Genetics An Introduction

Genetics Genetic information directs cellular function, largely determines an organism’s external appearance and serves as a link between generations in every species

What does genetics mean? Genetics is the branch of biology concerned with heredity and variation Involves the study of cells, individuals, their offspring and the populations within which organisms live Geneticists investigate all forms of inherited variation as well as the molecular basis underlying such characteristics

What is the center of heredity in a cell? Eukaryotic organisms the nucleus contains the genetic material in the form of genes present on chromosomes

What is a gene? The gene is a functional unit of heredity residing at a specific point along the chromosome An informational storage unit capable of undergoing replication, expression, and mutation The gene has been found to be a very complex element The length of DNA that specifies a product or action

What is a chromosome? A long, usually circular DNA molecule organized into genes Most eukaryotes have many chromosomes that are composed of linear DNA molecules intimately associated with proteins These contain many nongenic regions

How many chromosomes does an organism have? Members of most eukaryotic species have a specific number of chromosomes in each somatic cell Diploid number (2n) Example: humans have a diploid number of 46 These chromosomes are found to occur in pairs Homologous chromosomes: the members of each pair are identical in their length and in the location of the centromere Also contain the same sequence of gene sites

Chromosomal Theory of Inheritance Inherited traits are controlled by genes that reside in chromosomes, which are faithfully transmitted through gametes to future generations

History of Genetics

Prehistoric Times We don’t know when people first recognized the existence of heredity Various types of archeological evidence provides insight Evidence documents the successful domestication of animals and cultivation of plants thousands of years ago Between 8000 and 1000 BC: horses, camels, oxen, and various breeds of dogs were domesticated to serve various roles 7000-5000 BC: cultivation of plants (maize, wheat, rice, date palm) This supports the hypotheses that our ancestors learned that desirable and undesirable traits are passed to successive generations By influencing their breeding, many desirable varieties of animals and plants could be obtained

The Greek Influence During the Golden Age of Greek culture, philosophers directed much attention to heredity Focus: how heredity relates to the origin of humans and to their reproduction Evidence: writings of Hippocratic school of medicine (500-400 BC) and Aristotle (384-322 BC)

1600-1850: The Dawn of Modern Biology 300BC-1600AD: Theoretical understanding of genetics was not extended by significant new ideas Roman times: plant grafting and animal breeding were common Middle Ages: aware of the impact of heredity on organisms they studied 1600s: William Harvey wrote a treatise on reproduction and development (patterned after Aristotle’s work) He is credited with the earliest statement of the theory of epigenesis An organisms is derived from substances present in the egg, which differentiate into adult structures during embryonic development

1600-1850: Continued The theory of epigenesis as written by Harvey conflicts with the 17th century theory of preformation Sex cells contain a complete miniature adult called homunculus Preformation was popular into the 18th century Casper Wolff and others clearly disproved the preformation theory He was convinced that several structures were not initially present in the earliest embryos he studied, but instead were formed later during development 1808: John Dalton expounded his atomic theory All matter is composed of small, invisible units called atoms

1600-1850: Continued 1830: Matthias Schleiden and Theodor Schwann proposed cell theory All organisms are composed of basic visible units called cells, which derived from similar preexisting structures The fixity of species was also prevalent Animal and plant groups remain unchanged in form from the moment of their appearance on Earth Supported by Carolus Linnaeus Work of Joseph Gottlieb Kolreuter and his work with tobacco

Charles Darwin and Evolution Darwin published in 1859 the book On the Origin of Species Statement of his evolutionary theory Observations convinced him that existing species arose by descent with modification from other ancestral species Led to his formulation of the theory of natural selection 1868: published Variations in Animals and Plants under Domestication Attempted to provide a more definitive explanation of how heritable variation arises gradually over time

Mendel As Darwin’s work ensued, Gregor Johann Mendel conducted his experiment between 1856 and 1863 Published in 1866 Demonstrated a number of statistical patterns underlying inheritance and developed a theory involving hereditary factors in the germ cells to explain these patterns Early part of the 20th century: chromosomes were discovered and support for the epigenetic interpretation of development had grown considerably