Armand Leroi is a developmental and evolutionary biologist, who writes about the biological implications of various birth defects and human variants, as well as tracing the historical records and mythology surrounding certain identifiable syndromes. Much of it was review of basic information on developmental biology and teratology that I learned many years ago, but this was an engaging format for review of material that still fascinates me.
Each chapter describes and discusses a different kind of mutation, the first being conjoined twinning. Apparently the oldest depiction of conjoined twins comes from a Neolithic shrine in Anatolia. In the Renaissance, the birth of conjoined twins was sometimes taken as an allegory for political union. Scientifically, much of the debate has been to determine whether conjoined twins are the result of incomplete monozygotic twinning (a single embryo splits into two) or of fusion between two separate embryos. The author explains why neither of these theories fits the bill, and why an aberration of embryonic organization is more likely to be the culprit.
Situs inversus (organs arranged in the mirror image of the usual situs solitus positioning) also may come into the mix, as in twins fused side to side (rather than head to head, for example), 50% of right-side twins (from the twins' perspective) have situs inversus. Situs inversus is also a symptom of Kartagener's syndrome, appearing in 50% of afflicted individuals who are also plagued by a weak sense of smell and sterility. What common thread ties these symptoms together? A mutation in some one of the genes coding for the protein complex called dyenin, which leads to defective cilia (little, beating, cellular "hairs"), which render sperm immotile, and the lungs and bronchi unable to clear irritating particles, as well as rendering the developing embryo unable to tell the difference between right and left. With no right/left "awareness," the positioning of developing organs is a random occurrence.
Other chapters cover the cyclops in myth and reality (a malformation of the face and brain which is incompatible with life), sirenomelia (fusion of the lower limbs), limb deformities, dwarfism, albinism, hermaphroditism, hypertrichosis (excessive hairiness), and more.
Perhaps most interesting of all is his final discussion of the biology of beauty. The author observes that as much as the idealized vision of beauty may differ between cultures, a common thread is not simply health, but facial and bodily evidence for a lack of deleterious mutations.