Dave Curtis

Let’s say that the aim of eugenics is to intervene at a societal level to improve the genetic stock of the population, for example to eliminate undesirable characteristics or to produce average increases in the values of desirable traits. 3/n

Animals are bred in controlled environments and have short generational times with large numbers of offspring. In these circumstances selective breeding can produce desired changes in a small number of specific traits such as milk yield or racing performance. 4/n

So why wouldn’t it work in humans? Let me start by saying that there have been tremendous advances on our knowledge on this subject in just the last couple of years and our understanding has changed a lot. 5/n

My claims are based on results of genetic epidemiological studies of hundreds of thousands of people, such as UK Biobank, and sequencing studies of many thousands of people. These results have emerged recently and many commentators may not fully appreciate them. 6/n

There are a number of different kinds of reason why eugenics would not work. One is that humans have long generational times and small numbers of offspring. This would make any selective breeding process extremely slow. 7/n

Another reason is that humans are exposed to very different environments, so most of trait variation is not due to genetic factors but to differences in environment. One consequence is that it makes it hard to identify subjects who have desirable genetic characteristics. 8/n

We can now measure genetic potential directly from genetic markers and what we know from this is that these genetic predictors perform extremely badly. We can also tell that there are many important, very rare genetic variants which we will never be able to identify. 9/n

So we have an absolute ceiling on our ability to assess an individual’s genetic fitness from either their current performance or from their genome and we know that the potential ability to do this is extremely low, far too low to be useful for selective breeding. 10/n

We should bear in mind that harsh selection pressures have been acting on humans up to the present and that there may be very little scope for overall improvement. In any event, we can confidently say that selective breeding to improve desirable traits is not practicable. 11/n

What about removing deleterious characteristics? This was a prominent aim of previous eugenic enterprises. Recent work has shown many cases of severe intellectual disability are due to what are called de novo genetic mutations. 12/n

A de novo mutation is one which happens around the time the embryo is formed and means that the child has a new genetic variant which was not present in either parent. So a child with severe learning disability can be born to perfectly healthy parents. 13/n

Many other cases of severe intellectual disability occur as a result of recessive variants, where each healthy parent carries one copy of the variant but only the child who inherits two copies of it is affected. 14/n

With a recessive disease it may be possible to eliminate cases of the disease from the population using a combination of carrier testing, prenatal screening and selective termination. However this is not eugenics because the variants are still present in the population. 15/n

Everybody carries variants which are harmless but which would have a damaging effect if a child inherited another copy from the other parent. So selective breeding cannot eliminate carriers of recessive conditions from the population because everybody is a carrier. 16/n

Selective breeding cannot eliminate cases which are due to de novo mutations nor those due to recessive effects. Parents of children with de novo mutations are themselves genetically normal. Recessively acting variants are carried by everybody. 17/n

De novo and recessive effects account for a large proportion of genetic causes of intellectual disability and also to a lesser extent contribute to many other conditions, including autism and schizophrenia. 18/n