Question of the Week

Is modern medicine affecting the human gene pool?

Sat, 17th Sep 2011

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Question

Derek, Japan asked:

Over the past 100 years, and much more so in the past 40 years, medical science has been able to save an increasing number of babies (and mothers) being born that would have not survived if they had been born at any other time in human history. Complications that would have either ended the pregnancy, killed the baby during childbirth, and/or killed the mother are now easily overcome. Is this reducing the quality of the entire human race's gene pool? Due to medical science, the strongest are no longer the only ones to survive today. It seems to me like almost everyone I know has some type of health issue. Is this due to our "diluting" of the gene pool, and is it going to get worse as medical science gets better?

 

 

Answer

We put this question to Professor Bill Amos, from the Department of Zoology at Cambridge University...

Bill -   This is an interesting question but I'm afraid many of the aspects really are quite unresolved.

Perhaps we could start with the problems with births.  Humans of course have been evolving larger and larger brains for a while now which gives them larger skulls and this of course can present problems during the birth process.  These days, we can use caesarean section but the key point here is that it’s only going to become an increasing problem if those children born by caesarean section are born, grow up and have larger families than on average.

An Intensive Care Unit (ICU)This is a recurrent theme, so for example individuals with spina bifida – this is a rare genetic disease.  If they also grow up, again, this is only going to become an increasing problem if they themselves then have larger families, and it’s almost certainly not the case that this would happen with most genetic defects.

I think perhaps more interesting is the question of the immune function, that genes that help us fight disease.  And here, I think there may be an interesting issue.  In developing countries where there's much less access to medical treatment and antibiotics, many children die through infections that are potentially preventable.  This, in theory, removes some of those individuals with weaker or poorly attuned immune systems from the population, but in the western culture where more medicine is present, these individuals would grow up.  So what happens to them in western culture?  My best guess is that these are the people who are likely most prone to asthma and allergies since these are potentially reflections of a poorly tuned immune system and that is what we might predict these have.

Diana -   So modern medicine might change who lives and who dies, but as the human population is so large, the overall effect on the gene pool shouldn’t be species altering and this is because the number of people either being born by caesarean section or surviving in spite of inherited diseases aren’t likely to breed in greater numbers than the rest of the population.  Human immune function however may change through time.

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Excellent question, and something I often have wondered myself.  I look forward to reading the "answer".

There would have to be enough data now to study 2 or 3 generations after a premature birth, complicated birth, or low birth weight.  What about sperm motility?  There is now technology to essentially inject DNA from a low motility sperm directly into an ovum.  Do second generation fathers from this procedure also have issues with low sperm counts and low sperm motility?

Some modern treatments like the use of RhoGAM after the birth of a first child to prevent problems with a second child birth saves many children, but not necessarily with negative gene consequences.

Of course, on the positive side, we now have the technology to abort fetuses with major chromosomal abnormalities before they are born.  And, sperm and egg banks allow parents in certain situations to select what they feel are positive attributes for their children.  Will longevity be a feature that will be selected for in the future?

You might also ask whether this is a "modern" issue, or whether it is an evolutionary issue that as already been occurring for hundreds of thousands of years, perhaps millions of years.  Humans have already evolved to have one of the most complicated birth procedures of all animals, some of the most helpless newborns, and a very long developmental period.  Most cultures use some form of midwives.  How old and "universal" is the use of hot water in conjunction with childbirth?  Anyway, the issues that have led to a difficult childbirth and helpless infants are the same issues that have led to intelligence and adaptability of the species. CliffordK, Sun, 11th Sep 2011

While individuals with a given genetic defect wouldn’t be actively selected for, wouldn’t spontaneous random mutations tend to deteriorate functional genes over time in the absence of selective disadvantage to genetic defects?

Also, I'm confused as to why individuals with a poorly tuned immune system would be more selected for than, say, individuals with genetic spina bifida, in western populations. Lightbulb, Sat, 24th Sep 2011

I somewhat disagree with the stated answer. Looking at genetic disease statistically, many are becoming manageable and offsetting what was in the past a certain death sentence. Some diseases such as sickle cell, Huntington's, and FH are statistically significant and are becoming increasingly manageable. Not to mention if the future of medicine provides increased relief, you could certainly see the retention of genes that otherwise would be eliminated by early death. The Penguin, Tue, 18th Oct 2011

Sickle Cell disease is a somewhat unique disease.

It is a recessive disease that is very different in the heterozygous and homozygous forms.

It is believed that the heterozygous sickle cell trait provides some protection against Malaria.  So, improving the lives of the homozygous children may not greatly affect the gene pool as the numbers of carriers of the heterozygous trait are selected for by a common disease in Africa.   In fact, the prevalence of the sickle cell genes may be negatively impacted by aggressively treating Malaria, as well as genetic counseling for carriers of the trait.

Perhaps that is a point when considering genetics.

A recessive trait can be maintained in a gene pool by heterozygous carriers.  If on average a recessive trait is passed on to half of the children, then in a family of two, on average only ˝ of the offspring will be a carrier.  Larger families would spread the genes quicker throughout society.  While a homozygous individual does pass on the genes to 100% of the offspring, those would be few individuals for a rare recessive disease.  The disease is only amplified if there is some selective pressures to increase its prevalence. 

Nonetheless, it sets a dangerous precedence to select traits that would otherwise have not continued in the gene pool.

A lethal dominant gene would die out quickly without medical intervention.      CliffordK, Fri, 21st Oct 2011

You could take this a step further and ask the question what external factors might effect the human gene pool. 

For example would the invention of spears effect the human gene pool? 

Today with our population so large any kind of external factor among such a large number of people would probably go unnoticed. What if you asked the same question about medicine if we had a much smaller population, say 1 million people?
If the response is well yes we have had an effect on the gene pool then even though this effect is so dilute amongst the actual population then it must have had some impact even if minuscule. 
Aaron_Thomas, Sat, 22nd Oct 2011

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