Heterosis

Most of the maize varieties grown in the U.S. are hybrids between two parental inbred lines of contrasting pedigree. One of the reasons for using such “wide crosses” is to obtain heterosis. Heterosis, also known as hybrid vigor, is a phenomenon where an organism has greater vigor than its distantly related parents (and is the opposite of inbreeding depression). In maize, for example, the hybrid offspring of two inbred lines that descend from the two major ancestral pools of maize will usually be taller and have larger ears (cobs) than either of its parents. Although the phenomenon of heterosis was first described over a century ago, and has been exploited in crop breeding ever since, scientists still do not fully understand why it happens. The molecular basis of heterosis remains a hot topic of current research. Currently, scientists believe that heterosis is mainly caused by healthy alleles across numerous genes from one parent compensating for dysfunctional alleles at those genes from the other, distantly related parent (and vice versa for a second set of genes). You might think of heterosis as having “the best of both worlds” in a single plant!