5. RNA Flouts Rules of Heredity
Minoo Rassoulzadegan's mice are unruly. In blatant violation of the laws of genetics—described a century ago by famed monk-scientist Gregor Mendel—they inherit their parents' coloring without inheriting the genes that cause it. Rassoulzadegan, a geneticist at the University of Nice, is not the first scientist to marvel at this apparent breakdown in the rules of DNA. "Geneticists have noticed strange, non-Mendelian things ever since Mendel," says geneticist Robert Martienssen of Cold Spring Harbor Laboratory. "We've been wondering what this weird stuff was for decades." In 2006 a suite of experiments by Rassoulzadegan and others have started clearing up the mystery surrounding epigenetics, the inheritance of traits that cannot be explained by DNA alone.
Rassoulzadegan stumbled upon her rodent scofflaws after altering a gene in gray mice so that their feet and the tips of their tails turned white. The big surprise came in the next generation. Some offspring also had white spots, even though they didn't inherit the mutated gene and so should have been all gray. When she looked for the cause, Rassoulzadegan found unusual amounts of RNA in the sperm of the mutant parents. She then injected RNA from the brains and sperm of those mice into ordinary gray mouse embryos. Many of the RNA-injected embryos likewise grew into white-tailed adults, regardless of the coloration written in their DNA.
In an experiment published two months after the mouse work, Vicki Chandler of the University of Arizona at Tucson found the same process at work in plants. She was investigating the cause of a perplexing phenomenon in maize in which the light-purple color of the stalk is passed on to offspring even when the gene sequence that caused it in the parent is not. She, too, traced the cause to RNA.
