8.2 Genetics, Conception, and Prenatal Development

Chromosomes and Genes

You are a walking, talking collection of biological building blocks called cells. But you weren’t always this way. You began as a single cell. Then that cell divided into two daughter cells, and those daughter cells divided, and their daughters divided, until eventually there were trillions of cells (Sherwood, 2010).

With the exception of red blood cells, every cell in the human body has a nucleus at its center. Within this nucleus is material containing the blueprint or plan for the building of a complete person. (But as you will soon learn, these plans are easily altered by the environment.) This material is coiled tightly into 46 chromosomes, the inherited threadlike structures we get from our biological parents—23 from our biological mother and 23 from our biological father. A chromosome contains one molecule of deoxyribonucleic acid (DNA). Looking at the DNA molecule in Figure 8.1, you can see a specific section along its length has been identified. This section corresponds to a gene, and each gene encodes a particular protein. The proteins encoded by genes determine the texture of your hair, the color of your eyes, and some aspects of your personality. Genes influence nearly every dimension of the complex living system known as YOU.

FIGURE 8.1Chromosomes, DNA, and GenesEvery cell in your body, except red blood cells and sex cells (sperm or egg), contains a full set of 23 chromosome pairs like those shown in the photo below. These 23 chromosome pairs contain the full blueprint for you as a complete, unique person. The primary component of each chromosome is a single, tightly wound molecule of DNA. Within that DNA are around 21,000 genes, each determining specific traits such as hair texture.

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How exactly did you get your genes from your biological parents? As we mentioned above, genes are found in chromosomes, and chromosomes are what we inherit from our biological parents. (Both the sperm and egg contain 23 chromosomes, which explains why we have 23 pairs.) The 23rd chromosome pair, in particular, determines genetic sex (the XX for a female, and XY for a male; Chapter 10).

These 23 chromosome pairs are unique to you and are known as your genotype. Genotypes do not change in response to the environment, but they do interact with the environment. Because so much variability exists in the surrounding world, the outcome of this interaction is not predetermined. The color and appearance of your skin, for example, result from an interplay between your genotype and a variety of environmental factors including sun and wind exposure, age, nutrition, and smoking—all of which can impact how your genes are expressed (Kolb & Whishaw, 2011; Rees, 2003). The results of this interaction are the observable characteristics of an individual, or phenotype. A person’s phenotype is apparent in her unique physical, psychological, and behavioral characteristics (Scarr & McCartney, 1983).

You might be wondering what all this talk of genotype and phenotype has to do with psychology. Our genetic make-up influences our behavior, and psychologists are interested in learning how genes might do this. Consider schizophrenia, a psychological disorder (Chapter 13) with symptoms ranging from hallucinations to emotional problems. A large body of evidence now suggests that a person’s genotype may predispose him to developing schizophrenia. Researchers report heritability rates as high as 80 to 85% (Craddock, O’Donovan, & Owen, 2005; Tandon, Keshavan, & Nasrallah, 2008). But the expression or manifestation of the disorder results from a combination of genotype and experience, including diet, stress, toxins, and early parenting (Champagn & Mashoodh, 2009; Zhang & Meaney, 2010). Identical twins, who have the same genotype, may display different phenotypes, including different expressions of schizophrenia if they both have this disorder. This is because schizophrenia, or any psychological phenomenon, results from complex relationships between genes and environment.

We know that environmental factors influence the expression of genes, and how this occurs is the focus of epigenetics, a field that examines the processes involved in the development of phenotypes. As mentioned previously, the environment cannot change our genetic make-up, but it does impact the expression of our genes. For example, maternal behavior during pregnancy can determine which genes are expressed by a fetus (Masterpasqua, 2009; Petronis, 2010). In one longitudinal study following 40 pairs of identical twins, the researchers found that gene expression differed significantly between twins. As the twins grew older, their lifestyles, susceptibility to diseases, and a variety of other characteristics became less and less similar, even though their genetic make-up remained identical (Fraga et al., 2005).

Seas of DNA The colored lights are an artistic representation of the human genome, the complete set of DNA found in nearly every cell in the body. Researchers with the Human Genome Project have decoded the entire human genome, which contains about 21,000 DNA segments known as genes (Pennisi, 2012, September 5). Genes are the blueprints for proteins that endow you with a unique set of traits, including eye color, hair texture, and—to a certain extent—psychological characteristics.

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Genes are behind just about every human trait you can imagine—from height, to shoe size, to behavior. But remember, you possess two versions of a gene from each chromosome pair: one from your biological mother and one from your biological father. Sometimes the genes in a pair are identical (two genes encoding dimples, for instance). In other cases, the two genes differ, providing conflicting instructions about the outcome (one encodes dimples, while the other encodes no dimples). Often one gene variant has more power than the other. This dominant gene governs the expression of the inherited characteristic, overpowering the recessive, or subordinate, gene in the pair. A recessive gene cannot overcome the influence of a dominant gene. For example, “dimples” are dominant, and “no dimples” are recessive. If one gene encodes for dimples and the other no dimples, then dimples will be expressed. If both genes encode for no dimples, no dimples will be expressed.

This all sounds relatively straightforward, but it’s not. Psychological traits—and the genetics behind them—are exceedingly complex. Characteristics such as intelligence and aggressive tendencies are influenced by multiple genes, most of which have yet to be identified.

Even if you could identify every gene associated with every trait, you would still face the daunting task of untangling the effects of heredity and environment. Take Jasmine’s son Eddie, who might be described as introverted. We could try to estimate the degree to which his introversion results from something in the environment (being the youngest in a family of very outgoing people, for example). We could also search for something in his genetic background that contributes to his introversion (perhaps his father is shy, and he passed this trait along to his son). The nature–nurture issue is complex yet vital for a thorough understanding of development. Now, let’s explore the biological beginnings of human development.

Can You Roll? Urban myth tells us that tongue-rolling ability is determined by the presence of a single, dominant gene (Starr, 2005, June 10). It’s probably more complicated than that, though. Many people who cannot roll their tongues as young children eventually learn to do so, suggesting that environmental factors also play a role in the development of this skill (Komai, 1951).

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From Zygote to Embryo to Fetus

As you may recall, Jasmine’s pregnancy with Jocelyn was not particularly difficult. She “didn’t feel” pregnant. But the changes occurring within her body were extraordinary. The making of a human being, from a single cell to a sensing, perceiving, aware individual, is nothing short of awesome. Let’s take a look at prenatal development, the 38–40 weeks between conception and birth.

In the beginning, before you became you, an egg (also known as an ovum) from your biological mother and a sperm cell from your biological father came together at the moment of conception. Together, the egg and sperm formed a single cell called a zygote, which is smaller than the tip of a needle. Under normal circumstances, a zygote immediately begins to divide into two cells, then each of those cells divides, and so on. This fast-growing mass of cells begins to move through the fallopian tube toward the uterus.

Conception sometimes results in twins or multiples. Identical or monozygotic twins develop from one egg inseminated at conception. This egg is fertilized by one sperm and then it splits, forming separate zygotes. Monozygotic twins have identical sets of 46 chromosomes, as they originate from the same zygote; the resulting infants are the same sex and have almost identical features. Fraternal or dizygotic twins, on the other hand, occur when two eggs are inseminated by two different sperm, leading to the development of two zygotes. This can occur naturally, but assisted reproductive technology may increase the odds of a woman releasing more than one egg (Manninen, 2011). Twins and multiples resulting from these distinct sperm–egg combinations are like other biological siblings; they share around 50% of their genes.

From conception to the end of the 2nd week is the germinal period, during which the rapidly dividing zygote implants in the uterine wall. Between the 3rd and 8th weeks of development, the growing mass of cells is called an embryo. The embryo is protected in the amniotic sac and eventually receives nourishment, hydration, and oxygen through the umbilical cord, which is attached to the placenta. For the most part, the placenta ensures that the blood of the mother and the baby do not mix and disposes of carbon dioxide and waste. Until now, all the cells have been identical. But during this embryonic period, the cells differentiate and the major organs and systems begin to form (Figure 8.2). This differentiation allows for the heart to begin to beat, arms and legs to grow, and the spinal cord and intestinal system to develop by the end of this period. But less than half of all zygotes actually implant in the uterine wall (Gold, 2005). Of reported pregnancies, around 21% end in a miscarriage (Buss et al., 2006), many of which result from genetic abnormalities of the embryo (Velagaleti & Moore, 2011).

FIGURE 8.2Prenatal Development and Periods of Critical GrowthDuring prenatal development, individual structures form and are fine-tuned at different times. As each structure is being established, it is particularly vulnerable to alterations. Once their critical periods are complete, the structures are fully established.

LEFT: OMIKRON/SCIENCE SOURCE; CENTER: ANATOMICAL TRAVELOGUE/SCIENCE SOURCE; RIGHT: NEIL BROMHALL/SCIENCE SOURCE

Dad and Daughter A father enjoys time with his 25-year-old adopted daughter. The girl has fetal alcohol syndrome, a condition that results from exposure to alcohol during fetal development. The symptoms of fetal alcohol syndrome—which can be avoided by abstaining from alcohol during pregnancy—include delays in physical growth, learning disabilities, and problems with anxiety, attention, and impulse regulation (Mayo Clinic, 2011, May 21).

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The embryo may be safely nestled in the amniotic sac, but it is not protected from all environmental dangers. Teratogens are agents that can damage a zygote, embryo, or fetus (TABLE 8.1). Radiation, viruses, bacteria, chemicals, and drugs are all considered teratogens. The damage depends on the agent, as well as the timing and duration of exposure, and can result in miscarriage, decreased birth weight, and heart defects. One well-known teratogen is alcohol, which can lead to fetal alcohol spectrum disorders (FASD). In particular, fetal alcohol syndrome (FAS) is the result of moderate to heavy alcohol use during pregnancy, which can cause delays in normal development, a small head, lower intelligence, and distinct facial characteristics (for example, wide-spaced eyes, flattened nose). Researchers continue to debate what constitutes an acceptable amount of alcohol use during pregnancy, but they do agree even a small amount poses risks (Nathanson, Jayesinghe, & Roycroft, 2007; O’Brien, 2007).

Between 2 months and birth, the growing human is called a fetus (Figure 8.2). During the fetal period, the developing person grows from the size of a pumpkin seed to a small watermelon, the average birth weight being approximately 7 pounds (by North American standards). The fetus rapidly gains weight and begins to prepare for birth, and it already has clear sleep–wake cycles (Suwanrath & Suntharasaj, 2010). Under normal circumstances, all organs, systems, and structures are fully developed at birth, and the brain is approximately one-quarter the weight of an adult brain (Sherwood, Subiaul, & Zawidzki, 2008).

If you step back and contemplate the baby-making phenomenon, it’s really quite amazing. But many more exciting developments are in store. Are you ready for some shrieking, babbling, and a little game of peekaboo? And then perhaps some talking, toddling, and tantrums? Let’s move on to infancy and childhood.