So Plomin expanded the search. Rather than looking at a predefined set of genes, he mapped thousands of genetic markers sprinkled across the chromosomes of his subjects. If a marker turned up frequently in high- or low-scoring students, there might be an intelligence-linked gene not far away. He and his colleagues added more children to their study so that they could detect genes with weaker effects. At one point in the research, Plomin thought he had found an authentic link between intelligence and a gene known as IGF2R that encodes a growth factor receptor which is active in the brain. But when he and others tried to replicate the result, they failed. “It doesn’t look like that has panned out,” he says.

Plomin suspected that he needed more genetic markers to find intelligence genes. When eggs and sperm develop, their chromosomes swap segments of DNA. The closer two segments of DNA are to each other, the more likely they are to be passed down together. But in Plomin’s early studies, millions of DNA nucleotides separated each pair of markers. It was possible that intelligence genes were so far from a genetic marker that they were sometimes getting passed down together and sometimes not. He needed a much denser set of genetic markers to reduce the chance of this happening.

   
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