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Modern Genetic Analysis Chapter 1. Genetics and the Organism
Introduction

Why study genetics? There are two basic reasons. First, genetics has come to occupy a pivotal position in the entire subject of biology. For any serious student of plant, animal, or microbial life, an understanding of genetics is thus essential. Second, genetics, like no other scientific discipline, has become central to numerous aspects of human affairs. It touches our humanity in many different ways. Indeed, genetic issues seem to surface daily in our lives, and no thinking person can afford to be ignorant of its discoveries. In this chapter we take an overview of the science of genetics, showing how it has come to occupy its crucial position. In addition we provide a perspective from which to view the subsequent chapters.

First we need to define what genetics is. Some define it as the study of heredity, but hereditary phenomena have been interesting to humans since before the dawn of civilization. Long before biology or genetics existed as the scientific disciplines we know today, ancient peoples were improving plant crops and domesticated animals by selecting desirable individuals for breeding. They also must have puzzled about the inheritance of individuality in humans, and asked such questions as "Why do children resemble their parents?" and "How can various diseases run in families?" However, genetics as a set of principles and analytical procedures did not begin until the 1860s when an Augustinian monk named Gregor Mendel (Figure 1-1) performed a set of experiments that pointed to the existence of biological elements called genes. The word genetics comes from "genes," and genes provide the focus for the subject. Whether geneticists study at the molecular, cellular, organismal, family, population, or evolutionary level, genes are always central in their studies. Simply stated, genetics is the study of genes.

What are genes? Genes are composed of a threadlike double-helical macromolecule called deoxyribonucleic acid, abbreviated DNA. DNA, the hereditary material that passes from one generation to the next, dictates the inherent properties of a species. The information encoded in DNA is in the form of a sequence of chemical subunits called nucleotides. Each cell in an organism typically contains one or two sets of the basic complement of DNA, called a genome. The genome itself is made up of one or more extremely long molecules of DNA that are assembled into structures called chromosomes. Genes are simply the functional units of chromosomal DNA. Each gene not only encodes the structure of some cellular product, but also bears control buttons that determine when, where, and how much of that product is synthesized (Figure 1-2). Most genes encode protein products. Proteins are the most important determinants of the properties of cells and organisms: when you look at an organism, what you see is either a protein or something that has been made by a protein. The way genes encode proteins is indirect and involves several steps. The first step is to copy (transcribe) the information encoded in the DNA of the gene as a related but single-stranded molecule called messenger RNA. Subsequently the information in the messenger RNA is translated (decoded) into a string of amino acids called a polypeptide. The polypeptides, on their own or by aggregating with other polypeptides and cell constituents, form the functional proteins of the cell. Since the properties of cells determine the properties of an organism, there is a direct link between the genes and the structure and function of an organism. In complex organisms chromosomes generally number in the order of tens, but genes number in the order of tens of thousands.

Armed with these definitions of genetics and genes, we can now go on to study and understand how these subjects have become so important. We will start with the impact of genetics on our own species.

© 1999 by W. H. Freeman and Company. All rights reserved.