isolated mitochondrial dna and treated the cheek cells with chelex, which binds to mg2+. why is this important to the dna isolation?

Is important to inhibit the activity of DNases

Explanation:

The magnesium ions are cofactors of multiple enzymes like deoxyribonucleases also called DNases  (cut the DNA)  that helps in activating the enzymes for its functions. Chelex catches magnesium ions and inactivates the DNases. Is important in DNA isolation for the inactivation of DNases that can degrade the DNA that we want to isolate.

this is true, assuming that the allele is completely recessive and incomplete dominance is not a factor.