Dr. P. Saranraj M.Sc., M.Phil., Ph.D., NET
Department of Microbiology
Sacred Heart College (Autonomous)
Tirupattur, Vellore District.
Genetics is the study of genes, heredity and genetic
variations in living organisms.
Genes (Wilhelm Johannensen, 1909) – A hereditary factor that
constitutes a single unit of hereditary material. It corresponds to a segment of DNA
that codes for the synthesis of a single polypeptide chain.
Heredity - The passing of phenotypic characters from parents to their
offspring, either through asexual reproduction or sexual reproduction.
Genetic variation - It refer the differences between individuals or
differences between populations.
The term “Genetics” was first proposed by the English biologist
William Bateson (1861 - 1926) in 1909.
Genetics is relevant to all fields of biological disciplines includes
Medicine, Agriculture, Anthropology, Biochemistry,
Microbiology, Biotechnology, Ecology, Physiology and
3. Mid 1800’s Discoveries
Four major events in the mid-1800’s led directly to
the development of Modern Genetics.
1859: Charles Darwin publishes “The Origin of
Species” which describes the “Theory of Evolution” by
1866: Gregor Mendel publishes experiments in “Plant
Hybridization” which lays out the basic theory of genetics.
It is widely ignored until 1900.
1871: Friedrich Miescher isolates “Nucleic acid”
from pus cells.
1888: Waldeyer coined the term “Chromosomes”
4. Major Events in the 20th
1900: Rediscovery of Mendel’s work by Robert Correns,
Hugo de Vries, and Erich von Tschermak.
1901: Hugo DeVries coined the term “Mutation”
1902: Archibald Garrod discovers that “Alkaptonuria (Black
urine and Black bone caused by mutation in HDG Gene)”, a
human disease, has a genetic basis.
1909: The term “Genetics” was first proposed by the English
biologist William Bateson.
1909: The term “Gene” was first introduced by Wilhelm
1910: Genetics became an independent scientific field in 1910
with the study of the fruit fly (Drosophila melanogaster) by
Thomas Hunt Morgan at Columbia University in New York.
1918: R. A. Fisher begins the study of quantitative genetics by
partitioning phenotypic variance into a genetic and an
5. 1926: Hermann J. Muller determined the “Spontaneous
Mutation” in Drosophila and the mutation can be induced by
Roengten rays (X rays).
1928: F. Griffith proposed the “Transforming Principle” which
shows “DNA – as a Genetic material”.
1940: Lederberg and Tatum demonstrated the “Genetic
Recombination” in Bacteria.
1941: C. Auerbach and J.M. Robson observed the
mutation inducing chemicals.
1941: Beadle and Tatum proposed “One Gene One Enzyme
Theory” (One gene is responsible for the formation of one
1944: Oswald Avery, Colin MacLeod and Maclyn
McCarty show that DNA can transform bacteria, demonstrating
that DNA is the hereditary material.
6. 1947: Delbruck and Bailey demonstrated the
“Genetic Recombination” in virus.
1952: Hershey and Chase proved that genetic
information is transferred by DNA alone in
1953: James Watson and Francis Crick determine
the structure of the DNA molecule, which leads directly to
knowledge of how it replicates.
1955: F. Sanger determined the sequence of amino
acid in Insulin.
1966: Marshall Nirenberg solves the Genetic code,
showing that 3 DNA bases code for one amino acid.
1972: Stanley Cohen and Herbert Boyer combine
DNA from two different species in vitro, then transform it
into bacterial cells: first DNA cloning.
1977: Maxam – Gilbert developed DNA Sequencing.
2001: Sequence of the entire human genome is
8. PREFORMATION THEORY
The “Theory of Preformation” believes that the
organism is already present, i.e., preformed in the sperm or
egg in a miniature form called Homunculus.
Sperms were observed for the first time by Leeuwenhoek,
Preformation theory was given by two Ditch Biologist
Swammerdam and Bonnet (1679).
It was believed by a number of workers of that period like
Hartosoeker (1694) and Dalepatius (1694).
It was supported by Roux as late as 1888 but rejected by
many scientists because it was not scientifically proved.
9. THEORY OF EPIGENESIS
Proposed by a German biologist Wolff (1738-1794).
Epigenesis is the process by which plants, animals and
fungi develop from a seed, spore or egg through a
sequence of steps in which cells differentiate and organs
Also called as “Neoformationism”
The theory states that the egg and sperms are
The differentiation into various organs/parts takes place
only after fertilization in the zygote resulting in the
development of adult tissues and organs.
This concept is universally accepted.
10. INHERITANCE OF ACQUIRED
The Inheritance of acquired characteristics is
a hypothesis that physiological changes acquired over
the life of an organism (such as the enlargement of a
muscle through repeated use) may be transmitted to
It is also commonly referred to as the Theory of
Adaptation equated with the Evolutionary theory
of French Biologist Naturalist Jean – Baptiste Lamark
known as “Lamarckism”.
11. GERMPLASM THEORY
Germ plasm or Polar plasm is a zone found in the cytoplasm
of the egg cells of some organisms.
Germ plasm Theory states that the body of multicellular
organisms are formed of two type of cells, namely “Somatic
cells” and “Germ cells”.
The Germ plasm Theory was proposed by August Wisemann in
The main bulk of the body is formed of Somatic cells. The
Somatic cells disappear with the death of the animal. So any
changes affecting these cell is not heritable.
The cytoplasm of the Somatic cell is called Somatoplasm.
12. The reproductive cells (Gametes) are called “Germ
The Germ Cells are carried to the descendants,
generation after generation. So, any change affecting these
cells is inherited.
14. COMMON TERMINOLOGIES IN GENETICS
Trait – A distinguishing quality of characteristics, typically one
belonging to a person. In science, the word Trait refers to a
characteristic that is caused by Genetics.
Dominant – Traits that are expressed.
Recessive – Traits that are covered up.
Alleles – Different forms of characteristics.
Homozygous – Two same Alleles.
Heterozygous - Two different Alleles.
Probability – Chances or percentages that something will occur.
16. GREGOR MENDEL
Father of Genetics
Conducted his experiment with Pea (Pisum sativum L.)
They were easily available
They reproduce quickly
They show obvious difference in the traits.
Understood that something that carried traits from
one generation to the next – Factor.
17. MENDEL’S EXPERIMENT WITH PEA PLANT
(Pisum sataivum L.)
Gregor Mendel was one of the first to apply an experimental
approach to the question of “inheritance”.
For seven years, Mendel bred pea plants and recorded
inheritance patterns in the offspring.
Particulate Hypothesis of Inheritance - Characters of
parents pass on to their offspring separate and distinct factors
(today called genes) that are responsible for inherited traits.
Mendel probably chose to work with peas because they are
available in many varieties.
24. LAW OF INDEPENDENT ASSORTMENT
The principles that govern heredity were discovered by a monk named
Gregor Mendel in the 1860's. One of these principles, now called
“Mendel’s Law of Independent Assortment”.
It is also called as “Mendel’s Second Law” .
Mendel’s Law of Independent Assortment states that allele pairs
separate independently during the formation of gametes. This means that
traits are transmitted to offspring independently of one another.
Mendel formulated this principle after performing Dihybrid
crosses between plants that differed in two traits, such as seed color and
pod color. After these plants were allowed to self pollinate, he noticed
that the same ratio of 9:3:3:1 appeared among the offspring. Mendel
concluded that traits are transmitted to offspring independently.
26. The image shows a true-breeding plant with the dominant
traits of green pod color (GG) and yellow seed
color (YY) being cross-pollinated with a true breeding
plant with yellow pod color (gg) and green seeds (yy).
The resulting offspring are all heterozygous for green pod
color and yellow seeds (GgYy).
If the offspring are allowed to self pollinate, a 9:3:3:1
ratio will be seen in the next generation.
27. MONOHYBRID CROSS
It is a cross between two pure organisms in order to study
the inheritance of a single pair of alleles.
It produces a phenotypic mohohybrid ratio of 3: 1 in
It produces genotypic ratio of 1: 2: 1 in F2.
28. DIHYBRID CROSS
It is a cross between two pure organisms of a species in order
to study the inheritance of two pairs of alleles.
It produces a phenotypic di-hybrid ratio of 9: 3: 3: 1 in
It produces genotypic ratio of 1 : 2 : 1 : 2 : 4: 2 : 1 : 2.
29. BACK CROSS AND TEST
Back cross is the mating process between progeny and
dominant or recessive parent.
Test cross is the mating process between the progeny and
the recessive parent.
A back cross is the breeding of an F1 back to a homozygous
individual (either dominant or recessive).
A test cross involves breeding of a homozygous recessive
to the isolated trait.
30. GENOTYPE AND PHENOTYPE
Definition The genetic makeup of
an individual. Refers
to the information
contained on two
alleles in the cell.
of the genotype. An
Examples DNA and
Hair colour, Eye
colour, Weight and
Inheritance Inherited from parents
to young ones
Cannot be inherited
31. Genotype Phenotype
Depends upon The hereditary
information that was
given to an individual by
Genotype and the
influence of the
Contains All the hereditary
information of an
individual, even if those
genes are not
Expressed genes only
Determined by Genotyping – using a
biological assay, such as
PCR, to find out what
genes are on an allele.
(Inside the body)
Observation of the
individual (Outside the
32. ALLELE AND GENE
“Allele” refers to “A specific variation of a gene” (Example -
Blue eyes, Green eyes, Type A blood, Black skin and White
“Allele” also refers to “Different forms of characteristics”.
“Gene” refers to “A section of DNA that controls a certain trait”
(Example - Eye color, Blood type and Skin color).
Trait - A distinguishing quality of characteristics, typically one
belonging to a person.
33. PURE LINE
Pure line is a self pollinated descendent of a self
Pure line varieties are Homozygous (individual having two of
the same allele, whether dominant or recessive) and
Homogeneous (same or similar kind or nature) as they are
genetically similar and true breeding.
Pure line selection is a method in which new variety is
developed by selection of single best plant progeny
among traditional varieties.
Pure line selection method has many advantages. At the same
time it has disadvantages too.
34. Advantages of Pure Line Selection
Easy and cheap method of crop improvement
Rapid method, lines are usually genetically fixed and yield trials
can be immediately conducted.
Plants are uniform in performance and appearance.
Maximum possible improvement over the original variety
can be achieved.
Useful in improving low heritability traits.
Gives out uniformity in maturity, height etc.
35. Disadvantages of Pure Line Selection
Pure lines have poor adaptability due to narrow genetic base.
Superior genotypes can only be isolated from the mixed
population. This selection is powerless to bring changes in
hereditary factors i.e. to develop new genotype.
Mostly popular or in fact limited to self pollinated species
Time and space consuming.
More expensive yield trials.
36. INBRED LINE
Inbred strains (also called inbred lines [Plants] or linear
animals [Anmials]) are individuals of a particular species which are
nearly identical to each other in genotype due to
long inbreeding (production of offspring from the mating).
Inbred strains of animals are frequently used in laboratories
for experiments where for reproducibility of conclusions all the test
animals should be as similar as possible.
Certain plants including the genetic model
organism Arabidopsis thaliana naturally self pollinates, which
makes it quite easy to create inbred strains in the laboratory.
The best-known strains of laboratory animals are: (i) Rats & Mice
and (ii)Guinea pigs.
37. Inbreeding animals will sometimes lead to unwanted Genetic
drift or Allergic drift. The continuous overlaying of like
genetics exposes recessive gene patterns that often lead to
changes in reproduction performance, fitness, and
ability to survive. A decrease in these areas is known
as Inbreeding depression.
38. NORMS OF REACTION
The pattern of phenotypes produced by a given genotype
under different environmental conditions.
Norms of Reaction is also called as Reaction Norms.
One use of reaction norms is in describing how different
species especially related species respond to varying
The concept was introduced by Richard Woltereck.
changes in the phenotypic manifestation of genes was
For example, in the Chinese primrose the
color of the flowers varies from white at a temperature of
30 °C to pink at 20°C.