After the re-discovery of Mendel's work at the beginning of the 20th century, the accepted picture of genetics was that most organisms, at any given gene, are homozygous for a single dominant allele, whose phenotype corresponds to the "wild type" norm for the species, and  which is shared with other typical members of the same species.  Occasionally, a rare phenotypic variant arose as a mutation, the large majority of which were recessive and deleterious. For example, most Drosophila have red eyes: the discovery of a rare white-eyed fly was regarded as a mutant ("changed") that arose as a result of a mutation ("change") in a gene. Analysis of genetic crosses show the 'white' allele to be a sex-linked recessive to the dominant "red" allele responsible for the "wild type" red-eyed phenotype..

    In humans, many easily-recognized single-gene traits result in phenotypic traits such as alkaptonuria (accumulation of homogentisic acid, leading to darkened cartilage), albinism (absence of skin melanin), polydactyly (extra digits), achondroplasia (stunted limb growth), etc., all of which tend to be put in the medical category of "diseases". Thus we tend to think of these rare individuals as "mutants" and therefore "abnormal," in contrast to the majority of the population who (like ourselves) are "normal". We tend to associate genetic variation with disease, and view "mutation" as negative. All of these perceptions turn out to be inaccurate. Advances in molecular biology since the late 1960s have shown instead an enormous amount of genetic variation in most species, including our own. At the level of DNA sequences, it is likely that almost everyone is heterozygous at most gene loci, and (except for close relatives) is likely to differ from other members of the population. This being the case, the concept of a standard 'wild type' as "normal" has no meaning. Observable genetic differences occur as single-nucleotide polymorphisms (SNPs), alternative base pair differences at particular positions in the gene sequence. The variable effect of such SNP variation on phenotypes is one of the topics of molecular genetics. It remains the case that many gene variants do lead to medically deleterious conditions, and a great deal of time, money, and research is invested to understand and ameliorate such conditions.

    In modern usage, we avoid the use of 'mutant' to refer to individual humans (a comic book-style introduction to genetics headlines one cartoon"Queen Victoria was a mutant !"), though we may talk about 'mutant flies'. We also restrict the term 'mutation' to describe the molecular process by which gene variants are produced ("PKU results from mutations of the PAH locus"), or to characterize a newly-arisen sequence variant ("a splice-site mutation"). DNA mutation gives rise to SNP variation within populations. Observed SNP differences among individuals likely arose several or many generations previously in their ancestors, rather than as a result of mutation in a parental gamete. Rarer are genetic variants observed in particular families and close relatives, which can often be traced to a particular mutational event in a common ancestor. Famously, study of her pedigree (family history) shows that Hemophilia in the descendants of Queen Victoria is due to a novel mutation in HM's germ cells, which was inherited as a SNP in her male and female descendants, and which because it is sex-linked shows up only in male descendants.

    Conscious or unconscious acceptance of inaccurate attitudes about "What is Normal ?" with respect to heritable variation have led to misinformed social policy, with disastrous consequences. The term "eugenics" was introduced in the late 19th century (prior to any knowledge of genetics)  to describe efforts to improve the human species by encouraging reproduction of persons with "good heredity," as evidenced by superior intelligence, general health, higher social status, or (lighter) skin colour, etc. In the absence of any knowledge of genetics, early "positive eugenic" practices were those of livestock breeding, e.g., if 'like begets like' then persons of 'good stock' should be encourage to breed with others like themselves, produce more children, and hope for the best.

    Generalizing on Mendel's experiments with peas, many early geneticists assumed that variation for almost any phenotypic trait (Mendel's "Unit Character", such as pod color or seed shape) could be explained as the result of single-gene inheritance with one dominant and one recessive allele. For example, the US Navy in 1919 commissioned a study by the Eugenics Records Office on the genetics of "leadership ability" and "love of the sea" ("thallasophilia"), so as to select cadets for the US Naval Academy, based on their pedigrees. Not surprisingly, 'leadership' was found to be dominant, and 'sea lust' recessive and sex-limited, like beard growth. (This would explain the smaller number of lady Admirals). Textbooks showed pedigrees of the JS Bach family to demonstrate inheritance of musical ability, as shown by his multiple talented offspring.  State fairs awarded blue-ribbons to 'fitter families' with sturdy, healthy children, judged in exactly the same way as superior livestock. Such efforts confuse heredity with familiality, the tendency of family members to resemble each other because of their shared home environment. For example, historically it has been the case that boys follow their fathers into naval and army careers, and the Bach children received music lessons from Dad. As a reductio ab adsurdum, it has been shown that admittance to medical school behaves like a single-gene Dominant: applicants with two MD parents are almost invariably admitted, and those with one MD parent are admitted preferentially over those with none. If A is a rare and dominant allele of a gene for "Medical Aptitude", Aa x Aa => 3/4 , and Aa x aa => 1/2 "A" pre-meds.

    Eugenics unfortunately had its dark side. The massive casualties of World War I were seen in some Western countries as having "negative eugenic" consequences, as officers and soldiers killed tended to be young, physically fit, reproductive males of superior intelligence, leaving behind those unqualified for military service to do the breeding. Studies of certain notorious 'feeble-minded' and (or) 'criminal' families (notably, the Kallikaks) concluded that such traits ran in families, as expected for hereditary traits, again ignoring effects of familial poverty, malnutrition, and social stigmatization ('Kallikak' children were habitually shunned).

    The popularity of eugenic thinking reinforced post-World War I anti-immigrant sentiment in the US and Canada. With the introduction of IQ tests for educational tracking, low scores of new immigrants were ascribed to poor genetics, even though the tests were typically administered in English to non-English-speakers. US Army recruits were administered written IQ exams en masse, where many had never held a pencil. "Negative eugenics" sanctioned by law led to efforts to discourage or actively interfere with the marriage of persons perceived to have "bad genes".  In the 1920s and '30s, many US states and as well as Alberta and British Columbia passed laws permitting compulsory sterilization of thousands of persons with a variety of conditions considered "hereditary" on the basis of little or no evidence. This included traits such as epilepsy, mental retardation, "congenital" criminality or pauperism, etc). The US Supreme Court approved this policy, a leading Justice Homes famously pronouncing, "Three generations of imbeciles are enough'"

    The Nazi regime in Germany adopted legislation directly from US laws, and extended them as early as 1932 to medically-sanctioned murder of institutionalized persons deemed ("Lebensunwertes Leben:" Life unworthy of Life) in its Aktion T4 extermination policy. Technical methods of large-scale execution developed in this program (including gassing) were extended directly to the murder of millions in the Holocaust of the 1940s.

    Recommended books include:

        J Cornwell, "Hitler's Scientists: Science, War, and the Devil's Pact"
        C Browning, "The Origin of the Holocaust"
        SJ Gould "The Mismeasure of Man," 2nd ed.