Twins and Genetics
Besides pedigree analysis, twin studies are a method often used to study human heritability. There are two types of twins:
- Dizygotic (or nonidentical) twins: these occur when two eggs are fertilized by two separate sperm cells, leading to two genetically distinct fetuses.
- Monozygotic (or identical) twins: these occur when a single egg is fertilized by a single sperm and splits into two embryos during early development.
Since monozygotic twins arise from a single egg and sperm cell, they are (with the exception of some rare mutations) genetically completely identical. Dizygotic twins, on the other hand, share only 50 percent of their genetic material, the same as any pair of siblings.
The frequency with which dizygotic twins are born varies among populations and tends to run in families. In contrast, monozygotic twins occur at a relatively stable frequency (about 4 twin pairs in every 1000 births) and there much less tendency to run in families.
The Notion of Concordance
Comparisons of dizygotic and monozygotic twins can be used to estimate the importance of genetic and environmental influence on the development of certain traits. This often happens by calculating the concordance for these traits. Twins are concordant when they share a trait, and discordant when they do not. The concordance is the percentage of twin pairs that are concordant for a certain trait.
Since monozygotic twins are genetically as good as identical, traits that are genetically influenced should have a higher concordance in monozygotic twins than in dizygotic ones. Thus, a higher concordance for a trait in monozygotic twins than in dizygotic twins is a sign that the trait is influenced genetically.
It is important to realize that a high concordance of a trait in monozygotic twins in itself is not a signal of genetic influence, as they were probably raised in a very similar environment. This high concordance may be the result of a genetic or environmental influence. It is only in the comparison with the concordance of the same trait in dizygotic twins that a purely genetic influence can be discerned.
An Example of the Use of Twin Studies: Obesity
Obesity is a serious public-health problem that tends to run in families and increases the risk of developing diabetes, high blood pressure, heart disease, and other medical conditions. This means that it could be subject to a large genetic influence, or that it could be largely environmentally defined, as family members tend to have similar diets and exercise habits.
In order to understand the genetic and environmental influences on the development of obesity, twin studies were performed. The results of this large scale investigation showed that monozygotic twins were consistently more concordant than dizygotic twins.
This indicates that genes indeed influence the variation in body weight. However, genes alone do not cause obesity. This is important to realize: one does not inherit obesity, but rather inherits a predisposition toward a particular body weight. This means that some are more at risk for developing obesity than others.
- 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.