A reproductive cell is called a gamete. Every person begins life as a single cell, called a zygote the zygote is the combination of two gametes, one sperm and one ovum. The zygote contains all a person’s genes, which affect every aspect of development lifelong. Many people have misconceptions about heredity, so the beginning of this chapter is crucial to understanding human development.

First, we review some biology. All living things are composed of cells. The work of cells is done by proteins. Each cell manufactures certain proteins according to a code of instructions stored by molecules of deoxyribonucleic acid (DNA) at the heart of the cell. These coding DNA molecules are on a chromosome.

Humans have 23 pairs of chromosomes (46 in all), which contain the instructions to make all the proteins that a person needs for life and growth (see Figure 3.1). The instructions in the 46 chromosomes are organized into genes, with each gene usually at a precise location on a particular chromosome. Humans have about 20,000 genes, each directing the formation of specific proteins, which are made from a string of 20 amino acids.

The instructions for making those amino acids are located on about 3 billion pairs of chemicals, called base pairs, arranged in precise order. A small variation—such as repeated pairs, or one missing pair—changes the gene a tiny bit. Although all humans have most of the same genes with identical codes, those tiny variations of certain genes make each person unique. Each variation of a particular gene is called an allele of that gene.

RNA (ribonucleic acid, another molecule) and additional DNA surround each gene. In a process called methylation, this additional material enhances, transcribes, connects, empowers, silences, and alters genetic instructions. This noncoding material used to be called junk—but no longer. The influences of this surrounding material “alter not the gene itself, but rather the regulatory elements that control the process of gene expression” (Furey & Sethupathy, 2013, p. 705). Methylation continues throughout life, from conception until death.

Obviously, genes are crucial, but even more crucial is whether or not a gene is expressed. The RNA regulates and transcribes genetic instructions, turning some genes and alleles on or off. In other words, a person can have the genetic tendency for a particular trait, disease, or behavior, but that tendency might never appear in that person’s life because it was never turned on. Think of a light switch: A lamp might have a new bulb and an electricity source, but the room stays dark unless the switch is flipped.

69

Some genetic activation occurs because of prenatal RNA, but some occurs because of later factors, biological (such as pollution) and psychological (as with social rejection). The study of exactly how genes change in form and expression is called epigenetics, with epi a Greek root meaning “around, above, below.”

Researchers who sought a single gene for, say, schizophrenia, or homosexuality, or even something quite specific such as memory for math formulas, have been disappointed: No such genes exist. Instead, almost every trait arises from a combination of genes, each with a small potential impact. Then epigenetics is crucial: Events and circumstances surrounding the genes determine whether or not genes are expressed or stay silent (Ayyanathan, 2014).

It is human nature for people to notice differences more than commonalities. All people are the same in many ways, not only with two eyes, hands, and feet, but also with the ability to communicate in language, the capacity to love and hate, and the wish for a meaningful life.

Nonetheless, each of us is distinct in small ways, not only because of our experiences but also because of our genes. Many scientists seek genetic causes for dissimilarities. Differences begin with alleles, some of which reflect transpositions, deletions, or repetitions of those 3 billion base pairs. Some genes are polymorphic (literally, “many forms”) or, more formally, single-nucleotide polymorphisms (abbreviated SNPs, pronounced “snips”). Studying the impact of those SNPs is crucial for understanding why one person is unlike another.

Polymorphic genes can have two, three, or more versions. Most alleles seem inconsequential; some cause minor differences, such as the shape of an eyebrow or the shade of skin; a few rare ones are devastating. Several destructive alleles in combination with specific epigenetic conditions make a person develop schizophrenia, or diabetes, or some other serious problem (Plomin et al., 2013).

70

Some variation springs from the simple fact that it takes two people to make one new person. Each parent contributes half the genetic material. Thus the 23 chromosomes from one parent, with their thousands of genes, match up with the 23 from the other, forming identical or nearly identical pairs. Chromosome 1 from the sperm matches with chromosome 1 from the ovum, chromosome 2 with chromosome 2, and so on through the 22nd pair.

On those chromosomes, each gene from one parent connects with its counterpart from the other parent, and the interaction between the two determines the inherited traits of the future person. Since some alleles from the father differ from the alleles from the mother, their combination produces a zygote unlike either parent. Thus each new person is a product of two parents but unlike either one.

Diversity at conception is increased by another fact: When a man or woman makes sperm or ova, and then their chromosomes pair up when they join, some genetic material is transferred from one chromosome to the other, a phenomena called jumping. As a result, “even if two siblings get the same chromosome from their mother, their chromosomes aren’t identical” (Zimmer, 2009, p. 1254).

All these kinds of genetic diversity help societies, because creativity, prosperity, and even species survival is enhanced when one person is unlike another, although there are benefits when genes are shared as well. There is an optimal level of diversity and similarity: Human societies are close to that level (Ashraf & Galor, 2013).

The entire packet of instructions to make a living organism is called the genome. There is a genome for every species and variety of plant and animal—even for every bacterium and virus. Knowing the genome of homo sapiens, which was decoded in 2001, is only a start at understanding human genetics, since each individual has a slightly different set of those 3 billion base pairs.

The particular member of each chromosome pair from each parent on a given gamete is randomly selected. A man or woman can produce 2²³ different gametes—more than 8 million versions of his or her own 46 chromosomes. If a given couple conceived a billion zygotes, each would be genetically unique because of the chromosomes of the particular sperm that fertilized that particular ovum. Interacting alleles, methylation, and other genetic forces add more variations to the zygote. Epigenetic influences silence some genes and allow expression of others. Nurture creates even more variations.

The genes on the chromosomes constitute the organism’s genetic inheritance, or genotype, which endures throughout life. Growth requires duplication of the code of the original cell again and again.

In 22 of the 23 pairs of chromosomes, both members of the pair are closely matched. As already explained, some of the specific genes have alternate alleles, but at least each of these 44 chromosomes finds its comparable chromosome, making a pair. Those 44 chromosomes are called autosomes, which means that they are independent (auto means “self”) of the sex chromosomes (the 23rd pair).

Each autosome, from number 1 to number 22, contains hundreds of genes in the same positions and sequence. If the code of the gene from one parent is exactly like the code on the same gene from the other parent, the gene pair is homozygous (literally, “same-zygote”).

71

However, the match is not always letter perfect because the mother might have a different allele of a particular gene than the father has. If a gene’s code differs from that of its counterpart, the two genes still pair up, but the zygote (and, later, the person) is heterozygous. This can occur with any of the gene pairs, on any of the autosomes. Which particular homozygous or heterozygous genes my brother and I inherited from our parents was purely a matter of chance, and had no connection to the fact that I am a younger sister, not an older brother.

However, for the 23rd pair of chromosomes, my sex reveals a marked difference between my brother and me. My 23rd pair matched, but his did not. In that he was like all males: When gametes combine to form the zygote, half of the time a dramatic mismatch occurs. In males, the 23rd pair has one X-shaped chromosome and one Y-shaped chromosome. It is called XY. In females, the 23rd pair is composed of two X-shaped chromosomes. Accordingly, it is called XX.

Because a female’s 23rd pair is XX, all of a mother’s ova contain either one X or the other—but always an X. And because a male’s 23rd pair is XY, half of a father’s sperm carry an X chromosome and half a Y. The X chromosome is bigger and has more genes, but the Y chromosome has a crucial gene, called SRY, that directs the embryo to make male hormones and organs. Thus, sex depends on which sperm penetrates the ovum—a Y sperm with the SRY gene, creating a boy (XY), or an X sperm, creating a girl (XX) (see Figure 3.2).

In some other species, sex is not determined at conception. In certain reptiles, for instance, temperature during incubation of the fertilized egg affects the sex of the embryo (Hare & Cree, 2010). However, for humans, the zygote is either XX or XY, a fact that in the past was not revealed to anyone until the baby was born. Pregnant women ate special foods, slept in a particular way, or repeated certain prayers, all in a vain attempt to control the sex of their fetus—who had been male or female at conception.

72

Today, sex can be known much earlier: Parents choose names, decorate nurseries, and bond with the future boy or girl—or reject him or her, as the Opposing Perspectives explains. A zygote’s sex cannot be changed.

74