The zipper effect: Why different positions along the chromosome suffer different selection pressures

Variability within diploid sexual populations comes from two ingredients: mutations and recombination (or crossing-over). On average, the first introduces genetic defects in offspring genomes, while the second is a mechanism which tends to eliminate them, continuously “cleaning” the population genetic pool from harmful mutations along the generations. Here, we propose that loci near the chromosome tips are more effectively cleaned by the recombination mechanism than loci near the chromosome centre. This result implies that clusters of neighbouring, orchestrated-functioning genes, supposed to be more robust against the effects of genetic mutations, are more likely found near the chromosome centres, while isolated genes are more likely found near the tips. We confirm the tip–centre asymmetry through a simple computer agent-based model. In order to test this effect in reality, we also analyse as an example the particular case of HOX genes distributed along the 24 human chromosomes and verify that indeed, most HOX genes belong to such clustered networks located near the chromosome centres. Accordingly, isolated HOX genes are located closer to the tips.