The Genetic Code

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 (sperm is an abbreviation of spermatozoon, plural spermatozoa, from the Greek word for seed) and one ovum (plural, ova, from the Greek for egg). 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.

The Moment of Conception This ovum is about to become a zygote. It has been penetrated by a single sperm, whose nucleus now lies next to the nucleus of the ovum. Soon, the two nuclei will fuse, bringing together about 20,000 genes to guide development.

LENNART NILSSON/SCANPIX

What Genes Are

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 instructions stored by molecules of deoxyribonucleic acid (DNA) at the heart of each 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 located at a specific spot on a particular chromosome. Humans have between 18,000 and 23,000 genes, and each gene directs the formation of specific proteins made from a string of 20 amino acids.

How Proteins Are Made The genes on the chromosomes in the nucleus of each cell instruct the cell to manufacture the proteins needed to sustain life and development. The code for a protein is the particular combination of four bases, T-A-G-C (thymine, adenine, guanine, and cytosine).

The instructions for making those amino acids are 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 (Shapiro, 2009). This nongenetic material used to be called junk—but no longer. Thousands of scientists are now seeking to discover exactly what these molecules do (Wright & Bruford, 2011; Volders et al., 2013). We do know that methylation continues throughout life, and it can alter a gene’s expression, not only prenatally but after birth as well. This is part of epigenetics, mentioned in Chapter 1 as well as later in this chapter.

Variations

Twelve of 3 Billion Pairs This is a computer illustration of a small segment of one gene. Even a small difference in one gene can cause major changes in a person’s phenotype.

HYBRID MEDICAL ANIMATION/SCIENCE SOURCE

It is human nature for people to notice differences more than commonalities. Hence many scientists seek genetic reasons for the fact that no two people look or act exactly alike. Differences begin with alleles, which can be caused by transpositions, deletions, or repetitions of those base pairs, making some genes polymorphic (literally, “many forms”) or, more formally, single-nucleotide polymorphisms (abbreviated SNPs, pronounced “snips”).

Usually genes have only one set of instructions (no alleles), but polymorphic genes can have two, three, or more versions. Most alleles seem inconsequential; some cause only minor differences (such as the shape of an eyebrow or the shade of the skin); and a few are notable, even devastating, but the devastating ones are rare. Several destructive alleles in combination with specific epigenetic conditions make a person develop schizophrenia, or diabetes, or whatever (Plomin et al., 2012).

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

Each gene pairs with a counterpart gene from the other parent, and the interaction between the two members of each pair determines the inherited traits of the future person. Since the alleles from the father often differ from the alleles from the mother, their combination may be different from either parent. Thus each new person is a product of both 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, so that “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 and even species survival is enhanced when one person is unlike another, although there are benefits when genes are shared as well (Ashraf & Galor, 2011).

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 bacteria and virus. Knowing the genome of the human species is only a start (it was decoded in 2001) at understanding the genetics of people, since each individual has a slightly different set of those 3 billion base pairs.