Early on, some scientists objected to the eugenicists' insistence that heredity overwhelmed environmental influences in shaping human life. Others objected to the eugenicists' methodologies, noting that family studies often relied on hearsay evidence and biased observation rather than direct, quantifiable, empirical measurements. Others challenged the eugenicists' reliance on phenotypic traits such as body form to diagnose presumed underlying genetic causes. Developments in genetics increasingly undermined this simplistic reasoning from phenotype to genotype.
Instead, genetic studies increasingly revealed the complex nature of most human phenotypic traits. Human traits rarely result from the action of single gene pairs, and expression depends on complex environmental influences. Moreover, many genes induce pleiotropic effects: that is, a single gene may influence more than one phenotypic characteristic. If multiple genes cause single traits, or if single genes are involved in many effects, then any attempt to "breed out" traits becomes virtually impossible. Moreover, if, as most eugenicists believed, negative traits are recessive factors in single-gene disorders, then most "bad" genes are harbored in apparently normal, heterozygous carriers. The Hardy-Weinberg theorem, formulated in 1908, made it clear that eugenic selection directed solely against affected individuals would barely reduce the incidence of a trait in the larger population. To decrease such defects by half would require forty generations (1,000 years) of perfect negative eugenics.
The Hardy-Weinberg theorem alone, unfortunately, did not dissuade most geneticists from eugenics. Many continued to believe, as geneticist Herbert Spencer Jennings wrote in 1930, that preventing the "propagation of even one congenitally defective individual puts a period to at least one line of operation of this devil. To fail to do at least so much would be a crime." Nevertheless, the most bigoted aspects of eugenics dwindled after 1946, as scientists recoiled from the horrors of Nazi atrocities.
The dream of improving human life through genetic intervention remains with us today. While genetic knowledge and technology have changed since the Holocaust, the cultural and political context surrounding the pursuit of genetic improvement has undergone even greater transformations. The goal of present genetic intervention is not group improvement, but individual therapy. Modern conceptions of individual, patient, and human rights reduce the risk of abuses committed in the name of eugenics. While negative eugenics has been largely eliminated as neither possible nor socially acceptable, positive eugenics are still considered desirable among
some people who propose genetic engineering for the development of children with superior traits. The ethical issues surrounding genetic engineering and cloning are still debated in light of the history of the eugenics movement. see also Cloning: Ethical Issues; Gene Therapy: Ethical Issues; Hardy-Weinberg Equilibrium; Inheritance Patterns; Reproductive Technology: Ethical Issues.
Gregory Michael Dorr
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All life on Earth is cellular and uses DNA to store genetic information. However, evidence suggests that, on ancient Earth, much complex chemical activity preceded cellular development, and it was probably not DNA-based at the start. What was the nature of this activity, and how did it lead to life? "Molecular evolution" is a term used to describe the stages that preceded the origin of life on Earth. The term implies that information-containing molecules were subject to the process of natural selection, whereby genetic structures were capable of both replication (the copying of specific nucleic acid sequences) and mutation. In addition, it is theorized that certain chemical reactions may have taken place on early Earth, before true evolution began, and some of these reactions may have helped to form these informational molecules that enabled replication and mutation.
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