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