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Genetic Variation, Disease Genes, and Risk Factors

What Is Individual Genomics and Personalized Medicine?


The characteristics that make us individuals span a wide range: obvious physical traits, such as eye, hair, and skin color, ear, head, and body shape, physiological traits, such as a tendency to gain weight, metabolize alcohol poorly, or have oily skin, health related traits, such as a tendency to develop certain types of diseases or respond well to particular drugs, personality features, like a tendency toward idiosyncratic behaviors or a knack for certain social skills, and intellectual abilities, such as impaired or exceptional cognitive or puzzle solving abilities. Some physical traits are generally fixed. However, many of the other social, personality, and intellectual attributes result from a combination of innate characteristics molded by a lifetime of experience and learning.

Genes Define You

While the causes for many of the more complex conditions, behaviors, and abilities may be numerous and difficult to define, some part are certainly a result of innate traits present in each individual. Since innate characteristics are largely the result of genetics, it is not overreaching to say that there is some genetic component in everything that makes us who we are.

Genes themselves do not actually do anything but simply carry information. The complete set of genes a person possess makes up their genome. This information is the source code to produce and regulate production of all the body's proteins, which form the biological structures and run the chemical reactions of the body's physiology. Basically, genes define every part of our selves, and little variations in each person's genetic code may produce very minor or very major differences in each of us.

The Limits of Genetic Determinism

For something to be genetically influenced, it is not that there is necessarily "a gene" for it. For example, genetics influences the tendency toward depression, cancer, and intelligence. While some features, such as eye color, are almost entirely determined by a few genes, for more complex attributes the genetic component is more indirect. In fact, many genes may produce a lot of small effects that come together in a certain environment to produce a trait. Even eye color is actuallynot so simple.

Most examples of genetic influence will be more complicated. For instance, preferences for certain foods may be influenced by the genetic differences in taste receptors producing a dislike of some foods, like cilantro, or a sensitivity to certain flavors, like bitter 6-n-propylthiouracil (PROP). However, the genetics of a person's brain chemistry may also affect their food preference similar to the how genetics influences a person's risk of becoming addicted to smoking. Finally, personality traits, such as openness to experience or extraversion may influence which foods a person prefers, if only by limiting the number they try. Genes for other individual traits may also have indirect roles too.

In the end, while it is clear there are obvious genetic influences on food preferences, actually sorting out which variations produce preferences for particular foods is likely to be almost impossible. This is especially true since personal experiences also play such a critical role with this sort of characteristic. After all, you can't have a preference for a food you have never tried, or you may have an aversion to a particular food as a result bad experience, such as a food poisoning incident, that overrides an internal inclination to it.

Assessing Genetic Risks

The more complex the trait, the more difficult it is to tease out the genetic influences. As a result, it is not really possible to definitively connect most traits, including diseases, with all the specific gene variations that control it. Particular genetics produces a tendency to manifest certain traits given the rights set of circumstances. What can actually be measured in many cases is how strong the tendency is to develop the trait when specific gene variants are present—in other words a genetic risk factor.

Essentially, a risk factor is the probability that a person with a specific set of gene variants will develop a certain trait, like a disease, when compared to other similar people without those same gene variants. The probability accounts for the fact that, while some genetics are present for development of a particular trait, other unknown factors—like additional unknown genetics or influences from the environment—are also required for the trait to actually appear. Since those other conditions are not known, the risk factor is really an estimation of how likely it is that the particular individual will either already possess or experience these unknown factors, and so, manifest the trait.

BRCA Genes and Breast Cancer Risk

For an example of genetic risk, take the specific variants of the BRCA1 and BRCA2 genes that have been linked to breast cancer. Women with specific variations of these genes have about a 60% chance of developing breast cancer, whereas the general rate of breast cancer is 12%. However, although women with these gene variants are much more likely than women without them to develop breast cancer, about 40% of women with these genes do not, in fact, develop breast cancer. Obviously, there must be more required to trigger breast cancers than just the type of BRCA gene a person has. Possibly other genes have an effect too, or maybe some common environmental factor triggers the cancer in combination with the BRCA genes.

Genetic Variation and Ancestry

Interestingly, the BRCA mutations are much more common in descendants of Ashkenazi Jews where they occur in 2.5 % of the individuals, as opposed to the general population where is only occurs 0.25% of the time. Since these genes obviously don't cause Jewishness, the higher frequency in the Jewish population is because the mutations likely originated and spread first in this somewhat segregated Eastern European founder population so it occurs more frequently in people with this ancestry.

Genetic association data is merely correlative—an observation. It suggests some connection between a genetic variant and a characteristic, but it does not show that a particular type of gene causes some effect. In fact, all heritable genetic variations arose at some point in human history and spread out from a specific population, so genetic variations are unevenly distributed throughout the world and among people with different ancestry. Different variations correlate with different ancestries and they provide a valuable tool to trace human development and global migration. This feature makes genetic variation vary useful as an indicator of an individual’s genetic ancestry.

Genes, Health, and Personalized Medicine

Since the sequencing of the human genome was completed in 2003, much of human genetic research has focused on identifying specific genetic differences associated with all sorts traits and especially for those directly related to healthcare. A better understanding of how an individual's health-related genetic predispositions can potentially provide practical information on lifestyle choices for optimal health, potential diseases to be most concerned about, and treatments that would be most effective when a condition arises.

However, we are really just at the beginning of the genomics age. The staggering task of associating the tens of millions of genetic variations with the multitude of diseases, characteristics, and other traits they influence has a long way to go. As more connections are drawn, the information becomes more useful and eventually it will provide a growing network of knowledge from which medical professional can extract practical guidance based on a patient's person genetic profile—truly personalized medicine.

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