AN EVOLUTIONARY PERSPECTIVE ON HUMAN HEALTH AND DISEASE

Durgavich’s initial hypothesis was a thought-provoking one: the unusually high rate of the Tay-Sachs mutation in Ashkenazi Jews today may relate to advantages the mutation gave this population in the past.

Of course, she’s not referring to the unfortunate individuals who inherited two copies of the mutation, but to those with only one faulty gene copy – such as herself. Could that one faulty copy have made the population more likely to survive, reproduce and pass on their genetic material?

This phenomenon is called “heterozygote advantage”, and it explains why sickle cell anaemia is more prevalent in African and Asian populations. Durgavich explains:

“In these geographic regions, malaria poses significant risks to health. The parasite that causes malaria, though, can only complete its life cycle in normal, round red blood cells. By changing the shape of a person's red blood cells, the sickle cell mutation confers protection against malaria. People with the mutation aren't less likely to get bitten by the mosquitoes that transmit the disease, but they are less likely to get sick or die as a result. Being a carrier for sickle cell anaemia is therefore the best possible genetic option in a malarial environment.

“Carriers are less susceptible to malaria, because they make some sickled red blood cells, but they make enough normal red blood cells that they aren't negatively affected by sickle cell anaemia.”

In the case of the Ashkenazi population, Durgavich posits that the Tay-Sachs mutation may have offered increased resistance to tuberculosis in past generations. According to research conducted in the 1970s, the Eastern European-born grandparents of a sample of American Ashkenazi children born with Tay-Sachs were much less likely to die from TB than non-Jews from the same area. Only one of the 306 grandparents in the study died of TB, despite TB causing up to 20% of deaths in large Eastern European cities during the early 20th century.

That’s quite a statistic, and one which offers a compelling case for why problematic versions of genes persist at high frequencies in certain populations. But why did this resistance mechanism proliferate among the Ashkenazi population in particular? Durgavich points to historical and cultural factors as one possible explanation:

“At various points in history this population was forced to live in crowded urban ghettos with poor sanitation. Ideal conditions for the tuberculosis bacterium to thrive. In these environments, where TB posed an especially high threat, those individuals who were not carriers of any genetic protection would have been more likely to die.

“This winnowing effect together with a strong cultural predilection for marrying and reproducing only within the Ashkenazi community, would have amplified the relative frequency of carriers, boosting TB resistance but increasing the incidence of Tay-Sachs as an unfortunate side effect.”

From this example, it’s clear to see how unravelling your own evolutionary past can glean useful insights. “Big picture” analysis equips us with the knowledge to make better-informed decisions, not only about our own health and lifestyles, but that of our offspring too. If, as a teenager, you knew for a fact that you were genetically disposed to developing life-ending lung cancer, would you still light that first cigarette?