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
A COSER LOOK AT GENES AND GENETIC ENGINEERINC ON THE ORIGIN OF SPECIES BY MEANS OF NATURAL SELECTION OR THE PRESERVATION OF FAVOURED RACES IN THE STRUGGLE FOR LIFE By CHARLES DARWIN ML A FELLOW OF THE ROYAL GPOLOGICAL LLssaaS Ire 8oCIETIES AUTHOR or JOURNAL or RESEARCES DURING H 3 S BEAGLE's voY AUM ROUND THE WORLD LON DON JOHN MURRAY ALBEMARLESTREET 1859 The right of Translation is reserved 14
GENETICS THE STUDY OF GENES but fully formed offspring were present in the sperm By the 19th century three theories on heredity prevailed within the scientific community pangenesis acquired character istics and blending inheritance Pangenesis held that every cell produced gemmules particles that embodied the cell s traits and that coalesced in the reproductive organs to form offspring The theory of acquired characteristics stated that traits acquired during an organism s life such as increased muscle from exercise were passed on to off spring Blending inheritance proposed that an organism s heritable characteristics are a blend of those in the parents For example offspring of a tall plant and short plant would be of medium height In his 1859 book on the Origin of Species English biologist Charles Darwin cited blend ing inheritance as a possible explanation for the variation observed in nature Variation in traits played a key role in Darwin's theory of In his book on the Origin of Species Charles Darwin proposed that blending inheritance could be a reason for variations in nature SSPL via Getty Images 15
