"modern medicine that will result in mankind accumulating more and more genetic defects."

Citation needed...

A lot of autosomal recessive genetic disease is a result of humans reproducing within their in-group, making the likelihood of two carriers of the same variant having offspring with that expressed variant. BRCA1 / BRCA2 in the Ashkenazi Jewish population are a common example. Nothing about medical genetic testing will directly cause more genetic defects.