Complex sentence: Spina bifida is the only example of a uniquely human non-heritable genetic "defect" offered by Pye despite his claim that there are "more than two dozen" such "defects".
Split into simpler sentences: Spina bifida is Pye's only specific example of a uniquely human non-heritable genetic "defect". This is despite Pye's claims that there are "more than two dozen" such "defects".