Besides pedigree analysis and twin studies, adoption studies are a third technique often used to study human inheritance mechanisms. This type of study is a very powerful tool to distinguish genetic and environmental effects on the development of a certain trait.
For various reasons, children can be separated from their biological parents after birth and adopted by individuals with whom they have no genetic relationship. They share no more genes than two random individuals. They do, however, share their environment. So, if an adopted person and his or her adoptive parents show similarities in a characteristic, these can be attributed to a common environment. If, on the other hand, the adopted individual and his or her biological parents share similarities in a characteristic, these can be attributed to genetic factors.
Thus, comparison of adopted people with their adoptive and biological parents can help to define the roles of genetic and environmental factors in the determination of human variation.
A First Example: Obesity
Similar to twin studies, adoption studies have been used to elucidate the causes of obesity. In 1986, a large-scale adoption study was published, in which information concerning the adult body weight and height of over 500 adopted individuals, their biological parents and their adoptive parents.
By measuring the body-mass index (adult body weight divided by the square root of the adult height), geneticists analyzed the relation between the adopted people and their biological and adoptive parents. Based on their BMI, the people included in the study were divided into four weight categories: thin, median weight, overweight and obese.
The results showed a strong correlation between the weight class assigned to the adopted individuals and their biological parents. Since the only connection between the adoptees and their biological parents are the genes they have in common, it was concluded that obesity is clearly influenced by genetic factors. Between the weight classification of the adoptees and their adoptive parents there was no clear association to be discovered.
A Second Example: Alcoholism
Another example of the successful use of adoption studies, was in the research surrounding the genetic factors influencing alcoholism.
A large adoption study, performed on 1775 Swedish adoptees, their adoptive and biological parents indicated that there are at least two distinct types of alcoholism. Type I alcoholics typically include individuals that develop problems after age 25, who lose control of the ability to drink in moderation. Type II alcoholics are mostly men who start drinking before age 25 that tend to be impulsive and aggressive while drinking.
The Swedish study found that alcohol abuse among biological parents was associated with increased alcohol use in adoptees. Type I alcoholism seems to require both genetic predisposition and environmental exposure, whereas type II seems to be highly hereditary.
It is important to realize that having a genetic predisposition for obesity or alcoholism does not mean that you are condemned or that you have no choice. It just means that you have an increased risk of developing these conditions. It does in no way imply that you are forced by your genes to drink or eat excessively. Both obesity and alcoholism are complex behavioral characteristics that are influenced by many factors.
- Maynard Smith, J. (1999) Evolutionary Genetics. Second Edition. Oxford University Press.
- Pierce, B.A. (2002) Genetics: A Conceptual Approach. First edition. W.H.Freeman Publishing.