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 regarded as 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.
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 this course. 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 a
mutation 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. However, observed SNP
differences among individuals likely arose at least
several (and probably many) generations previously in their
ancestors, rather than as a result of mutation in a parental
gamete. Rarer are genetic variants observed in 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 her germ cells, which is inherited as a SNP in her
male and female descendants, but 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 genetic 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
genes," as evidenced by superior intelligence, general
health, proper 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") could be explained as the result of
single-gene Mendelian 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 Naval Academy based on their
pedigrees. Not surprisingly, 'leadership' was found to be
dominant, and 'sea lust' recessive and sex-limited, like
beard growth (ever see a lady Admiral?). 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, boys often 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
rare and dominant, 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 these same laws, and extended
them in 1939 to medically-approved murder of institutionalized
persons ("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.