The problem with a lot modern science is that you're dealing with a virtually infinite numbers of variables both environmentally, behaviourally and genetically.

Short of having a thousand identical twins locked up in a room from birth you're swimming against a very strong tide. Long gone are the days when science was as simple as castrating a rooster and grafting his balls back: https://en.wikipedia.org/wiki/Arnold_Adolph_Berthold

https://en.wikipedia.org/wiki/Replication_crisis