The other day I mentioned “chimeras,” thinking of werewolves and centaurs. Turns out that humans can harbor two types of blood; it’s a condition called chimerism. One of the ways chimerism can occur is through blood cells from a “vanishing twin.”

20 to 30 percent of pregnancies that start out as twins end up as single babies, with one twin being absorbed by the mother during the first trimester.
Others researchers have found that in some cases, before the twin is absorbed, some of its cells enter the body of the other fetus and remain there for life. The cells can include bone marrow stem cells, the progenitors of blood cells.
Another route to chimerism is through the cells that routinely pass from a mother to fetus and remain there for life.

It’s being discussed within the context of the cyclist Tyler Hamilton’s recent two-year racing ban by the US Anti-Doping Agency. Interesting stuff.