The Wiley-Blackwell Handbook of Childhood Social Development. Группа авторов

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CHAPTER TWO Behavioral Genetics

       Darya Gaysina

      Why are there differences in children’s social development even from the first weeks of their lives? Why can these differences be seen among children growing up in the same family? What are the origins of individual differences in social development, as well as in other behavioral traits? These are the main questions that behavioral geneticists try to address.

      Nowadays, it is widely, even if not universally, accepted that individual differences in behavior are resulted from both genetic influences (Nature) and environmental influences (Nurture), that play an important role from the moment of conception throughout the life span. Moreover, Nature and Nurture do not act in isolation, but co‐act through complex gene–environment interplay, and jointly contribute to the whole variety of psychological characteristics (e.g., temperament, cognition, and emotions) of each person, and to individual differences in these characteristics that exist in the population.

Behavioral genetics is an interdisciplinary field that combines approaches of genetic and behavioral sciences to explore: the role of genetic factors; the role of environmental factors; and the role of gene–environment interplay, to better understand the origins of individual differences in social development and other psychological characteristics. This chapter focuses on key concepts in behavioral genetics, and on recent findings from genetically informative studies, such as twin, adoption, and molecular genetic studies, in relation to child social development. Specifically, we will explore relative contributions of genetic and environmental influences on typical and atypical child social development, and how these influences may change during development. We will discover how specific genetic factors contribute to child social development, and how these genetic effects can be modified by environmental influences. We will also find out how our genetic makeup shapes our environmental experiences. Finally, we will discuss how the findings from behavioral genetic research can be used in practice, in order to help children, families, and professionals working with children.

The Role of Genetic Factors in Social Development

      In the past few decades, tremendous progress has been achieved in our understanding of the role of genetic factors in human development. The breakthrough discovery of the human genome in the beginning of this century was a milestone in genetic investigations of human health and behavior. The human genome consists of a molecule of DNA (deoxyribonucleic acid), which is built with three billion nucleotide base pairs. There are four different types of nucleotides – adenine (coded by A), cytosine (C), guanine (G), and thymine (T). These four nucleotides are organized in a specific sequence (e.g., ACTTGACCC…). Approximately 2% of the sequence represents genes – the regions of DNA that encodes for proteins.

      The DNA sequence (including gene regions) is practically identical for all people, with less than 1% of the sequence varying among people. There are different types of DNA variation, also known as polymorphisms or mutations. The most common type of genetic variation is a change in just one nucleotide; this is called a single nucleotide polymorphism (SNP). SNPs occurs with a frequency of 1 in 1000 nucleotides. In addition, other more substantial changes can occur in the genome, including substitutions, insertions, and deletions of several nucleotides (Plomin et al., 2013). As a result of these DNA variations, each person has a unique genetic makeup. Any of the millions of polymorphisms in the human genome can contribute to individual differences in a particular human trait. Therefore, DNA variation is a major contributor to observed individual differences in various traits.

      As widely accepted today, psychological traits are influenced by multiple genetic variants with small effects (a phenomenon called polygenicity). In addition, every polymorphism may contribute to a large number of traits (a phenomenon called pleiotropy). In recent decades, the fast development of advanced technologies and analytical tools has made possible investigations of complex genetic architecture of psychological traits. However, progress with identification of genetic factors for various traits has turned out to be slower than initially expected.

      Earlier molecular genetic studies of social development used the candidate gene approach. These studies focused on one or a few SNPs (or other types of polymorphisms) in a specific gene of a biological pathway that is known to be implicated in a specific trait. Each SNP has 2 variants (alleles). If one allele (e.g., one form of a particular SNP) increases a chance of development of a particular trait, then we can identify the association of this allele with a specific trait by using simple statistical methods, such as linear regression analysis for a continuous trait, or logistic regression analysis for a binary trait. For example, in a genetic association study of antisocial behavior by using a case‐control design, the frequency of an allele associated with antisocial behavior will differ statistically between cases and controls.

Using the candidate‐gene approach, a number of genes, mainly involved in neurotransmitter pathways (serotonin, dopamine, and norepinephrine), have been studied for associations with antisocial behavior and/or aggression. There is some evidence for the role of the following genes in youth and/or adult antisocial behavior: Catechol‐O‐methyl‐transferase (COMT) (Qayyum et al., 2015), Monoamine oxidase A (MAOA), (Manuck et al., 2000), Serotonin transporter (SLC6A4), (Bellivier et al., 2000) and tryptophan hydroxylase (TPH), (Manuck et al., 1999). Fewer studies have been conducted in children. In one such example, an association was demonstrated between a polymorphism of the dopamine D4 receptor gene (DRD4) and maternal report of child aggression at age four (Schmidt et al., 2002). Evidence of associations of child aggression with common polymorphisms of SLC6A4 (Beitchman et al., 2003) and COMT (Caspi et al., 2008) have also been reported, although results are inconsistent (Schmidt et al., 2002).

      However, a systematic meta‐analysis of 185 studies of 31 candidate genes showed the absence of reliable associations between analyzed genetic variants and aggression (Vassos et al., 2014). Subgroup analysis, which took into account the severity of disorder, and sample characteristics, did not find any significant associations. There are a number of problems related to these reports of candidate gene associations, such as generally very low statistical power and difficulties with reproducibility (replication) of the initial findings.

      One of the latest developments in molecular genetic research is the Genome‐Wide Association Study (GWAS) approach, in which hundreds of thousands of SNPs are tested simultaneously for associations with a particular trait (Hirschhorn & Daly, 2005). Application of this method in large samples (tens or even hundreds of thousands of participants) allows for identification of multiple DNA markers of small effect. At the time of writing (January 2021), there have been 227,631 significant associations identified between common genetic variants (polymorphisms) and various traits. This list is continuously updated by the NHGRI‐EBI Catalog of human genome‐wide СКАЧАТЬ