6.3 Studying Twins and Adoptions Can Help Assess the Importance of Genes and Environment

Twins and adoptions provide natural experiments for separating the effects of genes and environmental factors in determining differences in traits. These two techniques have been widely used in genetic studies.

Types of Twins

Twins are of two types: dizygotic (nonidentical) twins arise when two separate eggs are fertilized by two different sperm, producing genetically distinct zygotes; monozygotic (identical) twins result when a single egg, fertilized by a single sperm, splits early in development into two separate embryos.

Because monozygotic twins result from a single egg and sperm (a single, “mono,” zygote), they’re genetically identical (except for rare somatic mutations), having 100% of their genes in common (Figure 6.11a). Dizygotic twins (Figure 6.11b), on the other hand, have on average only 50% of their genes in common, which is the same percentage that any pair of siblings has in common. Like other siblings, dizygotic twins may be of the same sex or of different sexes. The only difference between dizygotic twins and other siblings is that dizygotic twins are the same age and shared the same uterine environment. Dizygotic twinning often runs in families and the tendency to produce dizygotic twins is influenced by both heredity and environmental factors. There appears to be little genetic tendency for producing monozygotic twins.

Figure 6.11: Monozygotic twins (a) are identical; dizygotic twins (b) are nonidentical.

[Part a: f4foto/Alamy. Part b: Courtesy of Randi Rossignol.]

Concordance in Twins

Comparisons of dizygotic and monozygotic twins can be used to assess the importance of genetic and environmental factors in producing differences in a characteristic. This assessment is often made by calculating the concordance for a trait. If both members of a twin pair have a trait, the twins are said to be concordant; if only one member of the pair has the trait, the twins are said to be discordant. Concordance is the percentage of twin pairs that are concordant for a trait. Because identical twins have 100% of their genes in common and dizygotic twins have on average only 50% in common, genetically influenced traits should exhibit higher concordance in monozygotic twins. For instance, when one member of a monozygotic twin pair has epilepsy (Table 6.2), the other twin of the pair has epilepsy about 59% of the time; so the monozygotic concordance for epilepsy is 59%. However, when a dizygotic twin has epilepsy, the other twin has epilepsy only 19% of the time (19% dizygotic concordance).

Sources: (1 and 2) B. havald and M. hauge, U.S. public health Service Publication 1103 (1963), pp. 61–67; (3, 4, 5, and 6) B. havald and M. hauge, Genetics and the Epidemiology of Chronic Diseases (U.S. Department of health, Education, and Welfare, 1965); (7) J. S. Lawrence, Annals of Rheumatic Diseases 26:357–379, 1970; (8) G. C. Ebers et al., American Journal of Human Genetics 36:495, 1984.

The higher concordance in the monozygotic twins suggests that genes influence epilepsy, a finding supported by the results of other family studies of this disease. In contrast, the concordance rates of death from acute infection are similar in both monozygotic and dizygotic twins, suggesting that most death from infections has little inherited tendency. Concordance values for several additional human traits and diseases are listed in Table 6.2.

The hallmark of a genetic influence on a particular trait is higher concordance in monozygotic twins compared with concordance in dizygotic twins. High concordance in monozygotic twins by itself does not signal a genetic influence. Twins usually share the same environment—they are raised in the same home, have the same friends, attend the same school—and so high concordance may be due to common genes or to common environment. If the high concordance is due to environmental factors, then dizygotic twins, who also share the same environment, should have just as high a concordance as that of monozygotic twins. When genes influence the trait, however, monozygotic twin pairs should exhibit higher concordance than that of dizygotic twin pairs, because monozygotic twins have a greater percentage of genes in common. It is important to note that any discordance among monozygotic twins is usually due to environmental factors, because monozygotic twins are genetically identical. For example, for epilepsy, the concordance of monozygotic twins is considerably less than 100% (see Table 6.2), suggesting that in addition to genetic influences environmental factors also affect variation in this trait.

The use of twins in genetic research rests on the important assumption that, when concordance for monozygotic twins is greater than that for dizygotic twins, it is because monozygotic twins are more similar in their genes and not because they have experienced a more similar environment. The degree of environmental similarity between monozygotic twins and dizygotic twins is assumed to be the same. This assumption may not always be correct, particularly for human behaviors. Because they look alike, identical twins may be treated more similarly by parents, teachers, and peers than are nonidentical twins. Evidence of this similar treatment is seen in the past tendency of parents to dress identical twins alike. In spite of this potential complication, twin studies have played a pivotal role in the study of human genetics. TRY PROBLEM 30

A Twin Study of Asthma

To illustrate the use of twins in genetic research, let’s consider a study of asthma. Asthma is characterized by constriction of the airways and the secretion of mucus into the air passages, causing coughing, labored breathing, and wheezing (Figure 6.12). Severe cases can be life threatening. Asthma is a major health problem in industrialized countries and appears to be on the rise. The incidence of childhood asthma varies widely; some of the highest rates (from 21% to 27%) are found in Australia, The United Kingdom, Sweden, and Brazil.

Figure 6.12: Twin studies show that asthma, characterized by constriction of the airways, is caused by a combination of genetic and environmental factors. Inhalers are often used to deliver asthma medication to the lungs.

[Stockbyte/Getty Images.]

A number of environmental stimuli are known to precipitate asthma attacks, including dust, pollen, air pollution, respiratory infections, exercise, cold air, and emotional stress. Allergies frequently accompany asthma, suggesting that asthma is a disorder of the immune system, but the precise relation between immune function and asthma is poorly understood. Numerous studies have shown that genetic factors are important in asthma.

A genetic study of childhood asthma was conducted as a part of the Twins Early Development Study in England, an ongoing research project that studies more than 15,000 twins born in the United Kingdom between 1994 and 1996. These twins were assessed for language, cognitive development, behavioral problems, and academic achievement at ages 7 and 9, and the genetic and environmental contributions to a number of their traits were examined. In the asthma study, researchers looked at a sample of 4910 twins at age 4. Parents of the twins were asked whether either of their twins had been prescribed medication to control asthma; those children receiving asthma medication were considered to have asthma.

The concordance value for the monozygotic twins (65% among 1658 twin pairs) was significantly higher than that for the dizygotic twins (37% among 3252 twin pairs), and the researchers concluded that, among the 4-year-olds included in the study, asthma was strongly influenced by genetic factors. The fact that even monozygotic twins were discordant 35% of the time indicates that environmental factors also play a role in asthma.

Adoption Studies

Another technique used by geneticists to analyze human inheritance is the study of adopted persons. This approach is one of the most powerful for distinguishing the effects of genes and environment on characteristics.

For a variety of reasons, many children are separated from their biological parents soon after birth and adopted by adults with whom they have no genetic relationship. These adopted persons have no more genes in common with their adoptive parents on average than do two randomly chosen persons; however, they do share an environment with their adoptive parents. In contrast, the adopted persons have 50% of their genes in common with each of their biological parents but do not share the same environment with them. If adopted persons and their adoptive parents show similarities in a characteristic, these similarities can be attributed to environmental factors. If, on the other hand, adopted persons and their biological parents show similarities, these similarities are likely to be due to genetic factors. Comparisons of adopted persons with their adoptive parents and with their biological parents can therefore help to define the roles of genetic and environmental factors in the determination of human variation. For example, adoption studies were instrumental in showing that schizophrenia has a genetic basis. Adoption studies have also shown that obesity, as measured by body-mass index, is at least partly influenced by genetics (Figure 6.13).

Figure 6.13: Adoption studies demonstrate that obesity has a genetic influence.

[Redrawn with the permission of the New England Journal of Medicine 314:195, 1986.]

Adoption studies assume that the environments of biological and adoptive families are independent (i.e., not more alike than would be expected by chance). This assumption may not always be correct, because adoption agencies carefully choose adoptive parents and may select a family that resembles the biological family. Thus, some of the similarity between adopted persons and their biological parents may be due to these similar environments and not due to common genetic factors. In addition, offspring and the biological mother share the same environment during prenatal development. TRY PROBLEM 33