For the purpose of this article, eugenics is defined as “the belief that certain individuals should be killed, be forced to undergo sterilization or other be exposed to other coercive measures to prevented them from reproducing in order to protect the population from harm and to ensure the genetic quality of future generations”. I will occasionally attribute other beliefs to eugenics, such as beliefs in “racial purity” or that evolutionary beneficial implies moral, so let’s consider this a working definition for now. Yes, I am aware that there are people who support other forms of eugenics based on voluntarism etc. but those groups are not the target here.
As we shall see, there are many problems with eugenics. It is based on a multitude of scientific falsehoods, has huge practical problems, it is arguably not cost-effective and wildly unethical. Some of these points are somewhat overlapping, but they emphasize specific problems.
1. Eugenics is based on artificial selection, but this is in practice mainly useful for selecting genes with additive effects. However, most genes have interacting effects, making eugenics less efficient, although not impossible.
2. Eugenics is based on a naive view of development. There is hardly never a direct 1:1 relationship between one gene and one phenotypic trait. In general, most traits are polygenic (influenced by many genes) and most genes are pleiotropic (affect many different traits). It is more accurate to think of the situation as a huge, complex network of genes and gene products influencing each other. The heritability of personality traits and certain complex hereditary diseases tend to be moderate (calculated from twin and adoption studies). Using Genome-wide association studies to analyze hundreds of thousands of single nucleotide polymorphisms (SNPs), scientists have found that candidate SNPs can only account for a fraction of his heritability (“missing heritability problem”). This may be accounted for by rare gene variants that are unique for different populations, variation in copy number or genetic interactions.
3. Eugenics is based on a naive view of the power of genes. Genes tend to be risk factors for certain conditions, where environment can act as the trigger. A classical example is the condition know as phenylketonuria (PKU). The genetic risk factor is a mutated version of a gene coding for the enzyme known as phenylalanine hydroxylase that catalyze the hydroxylation of the amino acid phenylalanine to tyrosine. When this is non-functional, phenylalanine accumulates and is converted to phenylketones. This in turn causes mental retardation, brain damage and seizures. An incredibly successful treatment is a diet free of phenylalanine and monitoring of the blood levels of this amino acid. In this case, environmental interventions are more beneficial, cheaper and less unethical than eugenics.
4. If you imagine the general problem outlined in point 3, but instead think of it being hundreds of different genetic and environmental risk factors, then you have an approximate view of most complex human diseases.
5. Even for so called single gene disorders, an individual with one copy of the defect allele and one copy of the healthy allele may have a selective advantage. The classic example is that a person heterozygous for the allele that in the homozygous condition causes sickle-cell anemia has a higher resistance to malaria. The allele, although detrimental in the homozygous condition, is retained in the population by balancing selection. Eliminating gene variants that cause disease in the homozygous condition may lead to less prevalence of individuals with heterozygous advantage. Read more of this post