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LO 4 Examine the role genes play in our development and identify the biological factors that determine sex.

In order to understand human development, you must have a basic understanding of chromosomes and genes. 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. This material is coiled tightly into 46 chromosomes, the threadlike structures we inherit from our biological parents (23 from our father and 23 from our mother). A chromosome contains one molecule of deoxyribonucleic acid (DNA). Looking at the DNA molecule in Figure 8.1, below, you can see a specific section along its length has been identified. This section corresponds to a gene, and genes provide the instructions for making proteins. 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.

Your chromosomes, and all the genes they contain, come from your biological parents. In the moment of conception, your father’s sperm united with your mother’s egg to form a zygote, a single cell that eventually gave rise to the trillions of cells that now make up your body (Sherwood, 2010). Typically, both sperm and egg contain 23 chromosomes, so the resulting zygote has 23 pairs of chromosomes, or 46 total. The 23rd pair of chromosomes, also referred to as the sex chromosomes, includes specific instructions for the zygote to develop into a male or female. Let’s learn how this happens.

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SEX DETERMINATION The egg from the mother contributes an X chromosome to the 23rd pair, and the sperm from the father contributes either an X chromosome or a Y chromosome. If the sperm carries an X, the genetic sex is XX, and the zygote generally develops into a female. If the sperm carries a Y, the genetic sex is XY, and the zygote typically develops into a male. This designation of genetic sex is called sex determination, and it guides the activity of hormones that direct the development of reproductive organs and structures (Ngun, Ghahramani, Sánchez, Bocklandt, & Vilain, 2011). About 50% of sperm carry the X chromosome, and 50% carry the Y chromosome, which explains why about half the population is male (XY) and the other half is female (XX).

Genetic sex is established at the moment of conception and remains constant throughout life, but what happens next? In other words, how does the developing person acquire male or female reproductive anatomy? In a genetic male, the presence of the Y chromosome causes the fetal sex glands (also known as gonads) to become testes. If the Y chromosome is not present, as in the case of a genetic female, then the gonads develop into ovaries (Hines, 2011a). Both the testes and ovaries secrete sex hormones that influence the development of reproductive organs: androgens in the case of the testes and estrogen in the case of the ovaries. Testosterone, for example, is an androgen that influences whether male or female genitals develop. Beginning about 7 weeks after conception, the sex of the developing person can be determined through blood tests (Devaney, Palomaki, Scott, & Bianchi, 2011). By the end of the first trimester, genital anatomy can be determined by ultrasound (Scheffer et al., 2010).

DIFFERENCES OF SEXUAL DEVELOPMENT In some cases, hormonal imbalances and genetic abnormalities lead to differences of sex development (Topp, 2013). Such disparities result from “congenital conditions in which development of chromosomal, gonadal, or anatomic sex is atypical” (Lee, Houk, Ahmed, & Hughes, 2006, p. e488). According to the American Psychiatric Association, intersexual refers to having “conflicting or ambiguous biological indicators” of male or female in sexual structures and organs (2013, p. 451). Around 1% of infants are found to have differences of sex development at birth. They once might have been called “hermaphrodites,” but such terminology is outdated, derogatory, and misleading because it refers to the impossible condition of being both fully male and fully female (Vilain, 2008). There are various types of intersexual development (Table 8.1), but one example might be a genetic female (XX) or male (XY) who has sexual structures and organs that are ambiguous or inconsistent with genetic sex. In this case, we see that genes do not tell the whole story of human development. Let’s explore the relationship between genotype and phenotype.

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LO 5 Discuss how genotype and phenotype relate to development.

GENOTYPE AND PHENOTYPE Recall that most of the cells in your body have 23 chromosome pairs (46 chromosomes total). These chromosomes 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 expression or 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 makeup influences our behavior, and psychologists are interested in learning how this process occurs. Consider schizophrenia, a psychological disorder (Chapter 12) 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, 2008b). But the expression or manifestation of the disorder results from a combination of genotype and experience, including diet, stress, toxins, and early parenting (Champagne & Mashoodh, 2009; Zhang & Meaney, 2010). Identical twins, who have the same genotype, may display different phenotypes, including distinct 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. Understanding these relationships is the main thrust of epigenetics, a field that examines the processes involved in the development of phenotypes.

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DOMINANT AND RECESSIVE GENES Genes are behind just about every human trait you can imagine—from height, to shoe size, to behavior. But remember, you possess two copies of each chromosome (one from mom, one from dad), and therefore two of each gene. Sometimes the genes in a pair are identical (both of them encode dimples, for instance). In other cases, the two genes differ, providing dissimilar 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.

Now that we have a basic handle on genetics, let’s shift our attention toward the developmental changes that occur within the womb.

LO 6 Describe the progression of prenatal development.

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.

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THE ZYGOTE 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.

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 distinct 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.

GERMINAL AND EMBRYONIC PERIODS 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. During the germinal period, all the cells were identical. In the 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, the arms and legs to grow, and the spinal cord and intestinal system to develop. 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).

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TERATOGENS The embryo may be safely nestled in the amniotic sac, but it is not protected from all environmental dangers. Teratogens (tə-'ra-tə-jən) are agents that can damage a zygote, embryo, or fetus (Table 8.2). 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, heart defects, and other adverse outcomes. 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 that even a small amount poses risks (Nathanson, Jayesinghe, & Roycroft, 2007; O’Brien, 2007).

THE FETAL PERIOD 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).

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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? Let’s move on to infancy and childhood.

Question 1

1. Teratogens

2. Zygotes

3. Genes

4. Chromosomes

Question 2

Question 3

1. embryonic period

2. phenotype

3. germinal period

4. genotype

Question 4