A well written primer of our understanding of specific genes and their impact on sports performance—from natural physical traits (height, strength, achilles, calf size, etc) to training ability to pain thresholds and more. David tries to balance the nature (genes) vs nurture (environmental) factors. He also points out, repeatedly, that our understanding of impact of specific genes, specially in combinations, is still very incomplete.
Another takeaway, for me, is that much of what our understanding of these genes reveals is what separates the (say) Olympians from everyday people. The performance differences amongst the people at the top is too small to be explained by our current understanding of genes.
Finally, even at the very top, there are multiple paths to get there. The story of two high jumpers illustrated this well.