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Genetics of Disease - How Important Are Genes?

twins risk disorder component

Prior to searching for the genes involved in a disease and determining how those genes work in the various tissues of the human body, there must be clear evidence that genetics is involved in the disease. Genetic analyses are ethically and financially challenging, as well as quite laborious. Geneticists use several methods to evaluate whether or not genetics plays an important role in the etiology of a disorder before they begin a search for a gene. Some of these methods include familial aggregation, recurrence risks, and twin studies.

Familial aggregation can be established by obtaining a thorough family history on study participants. Individuals are simply asked if they have any other family members who have the disease. If individuals with the disease have a higher frequency of affected relatives compared with individuals without the disease, there is evidence of familial aggregation. Although familial aggregation for a disorder is consistent with the involvement of genetics, it may also reflect the presence of a common environmental factor to which all family members have been exposed (such as pesticides or contaminated drinking water).

Relative risk ratios are another method commonly used to determine if there exists a genetic basis to a disease. For example, in cystic fibrosis, an autosomal recessive Mendelian disorder, the risk of disease in the siblings of an affected individual is 1 in 4. The prevalence of the disease is about 1 in 1,600 in the general population.

In 1990 Neil Risch demonstrated that by comparing the risk of a disease occurring in the relatives of an affected individual with the risk of the disease occurring in the general population, one could measure the significance of the genetic component of the disease. The risk ratio, labeled λR, where R is the type of relative (such as sibling), is the ratio of the risk of disease in the relative of type R to the prevalence of the disease in the general population. Thus in cystic fibrosis, λs = 1/4 ÷ 1/1600 = 400. This means that the risk of developing cystic fibrosis is four hundred times greater for siblings of an affected individual than for an individual in the general population. This clearly demonstrates the effect that genetics plays in the development of cystic fibrosis.

As a general rule, the larger the λR, the stronger the genetic component. However, this ratio is extremely sensitive to the frequency of the disease in the general population. The more common the disorder, the lower the λR, although this does not necessarily preclude a genetic component to the disorder.

Twin and adoption studies provide a unique opportunity to tease apart the role of genetics and the influence of a common familial environment. Because twins were born at the same time, the environments to which they were exposed are very similar. This holds true for the prenatal environment, and often for the childhood environment, but rarely for adult environments.

Monozygotic (MZ), or identical, twins share 100 percent of their genetic makeup, and dizygotic (DZ), or fraternal, twins share on average 50 percent of their genetic makeup. Twin studies compare disease "concordance" in MZ twins with DZ twins. When both twins in a pair have the disease, the twins are said to be concordant. When one twin has the disease and the other does not, the twins are said to be discordant. If the disorder has a genetic component, then MZ twins will be concordant more often than DZ twins. The difference between the MZ and DZ concordance rates can be used to assess the strength of the genetic component.

In summary, geneticists are finding that some disorders have a large genetic effect and others have a small genetic effect. There are several methods geneticists can use to determine the size of the genetic effect of a disease. However, ultimately, researchers trying to fully understand genetic disorders must identify the genes that are involved and determine their function. The revolution in human genetics, primarily due to the successes of the Human Genome Project, has made and will continue to make an impact on scientists' ability to define the role of genetics in human disease.

Allison Ashley-Koch

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