Historically, the existence of hereditary diseases—diseases that are transmitted across generations from parents to offspring—has been used to justify eugenic sterilization policies. Although these past policies were often founded on a mistaken understand of hereditary disease, the recent mapping of the human genome and the advancement of technologies for studying human hereditary information has led to an increased and more intricate understanding of how and when humans can inherit various disorders. Because we can now identify many hereditary diseases before a fetus fully develops or before a person develops the disease, we have options for treating or avoiding such diseases. We can choose to abort a fetus with a life-altering hereditary disease, or use gene therapy to treat an existing hereditary disease or problematic gene locus. We thus have some measure of control over whether or not such diseases continue in our population, which can be seen as a kind of “modern eugenics”. However, the desirability of utilizing this control to treat and/or eliminate hereditary diseases is debatable, given the fact that what counts as a disease may depend on what traits a society values or disvalues.
Types of hereditary disorders Hereditary disorders are diseases that are transmitted genetically from parents to offspring. There are several different ways that this transmission can happen. Mendelian (single gene) disorders are hereditary diseases that are caused by a single genetic locus. The isolation to a single locus allows transmission of the genetic disease to follow Mendelian inheritance principles, which predict the expression of traits based on statistical rules and family history. Because humans inherit two copies of each gene (one from each parent), an individual can be either homozygous (inherit the same two copies of a gene) or heterozygous (inherit two different copies of a gene) for a given gene. Each copy is often associated with a degree of dominance or recessiveness: in a heterozygote, the copy that is expressed (or expressed to the greatest degree) is called the dominant allele, while the other copy in the heterozygote is the recessive allele. These ideas of zygosity and dominance are used to further classify Mendelian disorders.
Autosomal dominant disorders, such as Huntington’s disease and neurofibromatosis, require inheritance of only one copy of the disease-linked gene. The disease allele is dominant, and so transmission of the disorder to offspring can occur if only one parent is a carrier. Autosomal recessive disorders, like cystic fibrosis and Tay-Sachs, on the other hand, require two copies of the disease-linked allele. Individuals with autosomal recessive disorders are thus homozygotes with two recessive alleles. The parents of affected individuals are often unaffected and unaware that they are heterozygous carriers of the disease-linked allele.
While autosomal disorders affect both males and females, other Mendelian genetic disorders are considered sex-linked because they are caused by mutations on the X or Y chromosome. X-linked recessive disorders, such as red-green color blindness, typically occur in males. Because females have two X chromosomes, they can carry both an affected and an unaffected gene copy and thus be unaffected by the disease. Males who inherit an infected allele, however, will have only the disease-linked gene on their one X chromosome. X-linked dominant disorders affect both males and females, but are often more severe (even fatal) in males, since affected males lack a second, normal gene copy. Additionally, there are a few rare examples of Y-linked disorders, which can only be transmitted from father to son.
Hereditary disease and eugenics Although hereditary diseases are typically considered disorders that have a clear transmission pattern across generations, there are many other diseases, such as schizophrenia, autism, and heart disease, which have a genetic component but do not have a clear transmission pattern. Because such complex genetic disorders cannot be isolated to a single genetic locus or cause, are often influenced by both genetic and environmental factors, and may be caused by new mutations, we cannot predict with certainty the chance of passing such diseases on to future generations. They are thus not transmitted in the same sense that Mendelian disorders are transmitted. This is an important distinction to make because many conditions that were considered cause for eugenic sterilization were not, strictly speaking, hereditary diseases. Historically, proponents of eugenics policies commonly called for and justified sterilization for conditions such as criminality and low IQ by appealing to the hereditary nature of these conditions. Sterilization was argued to be a means of preventing the transmission of these undesirable characteristics across generations. However, most of these traits are not actually hereditary—that is, they are not traits that are transmitted from parent to offspring. They may have heritable components (in that genetics might play a role in whether an individual is high or low on that trait), but they are not inherited in way that proponents of eugenics tended to claim, and so cannot be easily eliminated from the population via sterilization policies.
For example, Alberta had a legislated eugenics program that appealed to the notion of hereditary diseases. Inspired by the eugenics movement, the Sexual Sterilization Act of 1928 was intended to reduce the prevalence of mental disease in the population by preventing people with mental diseases from procreating, and thus transmitting their disease to their offspring. The knowledge that certain diseases can be hereditary thus played a great role in the passage and acceptance of the Act. However, most of the traits (such as “mental defect” and “psychosis”) that the Act stated as cause for sterilization are not considered hereditary today (nor are they considered to be diseases). Interestingly, though, the 1942 amendment to the Act specified Huntington’s disease as a reason for sterilization. Huntington’s disease is now considered to be a hereditary disease caused by an autosomal dominant mutation, and, because of the 50% inheritance rate associated with the disease, it is a mutation that some individuals choose to be genetically tested for.
Genetic testing is now a common way of discovering whether an individual has a hereditary disease or is at risk for passing a hereditary disease on to his or her offspring. Genetic testing is commonly used prenatally to identify whether or not a fetus has a genetic or chromosomal condition, but it can also be used in adults to test for genes that have been linked to the development of certain complex diseases. Our ability to identify the genes that can cause hereditary diseases gives us a way to shape our evolutionary course. By selecting to implant only those embryos that are clear of known genetic abnormalities, by aborting fetuses that test positive for hereditary diseases, and by choosing mates based on their genetic makeup so as to avoid the possibility of having offspring that could inherit a hereditary disease, we could eliminate the chance of having a child with a genetic disease, and possibly eradicate certain diseases altogether. Genetic testing can also be used to identify hereditary diseases that may be treated via gene therapy—replacing or overcoming the defective genes. Thus, our control over hereditary diseases could potentially eliminate suffering caused by having certain genetic disorders.
However, what counts as a disease or disorder has a large social component to it: diseases are seen as negative or dysfunctional traits, but what exactly is considered negative or dysfunctional depends on what a society values and disvalues. Because many traits are at least somewhat heritable, what counts as a hereditary disease depends in part on what traits a society views negatively. For example, evidence suggests that homosexuality is somewhat heritable, and, indeed, negative views of homosexuality led it to be considered a disease in the past. As seen in historical cases of eugenics policies, such as Alberta’s Sexual Sterilization Act, the label of “hereditary disease” can be flexible, and can depend in part on the current moral values of the society. Screening embryos for hereditary diseases could therefore prevent the birth of offspring with traits that, in the future, might not be considered diseases. Thus, although our increased understanding of hereditary diseases grants us some control over the prevalence of these diseases, we should still, as a society, use caution when deciding just when to exercise this control.
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