There are lots of things wrong with Mukherjee's best-selling book The Gene. I've listed a few things that I know about [What is a "gene" and how do genes work according to Siddhartha Mukherjee?]. Others have come up with different problems.The biggest problem is that Mukherjee misrepresents the current state of knowledge in genetics, biochemistry, and molecular biology. His misleads his readers by promoting silly viewpoints that conflict with the consensus view. He doesn't mention that there are other views that are well supported by tons of scientific evidence.
The best example is regulation of gene expression. He fails to explain the standard textbook understanding of transcriptional regulation by transcription factors—a view that's solidly backed by decades of work in biochemistry, developmental genetics, molecular biology, and genomics. Instead, he promotes a flaky epigenetic theory that, according to him, threatens to overthrow Darwinian evolution.