When two genes are linked but are quite far apart so that crossing over occurs, their estimated map distance, based on recombination frequencies, is often an underestimation of their true map distance. What is the explanation for this underestimation? O The genes are located on different pairs of homologous chromosomes. O Some double crossover events go undetected since they do not lead to recombinant progeny. O There will be more single crossover events counted than actually have occurred, so the recombination frequencies will be reduced. o One of the genes is lethal when homozygous, which reduces the recombination frequency.