Print

CHAPTER 1 Introduction to Genetics Genetics is the study of heredity and variation in organisms. While every person is unique, it is also true that in some ways we resemble our close relatives more than we do a random stranger. This difference is due in part to shared genetic material among family members. Genes or combinations of genes are responsible for many traits, from eye color to whether or not your earlobes are attached, and some differences in genes or chromosomes are associated with particular diseases and conditions. This chapter pro- vides an overview of some basic facts about genetics and inheritance, plus brief overviews of the human genome project, the history of genetics, and the now-discredited field of eugenics. GENES AND CHROMOSOMES The basic unit of heredity is the gene. A gene is a small segment of genetic material occupying a specific location on a chromosome. Human beings have an estimated 20,000ă25,000 genes, and in most cases people have two copies of each gene, one inherited from each parent. A single gene may code for a specific characteristic or multiple characteristics, and multiple genes may combine to code for a single characteristic. Over 99 percent of genes are held in common by all people, with less than 1 percent differing between people. However, these small differences between people are sometimes crucial·for instance, mutations in cer- tain genes (sometimes as small as a change in a single base) may make the difference between a healthy person and a person who suffers from a serious disease.