2.1 Genetics and Environment

Chromosomes, DNA, and Genes

The first step in grasping the way genetics influence development is to understand chromosomes, DNA, and genes, and the way each functions within a cell–the focus of the field of molecular genetics. Cells duplicate themselves through a process of cell division called mitosis, which allows organisms to grow and replace old or damaged cells. Mitosis accounts for the replication of most types of cells in the body and ensures that each new cell includes all forty-six chromosomes, organized in twenty-three pairs located in the cell nucleus. A chromosome is a rodlike structure in the cell nucleus, composed of long molecules of DNA. DNA (deoxyribonucleic acid) molecules contain an individual’s genetic information; they coil around each other to form a double helix, a twisted ladderlike structure (Figure 2.2).

Figure 2.2 Chromosomes are located in the nucleus of the cell and are made up of the double-helix molecule of DNA. Genes are segments of the DNA sequence, made up of nucleic acid bases that provide the instructions for all the cells in the body. (attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

A human sex cell—the reproductive cell called a gamete—holds only twenty-three chromosomes, one from each pair of parental chromosomes. These cells replicate through a different type of cell division called meiosis. During meiosis, a sex cell, whether an ovum in a female or a sperm cell in a male, splits into two new cells that each also contain only twenty-three chromosomes. Thus, gametes contain only half the genetic information of each parent, ensuring that after fertilization of the ovum, the resulting offspring will have a total of forty-six chromosomes, twenty-three from the biological mother and twenty-three from the biological father. Which particular chromosomes these are is largely random: eight million different combinations are possible, contributing to the wide genetic variation seen among humans.

A human nonsex cell, called an autosome, is what is present in chromosome pairs one through twenty-two. The twenty-third pair—the sex chromosomes—may look the same or different (Figure 2.3). The sex chromosomes are called the X and Y chromosomes, based on their shapes. The combination of these chromosomes determines the child’s sex at conception: XX for a biological female and XY for a biological male.

Females have two X chromosomes, so each of their ova contains an X chromosome in the twenty-third position; males have both X and Y chromosomes, so each of their sperm cells contains one or the other, and thereby determines the baby’s sex.

Intersex is an umbrella term for people who have one or more of a range of variations in sex characteristics or chromosomal patterns that do not fit the typical conceptions of male or female; the prefix inter- means “between” and refers here to an apparent biological state “between” male and female. There are many causal factors that can make a person intersex; these are often referred to as differences in sex development (DSD), though there is debate over which DSDs make a person intersex. Genetically, the baby may have a different number of sex chromosomes. Rather than two X chromosomes (associated with females) or one X and one Y chromosome (associated with males), babies are sometimes born with an alternative number of sex chromosomes, such as XO (only one chromosome) or XXY (three chromosomes). In other cases, hormonal activity or even chance occurrences in the womb can affect the baby’s anatomy. The number of people born intersex has been expressed to be as high as 1.7 percent of births; however, based on different interpretations of what makes someone intersex, you may see other statistics or ranges quoted (Fausto-Sterling, 2000; Becker, Chin & Bates, 2022).

While some conditions or disorders associated with being intersex may require treatment or intervention, being intersex is not a disorder. The intersex community and many medical groups view surgery to support the assignment of an infant to a specific sex to be unethical and even abusive. For example, Medical ethicist Kevin Behrens (2020) argues that surgical interventions for intersex children should only be carried out when surgery serves the best medical interests of the child.

Figure 2.3 This image shows a full set of twenty-three pairs of chromosomes. Chromosomes one through twenty-two are the autosomes, and the chromosomes at the bottom right show the pairing for a biological female (XX) at conception. (credit: modification of work “Figure 7. Localization of Alu Sequences in Nuclei of Fibroblasts and Lymphocytes” by Andreas Blozer et al/PLOS Biology, CC BY 2.0)

The most commonly occurring twenty-third chromosome pairings are XX and XY (National Organization for Rare Disorders, n.d.; Genetic and Rare Diseases Information Center, n.d.). However, differences may occur during cell division that result in aneuploidy, an atypical number of autosomal or sex chromosomes that may result in various syndromes (Gottlieb et al., 2023; Skuse et al., 2018) (Table 2.1).

Our genetic inheritance is carried in the genes that make up our chromosomes. A gene is a segment of DNA that contains the instructions for making proteins that regulate the structure and functioning of the body. It’s estimated that humans have between 20,000 and 25,000 genes (National Human Genome Research Institute, 2022). The rungs of the chromosome’s ladderlike structure, which house the DNA sequence, consist of four nucleic acid bases: guanine (G), adenine (A), thymine (T), and cytosine (C). These nucleic acids pair up in specific ways (Klug et al., 2016). However, not all of our genes code for proteins; some regulate gene expression, or how genes work and interact with each other.

The complete sequence of an organism’s DNA is called its genome. Each human being shares about 99.9 percent of their genome with all other humans on the planet; the remaining 0.01 percent contributes to the differences we observe in physical and behavioral differences, as well as the variations in our risk of developing certain types of diseases (Duello et al., 2021; National Human Genome Research Institute, 2022).

Genetic Inheritance

Have you ever wondered why you share some physical characteristics with your biological parents but not others? Maybe you have your mother’s freckles, or perhaps a different eye color than either of your parents. How do these similarities and differences occur?

Recall that humans receive twenty-three chromosomes each from their biological parents’ sperm and ovum. During fertilization, these chromosomes fuse to create one cell with twenty-three pairs of chromosomes, each carrying two copies of each gene. The particular composition of the genes we inherit makes up our unique genotype, which cannot be directly observed. Some genes have a slightly different alternate forms, called an allele, and the specific alleles in our genotype can lead to differences in gene expression (National Human Genome Research Institute, 2022). These differences, in turn, result in a different phenotype, or set of characteristics that can be observed, such as hair or eye color. Biological parents may both give their child the same allele, or each parent may contribute a different allele.

The genes for certain traits, such as eye color, have both dominant and recessive alleles. If an allele is dominant, we need to inherit only one copy of it for the dominant trait, such as brown eyes, to be observed. However, we need two copies of a recessive allele, such as that for blue eyes, for the recessive trait to be observed. This pattern of inheritance is called the recessive/dominant pattern. If a child inherits the same eye color allele from both parents, the child is considered homozygous for this trait. The child who inherits different eye color alleles from both parents is heterozygous for this trait (Figure 2.4).

Figure 2.4 B denotes the dominant allele for brown eyes, and b denotes the recessive allele for blue eyes. Each pairing represents a different probability of inheriting eye color. There is a 25 percent chance of inheriting homozygous alleles resulting in brown eyes (BB), a 50 percent chance of inheriting heterozygous alleles (Bb or bB) resulting in brown eyes, and a 25 percent chance of inheriting homozygous alleles resulting in blue eyes (bb). (credit: modification of work “Heterozygous” by Darryl Leja, National Human Genome Research Institute/Wikimedia Commons, Public Domain)

Another inheritance pattern for traits such as hair texture, skin color, and height demonstrates incomplete or partial dominance, which occurs when an individual is heterozygous for a particular trait and neither allele is completely dominant or recessive. The result is incomplete dominance, an intermediate phenotype in which both alleles are expressed (Figure 2.5). For instance, if one parent is homozygous for curly hair and the other is homozygous for straight hair, incomplete dominance will result in the child’s having wavy hair, because alleles for both straight and curly hair are simultaneously expressed (Omoto & Lurquin, 2004).

Figure 2.5 If one parent is homozygous for curly hair and the other is homozygous for straight hair, their child will have wavy hair; this is a case of incomplete dominance. (credit: modification of work “ABO system codominance” by National Institutes of Health/Wikimedia Commons, Public Domain)

For other traits, a codominant inheritance pattern holds. With a codominant trait, an individual inherits both alleles of a gene, and each is fully expressed (National Human Genome Research Institute, 2024). For example, the ABO blood group has three alleles, A, B, and O, all encoded by the same gene. A child will inherit one of these alleles from each parent, which results in one of four possible phenotypes (A, B, AB, or O), commonly known as your blood type (Dean, 2005) (Figure 2.6).

Figure 2.6 A child whose biological parents are heterozygous for the A, B, O blood type alleles (AO and BO in this example) will have equal probabilities of expressing any of the four blood type phenotypes (A, AB, B, or O). (credit: modification of work “ABO system codominance” by National Institutes of Health/Wikimedia Commons, Public Domain)

Another factor that can influence the way genes are expressed is whether they are located on the X chromosome, meaning they are X-linked (Figure 2.7). For example, the genes identified for red-green colorblindness, a recessive trait, are located on the X chromosome. Biological males are more frequently affected than biological females by recessive X-linked traits such as red-green colorblindness. Females need to receive the recessive allele for this trait from both biological parents, whereas males need to receive it only from their biological mother.

Figure 2.7 Females who carry or inherit one X-linked red-green colorblind allele will not express the trait and are referred to as carriers. Males who carry or inherit the X-linked red-green colorblind allele and females who carry or inherit two copies will express the trait. (credit: modification of work “ABO system codominance” by National Institutes of Health/Wikimedia Commons, Public Domain)

The traits described rely on one or a few genes, but the vast majority of physical, behavioral, and health traits are a type of polygenic trait, meaning they are governed by multiple genes. Together, the effects of all the individual genes add up to create the observed phenotype. Scientists are beginning to identify which combinations of genes are associated with a range of complex traits and diseases such as cognitive ability, major depressive disorder, type 2 diabetes, and coronary heart disease (Tam et al., 2019). Not all the phenotypic variation observed is accounted for by genes, however, environmental factors also contribute to the expression of a trait. Complex traits and diseases are likely a result of multifactorial inheritance; that is, their occurrence depends on both genetic and environmental factors.

In addition, a single gene can influence multiple traits and produce a variety of phenotypic outcomes, a characteristic referred to as pleiotropy (Figure 2.8). For example, PKU (phenylketonuria) is a single-gene recessive disorder. This mutation results in the ineffective metabolism of phenylalanine (found in milk and other foods), bringing about a variety of issues if untreated, such as cognitive disability, eczema, and delayed growth (Elhawary et al., 2022; Targum & Lang, 2010). Another type of pleiotropic effect occurs when genes referred to as generalist genes affect different but related phenotypes. For example, the same genetic variants are shared across verbal and nonverbal cognitive abilities (Bearden & Glahn, 2017), and shared genetic variants are indicated across several psychiatric disorders (Cross-Disorder Group of the Psychiatric Genomics Consortium, 2019).

Figure 2.8 (a) Polygenic effects are observed when the products of several genes are combined to create a particular phenotype. Conversely, (b) pleiotropy effects occur when the products of a particular gene affect multiple phenotypes. (attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

Inherited and Genetic Disorders

Genes contribute to all aspects of human development, functioning, and trait expression, including inherited and genetic disorders. There are three main types of genetic disorders: single-gene disorders, chromosomal disorders, and multifactorial inherited disorders.

A single-gene disorder occurs in a single gene and can happen in two ways: (1) Individuals may inherit a pair of recessive alleles responsible for a disorder, or (2) mutations to the DNA sequence can result spontaneously, from errors during cell division or from exposure to environmental toxins or other hazards such as x-rays or pollution. Mutations occur when the DNA sequence of a gene is altered; this can occur in a variety of ways but most commonly through a change in a single nucleic acid base pairing substitution. Examples of single-gene disorders include PKU, Tay-Sachs disease, cystic fibrosis, and sickle cell anemia (Table 2.2).

A chromosomal disorder is due to errors during cell division and result in structural or numerical abnormalities. Structural abnormalities occur when sections within a chromosome are deleted, duplicated, or inverted (a section of the chromosome flips its orientation), or when sections are rearranged within or across the chromosome pairs. Numerical abnormalities happen during cell division and result in one of the twenty-three chromosome pairs containing only one chromosome (monosomy) or having an extra chromosome (trisomy). For example, individuals with Down syndrome will have three twenty-first chromosomes rather than two (Figure 2.9). Because each chromosome contains a large number of genes, deviation in chromosome number (more or less than two) will affect a range of characteristics.

Figure 2.9 Down syndrome is characterized by trisomy, the presence of an extra chromosome, on the twenty-first chromosome. The additional genetic material can affect physical and cognitive development and bring a higher risk of thyroid and heart disease. (credit: modification of work “Down Syndrome Karyotype” by National Human Genome Research Institute/Wikimedia Commons, Public Domain)

A disorder resulting from multifactorial inheritance occurs when both polygenic effects (controlled by many genes) and environmental effects contribute to a phenotype. Studies have identified several genes that contribute to the risk of developing type 2 diabetes, heart disease, obesity, autism spectrum disorder, and anxiety disorders (Dickerson & Dickerson, 2023; Genovese, & Butler, 2023; Li et al., 2024; National Human Genome Research Institute, 2020b). Many of these disorders are also influenced by multiple genes and a range of environmental influences, such as the prenatal environment, nutrition, socioenvironmental experiences, and environmental conditions.

Known inherited and genetic disorders can be screened for before pregnancy, prenatally, and shortly after birth. Table 2.2 offers an overview of some of these disorders.

Gene Environment Intersections

Genetic research has helped us understand human traits that rely on one or two genes. However, most human traits, such as personality traits, are more complex and result from a combination of genetic (nature) and environmental (nurture) influences. Understanding how, and how much, genes and environments influence gene expression together is the focus of the field of behavioral genetics. As described in 1.2 Themes of Development, heritability estimates indicate the degree to which genetics contribute to individual differences for a trait within a population. A high heritability estimate, near 100 percent, indicates that genetic effects explain a lot of variability within a population; a low heritability, closer to 0 percent, indicates that most of the variability is environmental (Mayhew & Meyre, 2017) (Table 2.3).

The goal of behavioral genetics is to explain 100 percent of the observed variance for a trait. Interpreting a heritability estimate also reveals to what degree environmental influences account for differences in a trait. For example, the heritability of intelligence is 50 percent, which means that genetics explains half the observed differences in this complex trait, and the remaining 50 percent is due to environmental effects. Note that heritability is a population statistic and cannot be applied to a specific individual. In other words, heritability can’t tell you how much your intelligence is due to the specific genes you inherited. Consider the examples in Table 2.3.

Heritability helps us understand understand the magnitude of individual genetic and environmental effects on a complex trait. But how do genes and environments work together to create the range of phenotypes observed for traits like personality or depression? The answer involves exploring the way genetic and environmental effects are intertwined via epigenetic effects, gene and environment interactions (G × E), and gene and environment correlations (rGE), and epigenetic effects.

Epigenetics

As you learned in 1.2 Themes of Development, epigenetics investigates the way environments, experiences, and behaviors influence the expression of inherited genes without altering the DNA sequence of those genes (National Human Genome Research Institute, 2022). How do epigenetic effects influence gene expression? One common way gene expression is regulated is by the addition and removal of chemical tags through the process of DNA methylation (Figure 2.10). The addition of methyl groups to the nucleotide base(s) of the gene, prevents expression of or silences the gene. Conversely, when a methyl group is removed from the nucleotide base(s) of the gene, it increases gene expression (Moore et al., 2012; Villicaña & Bell, 2021).

Figure 2.10 During DNA methylation, methyl groups are added to or removed from the nucleotide bases, influencing gene expression. (attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

Link to Learning

Watch this TED-Ed brief overview of epigenetics to learn more.

Epigenetic effects can be activated by chemical and environmental exposures such as (lead, air and water pollutants, PCBs, and BPA), diet and food availability (such as lack of folate, or prenatal malnutrition), lifestyle habits (such as smoking, high alcohol consumption, stress, and exercise), and a range of drug treatments (Toraño et al., 2016). These can occur throughout the lifespan and are linked to increased risks for chronic illness and disease and development of mental and physical health issues. Early studies of epigenetic effects examined the records of children whose mothers were pregnant during the Dutch famine of 1944–1945 during World War II (Heijmans et al., 2008). Prenatal exposure to the famine was linked to a greater risk for the development of type 2 diabetes and heart disease later in life (De Rooij et al., 2021). The children of mothers exposed to prenatal famine were larger at birth, while the children of fathers exposed to prenatal famine were heavier in adulthood (De Rooij et al., 2021). These studies indicate that epigenetic effects may be transmitted to future generations.

More recent investigations have focused on the effects of early childhood stress exposure. In children who had experienced earlier traumatic events, often called adverse childhood experiences, epigenetic effects were observed in areas of the brain that control the stress response. These children were more reactive to stress, even low levels, and more prone to depression and anxiety due to the epigenetic changes identified (Turecki & Meaney, 2016). Whether epigenetic effects are passed to the next generation in humans is just beginning to be examined. Although the chemical tags connected to epigenetic effects are usually removed during gamete production, research suggests some can be passed to future generations in animals (Gillette et al., 2018).

Regardless of whether epigenetic effects are passed to the next generation, providing health services and nutritional support to pregnant individuals, infants, and toddlers can reduce preventable illnesses with long-lasting implications that occur through epigenetic mechanisms (National Scientific Council on the Developing Child, 2010). For example, improved health care, exercise, and nutritional resources may serve as important environmental epigenetic factors that can improve mental and physical health. Nutrition and exercise show promise as protective factors in reducing risks of metabolic disease and other health risks (Abraham et al., 2023; Bekdash, 2024).

Gene and Environment Correlations

The understanding of how gene and environment interact suggests that our genes play a role in shaping our environments and experiences. Three types of genotype-environment correlations (rGEs) are evident across the lifespan: passive, evocative, and active (Plomin et al., 1977; Scarr & McCartney, 1983).

A passive genotype-environment correlation occurs when biological parents provide both the genes and environment for children. Therefore, children inherit a genotype that covaries, or varies in correlation with, their family’s environment. For example, if your inherited genes predispose you for musical ability, and you are raised by biologically related caregivers, you are also likely to be provided a family environment that supports musicality. For example, the father already had a drum set and the mother regularly plays ukulele. This type of genotype-environment correlation is observed during infancy and early childhood when children have little control over their environment.

An evocative (or reactive) genotype-environment correlation is evident when an individual’s genetically influenced traits or behaviors evoke a response from those around them. In other words, people evoke environmental effects that covary with their genetic predispositions. For example, an inclination for musical ability will likely be noticed by a music teacher, who works with the individual challenging music pieces and encourages trying out for the school orchestra. Evocative genotype-environment correlations steadily occur across the lifespan, from infancy through late adulthood.

An active genotype-environment correlation is present when our genes influence the experiences and environments an individual seeks out or selects to match their genetic predispositions. In other words, people construct environments that covary with their genotypes. This effect is also referred to as niche-picking, which means we actively choose the environment (niche) where we feel most comfortable. In this example, you recognize your musical ability and select environments and experiences that allow you to pursue personal and professional opportunities connected to music. Active genotype-environments increase over the lifespan as individuals gain more control over their environment.

The interplay between genes and environment is challenging to untangle. For any given human behavior, multiple connections and mutual influences between genotypes and environments work together to create the similarities and differences observed across human behavior (Figure 2.11).

Figure 2.11 Three types of genotype-environments are evident across the lifespan: passive, evocative, and active. (attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)