Geoffrey MillerVerified account

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