These kinds of chromosomal “package deals” could help explain the molecular mechanics of sexual attraction.

Unlike most genes, which are either dominant or recessive, MHC genes are co-dominant; both are expressed. If mom passes along MHC genes that differ from dad’s, the kids inherit a wider range of immune responses and stand a better chance of surviving a broad array of microscopic invaders. But even with fortuitous immunity combinations from mom and dad, offspring are still vulnerable. Parasites and pathogens, with their life spans measured in hours or days, have a decided evolutionary advantage. They can muster up new offense tactics hundreds of times faster than their longer-lived hosts can evolve effective defenses.

“This is the classic evolutionary arms race,” Anderson said.

Scientists have borrowed a character from Lewis Carroll’s Through the Looking Glass to describe it. Just as the Red Queen tells Alice she must keep running just to stay in one place and that to get someplace else she must run much faster, the Red Queen hypothesis says that hosts must constantly evolve new adaptations to outwit parasites and pathogens. What worked for mom and dad’s generation helps protect their young when confronted with similar threats, but it may provide little protection for sons and daughters confronted with new strains of harmful bacteria, viruses or fungi.