A Closer Loo AT GENES AND GENETIC ENGINEERINC a male gamete or sperm combines with a female gamete or egg during fertilization When defects occur in the genetic mate rial diseases known as genetic disorders result The cause can be mutations or altera tions in the DNA sequence of a single gene or defects in an entire chromosome Single gene disorders result from one or more mutations in a single gene that prevent it from acting properly Chromosome disorders occur from structural defects in a chromosome or from having too many or too few chromosomes Down's syndrome for example arises in peo ple with an extra chromosome 21 Genetic engineering involves the artificial manipulation of genes to change an organism Biologists first began experimenting with this process in the 1960s Initially genetic engineering referred to any method used for modifying organisms though heredity and reproduction This included artificial insemi nation and selective breeding In modern science however the term refers specifically to the field of recombinant DNA technol ogy In this process DNA from at least two sources is combined and then inserted into a host organism There the inserted DNA rep licates and functions along with the DNA of the host Io
INTRODUCTION Although some people worry that genetic engineering could have dangerous conse quences others tout its benefits including the creation of safer vaccines This debate will undoubtedly continue Nevertheless genetic engineering will remain an essential focus of genetics and further study will only expand its uses in agriculture medicine and other areas II
CHAPTER 1 GENETICS THE STUDY OF GENES hy do offspring resemble their parents Such resemblances are passed on relatively unal tered from generation to generation through a process called heredity The units of heredity are genes which are made up Children look like their parents because genes are passed on from one generation to the next a process called heredity iStockphoto Thinkstock 12
GENETICS THE STUDY OF GENEs of deoxyribonucleic acid DNA Genes are found on chromosomes which are threadlike parts inside every cell Encoded in every gene are biochemical instruc tions that determine the characteristics or traits of an organism Genetics is the study of genes how they operate and how they are transmitted from parents to offspring EARLr THEORIES OF HEREDITY Even before the beginnings of written history people were aware that certain traits could be passed from parent to offspring By selectively breeding animals or plants humans produced livestock and crops that could provide food pull plows and supply companionship and protection But while farmers and breeders learned to control the transmission of traits in agriculture the actual process of hered ity remained a mystery Many theories were advanced In ancient Greece it was thought that traits were transmitted through the blood and the word blood is still often used to denote ancestry In the 17th century some biologists held that female eggs contained miniature offspring with male sperm merely triggering embryonic development Other biologists proposed the opposite that tiny 13
