Carl Zimmer’s new book She Has Her Mother’s Laugh makes the subject of genes and heredity accessible to non-scientists. It covers many subjects, such as royal inbreeding, eugenics, mitochondrial DNA vs. chromosomal DNA, and the weird things that can happen along the way as a new human is constructed.
To focus in is to find chromosomes playing all sorts of tricks. Take, for example, chimeras. To the ancient Greeks, the Chimera was a fire-breathing hybrid monster; to a biologist, chimeras are organisms that comprise cells from two different individuals. Ranchers are familiar with one type of chimera, the freemartin, which results when a cow carries opposite-sex twins. Connected by a shared placenta, the fetal calves exchange stem cells. The bull calf grows up into a more or less normal bull, while the heifer—the freemartin—has undeveloped ovaries and exhibits masculinized behavior (and is particularly tasty on the grill). Where does one calf end and the other begin?
Zimmer describes a bizarre twist on the free-martin: a girl with different-colored eyes and ambiguous genitals who appeared at a Seattle genetics clinic. Her ovaries proved to have only XX chromosomes—typical female—but her other tissues were mixtures of XX and XY. Further analysis showed that she had started out as opposite-sex twins. But early in development, the two embryos had fused, becoming a single, highly unusual child. Like a verse from the old Ray Stevens novelty song “I’m My Own Grandpa,” this girl was her own twin brother.
But chimeras are not just oddities. You surely know one. In pregnant women, fetal stem cells can cross the placenta to enter the mother’s bloodstream, where they may persist for years. If Mom gets pregnant again, the stem cells of her firstborn, still circulating in her blood, can cross the placenta in the other direction, commingling with those of the younger sibling. Heredity can thus flow “upstream,” from child to parent—and then over and down to future siblings.
(Image credit: Mayuko Fujino)