Surgery and Healing in the Developing World - Dartmouth-Hitchcock

Metabolic Maladaptation
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cell anemia, there is no adaptive advantage of the homozygous patterns of the disease
no matter where its victim lives. Sickle cell anemia is a classic unmitigated
disease with enough associated morbidity that it should have been naturally selected
out of the population as distinctively disadvantageous.
Genetic Predetermined Disease
But two factors coincide to conserve this disease. First is the fact that it is not a
classic Mendelian dominant characteristic; it requires sickle hemoglobin inheritance
from both parents for the true sickle cell disease. The heterogyzote is not severely
effected by disease, and is instead characterized as having “sickle cell trait”. This
detectable abnormality confers a survival advantage on the person who bears it,
given the second factor in the environment-—malaria. The often lethal falciparum
malaria parasite cannot readily infest the red cells composed of sickle trait hybrid
hemoglobin, so this heterozygous condition confers a relative resistance upon the
carrier to malaria infestation. Sickle cell trait, therefore, is favored in tropical regions
of high falciparum endemnicity, whereas people with the sickle cell anemia are susceptible
to lethal anemia crises and people with normal hemoglobin are susceptible
to a lethal form of malaria. The sickle hemoglobin is a disease conserved, therefore,
because it confers adaptive advantage to coexistant falciparum malaria—-and the
incidence of the two diseases can be drawn on coterminus maps. 49
Acquired Disease
Two important distinctions are drawn in comparison with the “conserved diseases”
of sickle cell hemoglobin and hypothyroidism that make them remarkably
different. The hemoglobinopathies are familial and are inherited as genetic characteristics.
In fact, the genetic mutation in the case of sickle hemoglobin has been
defined down to a “point mutation” in a single base pair that gives rise to the synthesis
of the abnormal sickle hemoglobin. Penetrance and expression of the sickle gene
is high and sickle cell anemia “breeds true”, with little evidence of amelioration of
the disease state by other biologic adaptations. Hemoglobin status is genetically
determined, does not remarkably interfere with fertility and is congenital—-a term
that will be used to mean literally that one is “born with” the condition. Genetic
differences that can be passed on and environmental adaptive advantage are the two
kinds of ingredients evolutionary biology can work with over time to select in, or
out, those genetic predeterminants that are more, or less, adaptive.
Hypothyroidism is acquired. Within an adult individual’s lifetime—or, indeed,
within a short period of treatment—it can be abolished—and was, within a generation
of iodized salt introduction in the West, and within the period of observation
in this study. Earlier, and largely discredited, suggestions 50 held that there may have
been “a genetical [sic] factor in endemic goiter”, but this has been disproven by
several lines of evidence. First, hypothyroidism is a state of reduced fertility, with the
more profound the hypothyroidism, the less able the individual to reproduce. Second,
the disease is curable in the adult, and no transmission of the disorder then
occurs in the offspring.
But, is not cretinism congenital? Yes, in the sense that the term’s literal meaning
is to be “born with” the disorder; but it is not genetic and familial. Cretinism is a
disease acquired in utero, and such profound failure to develop may occur that cretinism
may be irreversible at the time it is recognized and treated after birth. Even
cretin women, if they are iodine repleted and should happen to become pregnant,
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