Choosy Eggs May Pick Sperm for Their Genes, Defying Mendel’s Law

In the winner-takes-all game of fertilization, millions of sperm race toward the egg that’s waiting at the finish line. Plenty of sperm don’t even make it off the starting line, thanks to missing or deformed tails and other defects. Still others lack the energy to finish the long journey through the female reproductive tract, or they get snared in sticky fluid meant to impede all but the strongest swimmers. For the subset of a subset of spermatozoa that reach their trophy, the final winner would be determined by one last sprint to the end. The exact identity of the sperm was random, and the egg waited passively until the Michael Phelps of gametes finally arrived. Or so scientists have thought.

Joe Nadeau, principal scientist at the Pacific Northwest Research Institute, is challenging this dogma. Random fertilization should lead to specific ratios of gene combinations in offspring, but Nadeau has found two examples just from his own lab that indicate fertilization can be far from random: Certain pairings of gamete genes are much more likely than others. After ruling out obvious...