Each one of us begins our developmental story at conception when the successful joining of an egg and sperm eventually produces an embryo. A lot happens during this beginning phase of life, including the influence of nature and nurture. Nature refers to the biological, genetic, and hereditary factors passed on to us by our parents and ancestors. Nurture refers to the environmental and social factors that influence development, such as nutrition, stress, or access to health care (Tabery et al., 2014; Hosken et al., 2019).

Many scholars support the idea that nature and nurture are both significant in development, and it would be hard to attribute a trait or characteristic to only one factor. Both are important, and they even impact each other in a process called bidirectionality (Lerner, 2006). This means that aspects of our genetic and hereditary factors may influence the kind of nurturing environment we need to thrive. Nurture may, in turn, influence how our genes impact our day to day interactions with ourselves and others. The influence works in both directions. Genes are the basic building blocks of heredity, which are made up of DNA.

Let’s use height as an example of how nature and nurture influence each other. You may have an idea about how tall you will be based on your genes. Your parents’ height is a great predictor of how tall (or not so tall) you will grow. However, height is a polygenic trait, meaning that it is not determined by a single gene, but rather a network of genes. In fact, most of our features are polygenic (Turchin et al., 2012). So, you may be able to estimate your genetic potential for height, but what about the impact of the environment?

Height is not determined at conception, or even birth. It’s more like genes create a range of what is genetically possible, and then your environment tips the scale one way or the other. With good sleep, nutrition, and regular exercise, you may grow to be on the high end of the genetic range. Low socioeconomic status or chronic stress may tip you the other way. These examples represent how nature and nurture constantly interact, expanding or contracting the possibilities of what may be for each individual.

It can be difficult, sometimes impossible, to know exactly how specific aspects of human development occur. Perhaps some of us are more sensitive to the influences of our genes, while others are more impacted by nurturing experiences (Sameroff, 2010). We may not all respond the same way to the same situation.

One theory that helps us understand the variations in human development is differential susceptibility. Differential susceptibility suggests that each person is born with a unique and varying degree of plasticity, which is the ability to be influenced by their environment (Belsky, 1997; Belsky et al., 2007). The level of plasticity depends on each individual, with some having a higher level than others. There is not a single factor that determines how plastic one is in their development. Instead, plasticity is influenced by our genetic make up and our environment.

For individuals who are more plastic, the conditions and experiences within their environment may have a greater impact on their development. In these cases, the effects of the good or positive inputs in a person’s environment are heightened, and the negative or detrimental inputs in a person’s environment, intensify the impact and degree of adversity (Jaffee et al., 2012). For individuals whose development is less plastic, the conditions of their environment and their experiences will have less of an impact on their overall development. However, this doesn’t mean that the quality of input doesn’t matter to the person and their overall well-being. It simply means that an individual with less plasticity may not fall as fast or as far in the face of adversity. We will have a deeper conversation about plasticity in Chapter 5.

Now that you have an understanding of the way nature and nurture work together, we will explore a little about our genetic makeup and the dynamic process of conception and prenatal development.

What Are Genes and Chromosomes?

Every single cell in our bodies, except for red blood cells (because they do not have a nucleus) contains deoxyribonucleic acid, or DNA, the unique material that serves as the building blocks for our genes. Chromosomes are thread-like structures that carry genetic information in the form of genes, which are made up of DNA. Imagine our DNA, genes, and chromosomes as a LEGO set. The DNA are the individual pieces. As with LEGO pieces, each piece of DNA serves a specific function in the design of the gene. The genes are like the blueprints for the buildings. In our bodies, genes contain the information to determine features like our hair color or whether our cheekbones resemble those of our mom or dad. Like the instructions for the LEGO set, the genes are how we get our unique look. Each completed minifigure or vehicle in the LEGO set is like a chromosome. When all 46 chromosomes come together, they represent a fully completed LEGO set, all of the people, buildings, and accessories in place. All of the pieces fit together to complete the puzzle.

Chromosomal and Genetic Abnormalities

In the United States, there are approximately 3 million babies born each year (Martin et al., 2021). The majority of these babies are born with the typical 46 chromosomes. Roughly 33 in every 1,000 babies, or about 3 percent, are born with a chromosomal abnormality. Chromosomal abnormalities are either structural or numerical. With a structural abnormality, a part of the chromosome does not duplicate correctly, resulting in missing information and an incomplete chromosome. Numerical abnormalities result when there are too many or too few chromosomes. Abnormalities may occur as early as conception. In many cases, a chromosomal abnormality will make it impossible for the fetus to survive, resulting in a spontaneous termination, more commonly called a miscarriage. According to the American College of Obstetricians and Gynecologists, 10 out of 100 known pregnancies end in miscarriage.

One risk factor that increases the risk of a chromosomal abnormality is the exposure to external factors that compromise fetal health, also known as teratogens. Some examples of teratogens are alcohol, smoking, exposure to pesticides and other chemicals, and use of pharmaceuticals, both prescription and illicit substances. Later in the chapter we will look more closely at how teratogens impact maternal and fetal health.

Genotype, Phenotype, and Heredity Factors

Do you like cilantro? You may love it and add it to all of your favorite dishes. You may hate it and think it tastes like soap. What you may not know is that the taste of cilantro is an inherited trait, determined by your genes. In fact, genes influence a lot of who we are, from what we look like to whether we are right or left handed. Even some of our preferences (such as whether you like cilantro) are impacted by our genetic makeup.

The genes we inherit from our parents make up our genotype. Because we get genetic material from both parents, it is possible to receive the same version of a gene from each. We call this a homozygous genotype. In this instance, we will display the characteristic because it is the only genetic option available. But it is also possible to get different versions of the same gene, such as the gene for eye color, which is called a heterozygous genotype. Eye color is an example of a trait that can be either homozygous or heterozygous. Blue eyes only occur if a person gets the same recessive gene from both parents. Brown eyes could result from either homozygous or heterozygous genes. If a person gets a recessive blue eye gene from one parent, but the dominant brown eye gene from the other, they would have brown eyes, but a heterozygous genotype for eye color. If a person receives the dominant brown eye gene from both parents, they would have brown eyes, and a homozygous genotype for eye color.

In some instances, such as eye color, we cannot display both genes, so one gene will be visible or active, while the other will not. Dominant and recessive genes influence which genes are expressed. The observable traits or characteristics of an organism that are expressed in our development are called the phenotype. Dominant genes express themselves in the phenotype, even when paired with a different version of the gene. Their silent partner gene is called recessive. Recessive genes express themselves only when paired with a similar version gene. If you sneeze when you are exposed to the sun, you can thank your parents—both of them—because photic sneezing is caused by a recessive gene! Red hair is also recessive, which means that in order for you to have it, you must receive the gene from both parents.

As we learned earlier in this chapter, many characteristics are polygenic, meaning they result from several genes. In addition, the dominant and recessive patterns described are usually not that simple either. Sometimes, the dominant gene does not completely suppress the recessive gene. This is known as incomplete dominance.

A Deeper Dive Into Genetics

Our understanding of genetics has evolved throughout centuries of scientific inquiry. Much of what we know about how genetic traits work can be traced back to the work of Gregor Mendel, who first identified the concept of dominant and recessive genes through observing hybrid pea plants. He discovered that when he bred certain species of plants together, specific traits seemed to be passed on more often than others (dominant) and that other traits were only passed on when both plants shared the same trait (recessive). Of course, humans are much more complicated than pea plants, but the concept proved to be true, albeit a little more nuanced.

Reginald Punnett was also an important figure in biogenetics. He created the first tool for predicting the probability of hereditary traits being passed on. The Punnett square uses a simple two by two square and basic multiplication to predict the offspring’s probability of a specific trait based on the possible combinations of two parents’ genetic information (figure 3.2).

Check out the link for more information on how dominant and recessive genes interact. You can even play with this Punnett square calculator [Website] and see probabilities of gene makeup.

Epigenetics

Earlier in the chapter, we learned about how nature and nurture interact to help shape our development. We learned that our genetic material is inherited (genotype) and that genes are expressed in several ways (phenotype). A gene may be turned on (expressed) or turned off (not expressed) based on environmental factors. To help us think about how epigenetics is related to gene expression, let’s think about baking cookies.

A recipe of chocolate chip cookies, using all of the same ingredients, can present differently each time. The phenotype of the cookie changes, even though the genes do not. How is this possible? The answer is the environment. In this case, the phenotype of a cookie will change based on the environmental changes in the ingredient butter. If you want a fluffy, soft cookie with a cake-like texture, it is best to soften the butter before mixing into the batter. But perhaps you prefer a cookie that is thin and chewy in the middle, but crispy around the edges. If so, you need to melt the butter before mixing the dough. Altering the butter (the environment) changes how the cookie develops in the oven and the finished product that comes out (phenotype), even though the ingredients (the genotype) stayed the same.

Our genetic material is determined at conception, but the conditions after conception may alter how genes present. Even though there is no actual DNA change, environmental conditions may prompt some genes to be switched on or others be switched off. The genetic potential is reacting to the environment. Without actually changing the DNA, the cells “remember” the environmental conditions and prepare future genes to respond, creating a heritable change that comes from outside of the gene itself. Some examples of epigenetic changes can be attributed to nutrition and diet, such as an increased risk of diabetes or obesity, while others can be attributed to chronic stress, such as increases in anxiety and attention deficit (Toperoff et al., 2012; Nilson et al., 2014). Like the butter in cookies, the DNA remains the same, but it may look and feel different based on external factors.

Research on mice who carry the agouti gene, a gene responsible for making the rodents yellow in color and more prone to diabetes and cancer, further demonstrates the impact of epigenetics on organisms. Researchers simply changed the mothers mice’s diet, feeding them methyl rich foods found in onions, garlic, and beets, which resulted in surprising changes to the offspring (Jirtle, 2014). The baby mice were thinner than the prior generation, with a brown coat versus a yellow coat, and were not as susceptible to disease. They also lived longer than the prior generation (Jirtle, 2014). These mice contained the same DNA as prior generations, but the change in diet was enough to “turn off” the negative impacts of the agouti gene. This research is important in helping us understand how maternal environments and exposure to both positive and negative factors can influence future generations.

Licenses and Attributions for Becoming You: How Human Development Begins

“Becoming You: How Human Development Begins” by Terese Jones is licensed under CC BY 4.0.

Figure 3.2. “Incomplete dominance punnett square” by Adabow, Wikimedia Commons is licensed under CC BY-SA 3.0 / A derivative from the original work.

Figure 3.3. Epigenetics and the Agouti Mice from the National Institute of Environmental Health Sciences, included under fair use.