appearance, but he shares his father’s quiet personality. Most of us learn early in life, and take it for granted, that children tend to resemble their parents. But to understand just how parents transmit their inborn characteristics and tendencies to their children, we must consider the human body at a molecular level.
Genetics
The human body is composed of trillions of units called cells. Within each cell is a nucleus that contains 23 matching pairs of rod-shaped structures called chromosomes (Plomin, DeFries, Knopik, & Neiderhiser, 2013). Each chromosome holds the basic units of heredity, known as genes, composed of stretches of deoxyribonucleic acid (DNA), a complex molecule shaped like a twisted ladder or staircase. Genes are the blueprint for creating all of the traits that organisms carry. It is estimated that 20,000 to 25,000 genes reside within the chromosomes and influence all genetic characteristics (Finegold, 2017).
Much of our genetic material is not unique to humans. Every species has a different genome, yet we share genes with all organisms, from bacteria to primates. We share 99% of our DNA with our closest genetic relative, the chimpanzee. There is even less genetic variation among humans. People around the world share 99.7% of their genes (Lewis, 2017). Although all humans share the same basic genome, every person has a slightly different code, making him or her genetically distinct from other humans.
Cell Reproduction
Most cells in the human body reproduce through a process known as mitosis in which DNA replicates itself, permitting the duplication of chromosomes and ultimately the formation of new cells with identical genetic material (Sadler, 2015). The process of mitosis accounts for the replication of all body cells. However, sex cells reproduce in a different way, through meiosis (see Figure 2.1). First, the 46 chromosomes begin to replicate as in mitosis, duplicating themselves. But before the cell completes dividing, a critical process called crossing over takes place. Chromosome pairs align, and DNA segments cross over, moving from one member of the pair to the other. Crossing over creates unique combinations of genes (Sadler, 2015). The cell then continues to divide. As the new cells replicate, they create gametes, containing only 23 single, unpaired chromosomes. Gametes are the cells of sexual reproduction: sperm in males and ova in females. Ova and sperm join at fertilization to produce a fertilized egg, or zygote, with 46 chromosomes, forming 23 pairs with half from the biological mother and half from the biological father. Each gamete has a unique genetic profile. It is estimated that individuals can produce millions of versions of their own chromosomes (National Library of Medicine, 2017).
Sex Determination
The sex chromosomes determine whether a zygote will develop into a male or female. As shown in Figure 2.2, 22 of the 23 pairs of chromosomes are matched; they contain similar genes in almost identical positions and sequence, reflecting the distinct genetic blueprint of the biological mother and father. The 23rd pair are sex chromosomes that specify the biological sex of the individual. In females, sex chromosomes consist of two large X-shaped chromosomes (XX). Males’ sex chromosomes consist of one large X-shaped chromosome and one much smaller Y-shaped chromosome (XY).
Figure 2.1 Meiosis and Mitosis
Figure 2.2 Chromosomes
Source: U.S. National Library of Medicine.
Because females have two X sex chromosomes, all ova contain one X sex chromosome. A male’s sex chromosome pair includes both X and Y chromosomes; therefore, one half of the sperm males produce contain an X chromosome and one half contain a Y. The Y chromosome contains genetic instructions that will cause the fetus to develop male reproductive organs. Thus, whether the fetus develops into a boy or girl is determined by which sperm fertilizes the ovum. If the ovum is fertilized by a Y sperm, a male fetus will develop, and if the ovum is fertilized by an X sperm, a female fetus will form, as shown in Figure 2.3. (The introduction of sex selection methods has become more widely available, and some parents may seek to choose the sex of their child. For more on this topic, see the accompanying feature, Applying Developmental Science: Prenatal Sex Selection.)
Genes Shared by Twins
All biological siblings share the same parents, inheriting chromosomes from each. Despite this genetic similarity, siblings are often quite different from one another. Twins are siblings who share the same womb. Twins occur in about 1 out of every 33 births in the United States (Martin, Hamilton, Osterman, Driscoll, & Drake, 2018).
Figure 2.3 Sex Determination
The majority of naturally conceived twins are dizygotic (DZ) twins, or fraternal twins, conceived when a woman releases more than one ovum and each is fertilized by a different sperm. DZ twins share about one half of their genes, and like other siblings, most fraternal twins differ in appearance, such as hair color, eye color, and height. In about half of fraternal twin pairs, one twin is a boy and the other a girl. DZ twins tend to run in families, suggesting a genetic component that controls the tendency for a woman to release more than one ovum each month. However, rates of DZ twins also increase with in vitro fertilization, maternal age, and each subsequent birth (Pison, Monden, & Smits, 2015).
Applying Developmental Science
Prenatal Sex Selection
Sperm cells can be sorted by whether they carry the X or Y chromosome. Through in vitro fertlization a zygote with the desired sex is created.
Brain light / Alamy Stock Photo
Parents have long shown a preference for giving birth to a girl or boy, depending on circumstances such as cultural or religious traditions, the availability of males or females to perform certain kinds of work important to the family or society, or the sex of the couple’s other children. Yet, throughout human history until recently, the sex of an unborn child was a matter of hope, prayer, and folk rituals. It is only in the past generation that science has made it possible for parents to reliably choose the sex of their unborn child. The introduction of sex selection has been a boon to couples carrying a genetically transmitted disease (i.e., a disease carried on the sex chromosomes), enabling them to have a healthy baby of the sex unaffected by the disease they carried.
There are two methods of sex selection: preconception sperm sorting and preimplantation genetic diagnosis (PGD) (Bhatia, 2018). Preconception sperm sorting involves staining the sperm with a fluorescent dye and then leading them past a laser beam where the difference in DNA content between X- and Y-bearing sperm is visible. PGD creates zygotes within the laboratory by removing eggs from the woman and fertilizing them with sperm. This is known as in vitro (literally, “in glass”) fertilization because fertilization
