Genetics and its history with gregor mendel law

1. Dr. Manoj Joshi

2. What is genetics ?
The branch of biology that deals with heredity, especially the mechanism
s of hereditary transmission andthe variation of inherited characteristics
among similar or related organisms.
People have known about inheritance for a long time by .
• Children resemble their parents
• Domestication of animals and plants, selective breeding for
good characteristics
• Sumerian horse breeding records
• Egyptian data palm breeding
• Bible and hemophilia

3. 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.
3. Organisms develop by expressing information carried in their
hereditary material.
4. The environment can’t alter the hereditary material in a directed
fashion.

4. Mid 1800’s Discoveries
 Three major events in the mid-1800’s led directly to the
development of modern genetics.
1. 1859: Charles Darwin publishes The Origin of Species, which
describes the theory of evolution by natural selection. This
theory requires heredity to work.
2. 1866: Gregor Mendel publishes Experiments in Plant
Hybridization, which lays out the basic theory of genetics. It is
widely ignored until 1900.
3. 1871: Friedrich Miescher isolates “nucleic acid” from pus cells.

5. 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.

6.  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.

7. 1953 James Watson
and Francis Crick
publish the double helix
model for DNA’s
chemical structure
1860s Mendel’s
work on peas
allows the
conclusion that
traits are inherited
through discrete
units passed from
one generation to
the next
1870s Friedrich
Miescher describes
nucleic acids
1910 Thomas
Morgan’s work
on fruitflies
demonstrates
that genes lie on
chromosomes
1940s Barbara
McClintock
describes
mobile genetic
elements in
maize
1958 Crick proposes
the ‘central dogma’ for
biological information
flow: that DNA makes
RNA makes protein
1909 The word ‘gene’
coined by Danish botanist
Wilhelm Johannsen
1944 Oswald
Avery shows
in bacteria
that nucleic
acids are the
‘transforming
principle’
1977 Phillip Sharp and
Richard Roberts find
that protein-coding
genes are carried in
segments
2001 initial results
from the Human
Genome Project
published
Genetics – history and key concepts…

8. 3 Laws from the 1860’s

9.  Principles of genetics were developed in the mid
19th century by Gregor Mendel an Austrian
Monk.
 Gregor developed these principles without ANY
scientific equipment - only his mind!
 Gregor experimented with pea plants, by crossing
various strains and observing the characteristics of
their offspring.

10. History cont…
Studied the following characteristics:
Pea color (Green, yellow)
Pea shape (round, wrinkled)
 Pea shape (round, wrinkled)
Flower color (purple, white)
Plant height (tall, short)

11. Law of Dominance
 Every gene has two alleles that can code
for a trait.
 One allele is dominant, meaning it will
always show.
 One allele is recessive, meaning it will
be masked by the presence of the
dominant allele.
 Hybrids will always show the dominant
phenotype.
Ex: PP = purple pp = white
Pp = purple

12. Mendelian Genetics …
TECHNIQUE EXPERIMENT
Mendel crossed two pure strains of pea plants with purple and white flowers and
discovered that the first filial generation were all purple

13. Law of Segregation
 Mendel arrived at this conclusion by performing monohybrid
crosses.
 These were cross-pollination experiments with pea plants
that differed in one trait, for example height of pea plant.
 The alleles for a trait separate when gametes are formed.
These alleles can then randomly united at fertilization.
Anaphase I of Meiosis Anaphase II of Meiosis

14. Mendelian
Genetics –
3:1 ratios …
Traits must come
in discrete units
– now called genes

15. Mendelian Genetics in humans …
1st generation
(grandparents)
2nd generation
(parents, aunts,
and uncles)
3rd generation
(two sisters)
Widow’s peak No widow’s peak
A dominant Mendelian trait
Ww ww
Ww Wwww ww
ww
wwWw
Ww
wwWW
Ww
or
Attached earlobe
1st generation
(grandparents)
2nd generation
(parents, aunts,
and uncles)
3rd generation
(two sisters)
Free earlobe
A recessive Mendelian trait
Ff Ff
Ff Ff Ff
ff Ff
ff ff ff
ff
FF/
or
FF
Ff
Many Mendelian
traits – some cause
disease e.g. cystic fibrosis

16. Law of Independent Assortment
 Mendel began to wonder what would happen if he studied plants that differed
in two traits (dihybrid).
 Data showed that traits produced by dominant factors do not have to appear
together in offspring.
 Ex: A green seed pod (dominant) could appear in a white-flowering pea plant
(recessive).
 In other words, dominant traits don’t have to travel together when traits are
passed from parents to offspring.
 Mendel’s law of independent assortment applies only to traits carried on
different chromosomes, i.e.unlinked genes
 independent assortment occurs as a result of the alignment of homologues
during metaphase I, determining which maternal and paternal chromosomes
assort to each daughter cell
 each pair of alleles separates independently of every other pair of unlinked
alleles during gamete formation

19. Dihybrid Crosses:
a cross that shows the possible offspring for two traits
Fur Color:
B: Black
b: White
Coat Texture:
R: Rough
r: Smooth
BbRr x BbRr
BR bR
br
bR
Br
BR
brBr
Then, find
the possible
genotypes
of the
offspring

20. Dihybrid Crosses:
a cross that shows the possible offspring for two traits
Fur Color:
B: Black
b: White
Coat Texture:
R: Rough
r: Smooth
BbRr x BbRr
BR bR
br
bR
Br
BR
brBr
BBRR BbRR BbRr
BBRr BBrr BbRr Bbrr
BbRR BbRr bbRR bbRr
BbRr Bbrr bbRr bbrr
BBRr

21. BR bR
br
bR
Br
BR
brBr
BBRR BbRR BbRr
BBRr BBrr BbRr Bbrr
BbRR BbRr bbRR bbRr
BbRr Bbrr bbRr bbrr
BBRr
How many of the
offspring would have a
black, rough coat?
How many of the
offspring would have a
black, smooth coat?
How many of the
offspring would have a
white, rough coat?
How many of the
offspring would have a
white, smooth coat? Fur Color:
B: Black
b: White
Coat Texture:
R: Rough
r: Smooth

22. BR bR
br
bR
Br
BR
brBr
BBRR BbRR BbRr
BBRr BBrr BbRr Bbrr
BbRR BbRr bbRR bbRr
BbRr Bbrr bbRr bbrr
BBRr
How many of the
offspring would have
black, rough coat?
How many of the
offspring would have a
black, smooth coat?
How many of the
offspring would have a
white, rough coat?
How many of the
offspring would have a
white, smooth coat?
Fur Color:
B: Black
b: White
Coat Texture:
R: Rough
r: Smooth
Phenotypic Ratio
9:3:3:1

23. More Practice…
 On the back of your notes…
In pea plants, yellow seeds (Y) are dominant over
green seeds (y), and rounded peas (R) are
dominant over wrinkled peas (r).
Cross a plant that is heterozygous for both traits
with a plant that is homozygous recessive for both
traits. Draw a Punnett square to show all possible
offspring, and determine the genotypic and
phenotypic ratios.

24. YyRr X yyrr
YR yR
yr
yr
yr
yr
yrYr
YyRr yyRr yyrr
YyRr Yyrr yyRr yyrr
YyRr Yyrr yyRr yyrr
YyRr Yyrr yyRr yyrr
Yyrr

25. Non-Mendelian inheritance…
Inheritance of traits (phenotypes) is often more complex than predicted by
simple Mendelian genetics
 Polygenic traits - those determined by more than one gene, vary in the
population along a continuum
o The sum of the effects of all the genes that contribute to the
phenotype (e.g. height, skin colour)
 Multifactorial traits - those that depend on the environment as well as the
genotype
o The sum of the effects of all the genes and the environmental
factors that contribute to the phenotype (e.g. height, skin colour)

26. Genetic Vocabulary
 Genetics: The scientific study of heredity
 Genes: Point on a chromosome that controls the trait.
 Allele: Alternate forms of a gene/factor. A or a
 Genotype: combination of alleles an organism has.
(genetic traits)
 Phenotype: How an organism appears. (physical traits)
 Dominant: An allele which is expressed (masks the
other).
 Recessive: An allele which is present but remains
unexpressed (masked)
 Homozygous: Both alleles for a trait are the same.
 Heterozygous: The organism's alleles for a trait are
different.

27. Genetic Vocabulary
 Probability : The mathematical chance that an event
will happen.
 Meiosis :The cell division that produces sex cells.
 Mutation : A change in the type or order of the bases in
an organism DNA: deletion, insertion or substitution.
 Natural Selection : The process by which organisms
with favorable traits survive and reproduce at a higher
rate than organisms without favorable traits.
 Evolution :The process by which population
accumulate inherited changes over time.

28. Punnett Squares
Genetic problems can be easily solved using
a tool called a Punnett square.
Tool for calculating genetic probabilities
A Punnett
square

29. Punnett Squares Vocabulary
 Punnett Squares : The chart used to show the
possible ways genes are combined when passed from
parents to offspring.
 Dominant genes are UPPER CASE (T)
 Recessive genes are lower case (t)
 Each parent has two genes for a trait. (TT), (Tt)
or (tt).
 There can be a possible four combinations for
each cross.

30. Punnett Squares Vocabulary Cont…
GENOTYPE: The gene combination
 Homozygous – Pure dominate or pure recessive
 Heterozygous – Hybrid a mixed
PHEOTYPE :The physical appearance
 Dominate trait
 Recessive trait
 Combination of two traits (incomplete dominance)

31. THANKS
for
kind attention