It deals with the topics to be covered in the microbial genetics and scietists contributions to the field.

1. Introduction to Microbial Genetics
DR. YALAVARTHI NAGARAJU

3. History of Genetics
Check what you know
 Children resemble their parents
 Domestication of animals and plants
 Selective breeding for good characteristics
 Ability to indentify a person as a member of a particular family by certain physical
traits

4. Old Ideas
Despite knowing about inheritance in general, a number of incorrect ideas had to be
generated and overcome before modern genetics could arise.
1. All life comes from other life: Living organisms are not spontaneously generated
from non-living material. Big exception: origin of life.
2. Species concept: offspring arise only when two members of the same species mate.
Monstrous hybrids don’t exist.

5. More Old Ideas
3. Organisms develop by expressing information carried in their hereditary material. As
opposed to “preformation”, the idea that in each sperm (or egg) is a tiny, fully-
formed human that merely grows in size.
4. The environment can’t alter the hereditary material in a directed fashion. There is no
“inheritance of acquired characteristics”. Mutations are random events.

6. More Old Ideas
5. Male and female parents contribute equally to the offspring.
 ancient Greek idea: male plants a “seed” in the female “garden”.
 alleged New Guinea belief: sex is not related to reproduction.

7. Different Old Theories explained the Similarities and
Dissimilarities between Individuals
 Blending theory
The mixture of sperm and egg resulted in progeny that were a “blend” of two parent’s
characteristics.
 Acquired characters inheritance (Jean Baptiste Lamarck)
Individuals inherit traits are strengthened by their parents
 Pangenesis (Charles Darwin)
The cells excreted gemmules then collected and concentrated in the reproductive organ.
Fathers and mother gemmules blended to form an embryo

8. Different Old Theories explained the Similarities and
Dissimilarities between Individuals
 Preformation and Epigenesis
Organism develop by expressing information carried in their heredity material
 Cell Theory (Hooke, Leuwenhoek, Schleiden, Schwann, Virchow)
1. All living things are composed of one or more cells
2. Cells are the basic units of structure and function in living things (Schleiden and
Schwann, 1839)
3. New cells are produced from existing cells (Virchow, 1858)

9. Mid 1800’s Discoveries
 Three 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 natural selection. This theory requires heredity to work.
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.

10. Gregor Mendel (1822-1884)
 Systematically recorded results of crosses
 Theorized on nature of hereditary material
 Postulate mechanism of transfer of "Elementen"
governing traits

11. Mendel’s Work with Peas
a. He selected strains that differed in particular
traits (e.g., smooth or wrinkled seeds, purple
or white flowers)
b. After making genetic crosses, he counted the
appearance of traits in the progeny and
analyzed the results mathematically.
c. He concluded that each organism contains two
copies of each gene, one from each parent,
and that alternative versions of the genes
(alleles) exist

12. 4. He deduced that the factors (now called genes) segregate randomly into gametes
(Mendel’s first law, the Principle of Segregation).
5. The two factors for a particular trait assort independently of factors controlling other traits
(Mendel’s second law, the Principle of Independent Assortment).
6. An example is seed color in peas:
i. True-breeding plants with yellow seeds (YY) are crossed with true-breeding plants with
green seeds (yy).
ii. The progeny (F1) have yellow seeds, and a heterozygous genotype (Yy).
iii. When the progeny self-pollinate, the F2 contains 3 yellow:1 green, with genotypic
ratios of 1 YY : 2 Yy : 1 yy.
Mendel’s Work with Peas

13. Major Events in the 20th Century
 1900: rediscovery of Mendel’s work by Robert Correns, Hugo de Vries, and Erich von
Tschermak.
 1902: Archibald Garrod discovers that alkaptonuria, a human disease, has a genetic basis.
 1904: Gregory Bateson discovers linkage between genes. Also coins the word
“genetics”.
 1910: Thomas Hunt Morgan proves that genes are located on the chromosomes (using
Drosophila).
 1918: R. A. Fisher begins the study of quantitative genetics by partitioning phenotypic
variance into a genetic and an environmental component.

14. Thomas Hunt Morgan
 Thomas Hunt Morgan: early 1900’s
 Worked at Columbia University; later at CalTech
 Studied fruit fly eye color, determining that trait was sex-linked
 Won the Nobel Prize in 1933 for his work on chromosomes and
genetics
 By this point, it was known that genetic material was located on
a chromosome
 This genetic material was in discrete units called genes
 It was NOT known whether the gene was simply a protein, or
whether it was composed of DNA

15. More 20th Century Events
 1926: Hermann J. Muller shows that X-rays induce mutations.
 1944: Oswald Avery, Colin MacLeod and Maclyn McCarty show that DNA can transform
bacteria, demonstrating that DNA is the hereditary material.
 1953: James Watson and Francis Crick determine the structure of the DNA molecule, which
leads directly to knowledge of how it replicates
 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.
 2001: Sequence of the entire human genome is announced.

16. James Watson and Francis Crick

17. James Watson and Francis Crick
 Used wire models to conform with the measurements that
Franklin and Wilkins had come up with
 Determined the structure to be a double helix
 Lead to understanding of mutation and relationship between
DNA and proteins at a molecular level
 1959 – “Central Dogma”
 DNARNAprotein

18. Central Dogma of Biology