The origin of chromosomes. I. Selection for linkage.

A model is analysed of cells containing independently replicating genes, which segregate randomly when the cell divides. We follow the fate of a primitive chromosome, in which two genes are linked, so that they replicate and segregate together. Such a chromosome increases in frequency in the population provided that (i) the two genes act synergistically, so that a cell containing at least one copy of each grows faster than a cell lacking one or other, and (ii) the number of genes per cell is small. The increase in frequency occurs even if the chromosome takes twice as long as an individual gene to replicate, giving a two-fold selective disadvantage within a cell. The increase occurs because a gene that is linked does not run the risk of finding itself, in the next generation, in a cell that lacks its synergistic partner.