Child Development From Infancy to Adolescence. Laura E. Levine
and reared apart by different families. Then we examine research that has used genome-wide association tests to locate genes associated with personality characteristics.
Studies of Adopted Children
One way to try to determine heritability is to study adopted children. Children who are adopted have birth parents from whom they inherit their genes and adoptive parents who provide the environment in which they grow up. To look at the relative contribution of genes and environment on whatever developmental outcome they are studying, researchers must have information about both the adoptive and the biological parents. They then look at the concordance rate, the degree to which a trait or an ability of a child is similar to that of their biological and adoptive parents. A higher concordance rate between child and birth parents shows the influence of the genes on that characteristic, while a higher concordance rate between child and adoptive parents shows the influence of the environment.
Concordance rate: The degree to which a trait or an ability of one individual is similar to that of another; used to examine similarities between twins and among adopted children and their biological and adoptive parents.
A variation on this approach has been to examine the similarity between parents and their adopted children as compared to their similarity to their biological children within the same family. One example of this type of research was carried out to examine the heritability of general cognitive ability. Petrill and Deater-Deckard (2004) found a modest correlation between mothers’ scores on the Stanford-Binet Intelligence Scale and their biological children’s scores, but essentially no correlation with their adoptive children’s scores. As you know from Chapter 2, correlational research cannot tell us about causation, but these findings would indicate that the genes the mother passed on to her biological child contributed to the similarity in their IQ, while the environment provided to the adoptive child did not have much influence.
Studies Comparing Identical and Fraternal Twins
Another approach to measuring the relative influence of genes and environment has capitalized on the fact that there are two types of twins. Fraternal or nonidentical twins are formed when a woman’s ovary releases two eggs during a menstrual cycle and each is fertilized by a different sperm. The resulting twins are referred to as dizygotic (DZ), because they develop from two (di) fertilized eggs (zygotes). They are only as genetically similar to each other as any other pair of siblings. Because they have about half their genes in common (see Figure 3.7), they don’t have an identical appearance. In fact, fraternal twins don’t even have to be the same sex. Because each egg is fertilized by a different sperm, one sperm can be carrying an X chromosome while the other sperm is carrying a Y chromosome. The tendency to have fraternal twins is genetically related so some families are more likely to have them than others. Researchers have identified a mechanism that helps explain why this is true. There are two single nucleotide polymorphisms (SNPs) that can affect the production and use of follicle stimulating hormone, the hormone involved with the release of an ovum each month. If a woman has one copy of each of these SNPs, it increases her chance of having twins by 29% (Mbarek et al., 2016).
Dizygotic (DZ) twins: Twins formed when a woman produces two ova or eggs, which are fertilized by two sperm; genetically DZ twins are as similar as any siblings.
Figure 3.7 Genetic similarities between siblings and identical and fraternal twins.
The second way twins develop occurs when a single egg is fertilized by a single sperm to form a zygote. The zygote begins replicating and producing additional cells, but early in this process, for reasons we don’t really understand, the ball of cells splits into two. Each ball of cells continues to develop prenatally to become one of two identical twins, referred to as monozygotic (MZ) twins because they are the product of a single (mono) fertilized egg (zygote). Because identical twins both have the same set of genetic material (including the information on chromosome pair 23), they are always the same sex and look much alike. This type of twinning occurs by chance, so the tendency to have identical twins does not run in families.
Monozygotic (MZ) twins: Twins formed when a woman produces one egg that is fertilized by one sperm and the resulting ball of cells splits to form two individuals with the same genes.
Twins and triplets. Identical twins are always the same gender, but fraternal twins can be the same gender or different. Triplets can be identical or fraternal, or a set of identical twins with a fraternal sibling.
©iStockphoto/digitalskillet
Barbara Penoyar/Photodisc/Thinkstock
©iStockphoto/digitalskillet
It has always been a puzzle why identical twins can have small differences in their basic appearance or develop different genetically based disorders. Scientists had previously assumed that the environment was responsible for these differences. However, researchers have discovered that even identical twins have small differences in the arrangement of their genes, and these differences may result in observable and sometimes significant differences (Morimoto et al., 2017).
Scientists have been able to look at concordance rates between identical twins and between fraternal twins to try to identify which behaviors or personality traits are linked with genetic inheritance. If identical twins, who share almost all their genes, are more similar to each other (that is, they have a higher concordance rate) on a trait such as shyness than fraternal twins, who share only half of their genes, the researchers conclude that genes play a role in determining whether someone is shy. Using this type of study to look at the causes of alcoholism, Pagan et al. (2006) followed identical and fraternal twins from adolescence through early adulthood. They found the age at which the twins began using alcohol was no more similar between identical twins than between fraternal twins, but identical twins were more similar to each other in terms of the amount they drank and whether they became problem drinkers in late adolescence and early adulthood. This finding provides evidence that genes play less of a role in the age teens begin to drink, but they play a stronger role at later ages in the development of problem drinking.
Research with twins has also examined the role of specific aspects of the environment rather than looking only at the effects of similarity in genes. For example, Kendler, Gardner, and Dick (2011) found that the genetic effects on alcohol use in adolescence were stronger when parental monitoring of their teens was low and when alcohol was easily available. This means that identical twins were more similar to each other in alcohol use than fraternal twins when all were allowed easy access to alcohol. Genetic effects were much more limited when parents kept a watchful eye on their teen and the peer environment did not promote alcohol use. In this case, there was little difference in concordance rate between identical and fraternal twins. These findings show that genes do not necessarily determine our destiny and helps identify the role of specific aspects of the environment that may influence when and how some genes are expressed.
Although behavioral genetics has shown that almost all behaviors studied have some genetic input, different traits and behaviors are more or less heritable (Dick & Rose, 2004). It is useful to discover which traits and behaviors are highly likely to develop from genetic input because this will aid in the search for the specific genes responsible. On the