This document provides an overview of Gregor Mendel's experiments with pea plants that laid the foundations for genetics. It discusses how Mendel studied seven traits in pea plants through controlled crosses between pure-breeding lines. His results demonstrated that traits are inherited as discrete units (now called genes or alleles) and showed dominance relationships. Mendel's work established the laws of segregation and independent assortment. Later researchers confirmed Mendel's findings through experiments with pea plants.
Basics of Undergraduate/university fellows
In supplementary gene action, the dominant allele of one gene is essential for the
development of the concerned phenotype, while the other gene modifies the expression of the first gene.
Allelic and Non-allelic interactions : Complete dominance; Incomplete dominance-in Four O'clock plant, Mirabilis jalapa and Snapdragon, Antirrhinum majus ; Co-dominance- MN blood group, AB blood group, Roan coat colour in shorthorn breed of cattle; Inheritance of Comb pattern in Poultry; Epistasis -Dominant - Fruit colour in Summer squash, Recessive - Coat colour in mice; Complementary gene interaction -Purple flower colour in Sweet pea (Lathyrus odoratus)
Basics of Undergraduate/university fellows
In supplementary gene action, the dominant allele of one gene is essential for the
development of the concerned phenotype, while the other gene modifies the expression of the first gene.
Allelic and Non-allelic interactions : Complete dominance; Incomplete dominance-in Four O'clock plant, Mirabilis jalapa and Snapdragon, Antirrhinum majus ; Co-dominance- MN blood group, AB blood group, Roan coat colour in shorthorn breed of cattle; Inheritance of Comb pattern in Poultry; Epistasis -Dominant - Fruit colour in Summer squash, Recessive - Coat colour in mice; Complementary gene interaction -Purple flower colour in Sweet pea (Lathyrus odoratus)
Gene interaction -Complementary, Supplementary,Dominant Epistasis, Recessive...Nethravathi Siri
Most of the characters of living organisms are controlled/ influenced/ governed by a collaboration of several different genes. • Numerous deviations have been recorded in which different kinds of interactions are possible between the genes.
Basics of Undergraduate/university fellows
Complementation between two non-allelic genes (C and P) are essential for production
of a particular or special phenotype i.e., complementary factor.
Two genes involved in a specific pathway and their functional products are required
for gene expression, then one recessive allelic pair at either allelic pair would result in
the mutant phenotype.
When Dominant alleles are present together, they complement each other to yield
complementary factor resulting in a special phenotype.
They are called complementary genes.
When either of gene loci have homozygous recessive alleles (i.e., genotypes of ccPP,
ccPp, CCpp, Ccpp and ccpp), they produce identical phenotypes and change F2 ratio
to 9:7.
We could define Mendel´s laws as the basic laws that talks about the inheritance of biological features that every human being has. They were created by Gregor Johann Mendel in 1865. Mendel created three laws: The law of Segregation, the law of Independent Assortment and the law of Dominance.
The rules of Mendel's inheritance: In a cross between pure contrasting traits, the dominant trait will be observed in the phenotype of the organism whilst the recessive trait will be concealed. Only a single gene copy is allocated in a gamete cell and this is carried out in a random manner.
Mendel’s Procedure: (i) Mendel observed one trait at a time. For example, he crossed tall and dwarf pea plants to study the inheritance of one gene. (ii) He hybridised plants with alternate forms of a single trait (monohybrid cross). The seeds produced by this cross were grown to develop into plants of Fillial 1 progeny or F 1 -generation
Gene interaction -Complementary, Supplementary,Dominant Epistasis, Recessive...Nethravathi Siri
Most of the characters of living organisms are controlled/ influenced/ governed by a collaboration of several different genes. • Numerous deviations have been recorded in which different kinds of interactions are possible between the genes.
Basics of Undergraduate/university fellows
Complementation between two non-allelic genes (C and P) are essential for production
of a particular or special phenotype i.e., complementary factor.
Two genes involved in a specific pathway and their functional products are required
for gene expression, then one recessive allelic pair at either allelic pair would result in
the mutant phenotype.
When Dominant alleles are present together, they complement each other to yield
complementary factor resulting in a special phenotype.
They are called complementary genes.
When either of gene loci have homozygous recessive alleles (i.e., genotypes of ccPP,
ccPp, CCpp, Ccpp and ccpp), they produce identical phenotypes and change F2 ratio
to 9:7.
We could define Mendel´s laws as the basic laws that talks about the inheritance of biological features that every human being has. They were created by Gregor Johann Mendel in 1865. Mendel created three laws: The law of Segregation, the law of Independent Assortment and the law of Dominance.
The rules of Mendel's inheritance: In a cross between pure contrasting traits, the dominant trait will be observed in the phenotype of the organism whilst the recessive trait will be concealed. Only a single gene copy is allocated in a gamete cell and this is carried out in a random manner.
Mendel’s Procedure: (i) Mendel observed one trait at a time. For example, he crossed tall and dwarf pea plants to study the inheritance of one gene. (ii) He hybridised plants with alternate forms of a single trait (monohybrid cross). The seeds produced by this cross were grown to develop into plants of Fillial 1 progeny or F 1 -generation
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KEY CONCEPTS
14.1 Mendel used the scientific approach to identify two laws of inheritance
14.2 Probability laws govern Mendelian inheritance
14.3 Inheritance patterns are often more complex than predicted by simple Mendelian genetics
14.4 Many human traits follow Mendelian patterns of
inheritance
12/15/2011
1
Genetic
Mendle’s law
Patterns of Inheritance
• Mendel’s Laws
• Variations on Mendel’s Laws
• The Chromosomal Basis of Inheritance
• Sex Chromosomes and Sex-linked Genes
12/15/2011
2
The Field of Genetics has Ancient Roots
• Hippocrates (father of medicine): particles from every part of the body travel to eggs
and sperm to be passed on
• Aristotle (philosopher): ‘potential’ rather than particles to produce body features
• 19th century biologists: blending- mom and dad’s traits blend like blue and yellow paint
Hippocrates Aristotle
Experimental Genetics Began in an Abbey Garden
• Modern genetics began in 1860s
• Gregor Mendel (monk in what was then Austria- now Czech Republic)
• Parents pass on discrete, heritable factors (1866)
• Heritable factors retain their individuality for generations (no blending)
• Studied garden peas
12/15/2011
3
• In a typical breeding experiment
– Mendel mated two different, true-breeding varieties, a process called hybridization
• The true-breeding parents
– Are called the P generation
• The hybrid offspring of the P generation
– Are called the F1 generation
– F2 generation comes next
Experimental Genetics Began in an Abbey Garden
• Cross: pollinating a flower of one variety with the pollen of another variety
Mendel chose to work with peas:
– Because they are available in many varieties
– Because he could strictly control which plants mated with which
– Because he could easily start his experiments with varieties that were “true-breeding”
12/15/2011
4
Some genetic vocabulary
Character: a heritable feature, such as flower color
Trait: a variant of a character, such as purple or white flowers
Gene: a discrete unit of hereditary information consisting of a specific
DNA (nucleotide) sequence on a chromosome
Allele: alternative version of a gene
Mendel’s Law of Segregation Describes the Inheritance of a Single Characteristic
Performed monohybrid crosses (only 1 trait differs between the varieties)
• When Mendel crossed contrasting, true-breeding white and purple
flowered pea plants
» All of the offspring were purple!!!
Mendel discovered:
A ratio of about three to one (3:1)
purple to white flowers,
in the F2 generation
When Mendel crossed the F1 plants –
Many of the plants had
purple flowers, but some
had white flowers
12/15/2011
5
Mendel’s Law of Segregation Describes the Inheritance of a Single Characteristic
Mendel developed a hypothesis to explain the 3:1 inheritance pattern that he observed
among the F2 offspring
Four Parts:
1. First, alternative versions of genes account for variations in inherited characters
which are now called alleles
Allele for purple flowers
Locus for flower-color gene
Homologous
pair of
chromosomes
Allele for white flowers
Mendel’s Model for Inheritance
Four Parts:
1. First, .
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2. State of genetics in early
1800’s
What is inherited?
How is it inherited?
What is the role of chance in
heredity?
3. Johann Gregor Mendel
(1822-1884)
Born to simple farmers in the Czeh Republic
1843: Augustinian monastery (Brno)
1851-53: University of Vienna; Physics
Institute
Mathmatics, chemistry, entomology, paleotology, botany,
plant physiology
1856-1863: Pea plant breeding experiments
1866: Published his findings
1900: Three botanists (De Vries, von
Tschermak & Correns) independently
conduct the same experiments, come
across Mendel’s paper and draw attention
to his work.
4. Mendelian genetics
• Mendel’s work
unnoticed until 1900’s
• Introduced concept of
“units of inheritance”
• When correlated with
cytological data →
Transmission
genetics was born
7. Why pea plants?
Easy to grow and hybridize
artificially
• Reproduce well.
•
Each seed is a new individual, can
measure the characteristics of a large
number of offspring after one breeding
season
• Grow to maturity in
single season
8. Mendel’s Approach
• Mendel obtained 34 different
varieties of peas from local
suppliers and examined the
characteristics of each
• He identified 14 strains
representing seven specific traits
each with two forms that could
be easily distinguished. He
spent two years making sure
these varities bred true.
Jos A. Smith
• He worked with these strains for
5 years, determining how each
character was inherited
9.
10. 1900 - Carl Correns, Hugo
deVries, and Erich von
Tschermak rediscover and
confirm Mendel’s laws.
Mendel published
in 1866, was not
appreciated in his
lifetime.
11. Mendel’s Approach Followed the Modern
Scientific Method
1.
Make initial observations about a
phenomenon or process
2.
Formulate a testable hypothesis
3.
Design a controlled experiment
to test the hypothesis
4.
Collect data from the experiment
5.
Interpret the experimental
results, comparing them to those
expected under the hypothesis
6.
Draw a conclusion and
reformulate the hypothesis if
necessary
One of Mendel’s
strengths was his
careful
experimental
design
12. Five Critical Experimental Innovations
• There were five features of
Mendel’s breeding experiments that
were critical to his success
• Controlled crosses
• Use of pure breeding strains
• Selection of dichotomous traits
• Quantification of results
• Use of replicate (repeated), reciprocal,
and test crosses
• Luck?
13. Controlled Crosses Between Plants
• Pea plants are capable
of self-fertilization and
artificial crossfertilization
• Self-fertilization occurs
naturally
• Cross-fertilization
involves removing the
anthers from a flower
and introducing pollen of
the desired type with a
small brush
From Peirce Genetics
14.
15. Pure-Breeding Strains to Begin
Experimental Crosses
• Mendel took 2 years
prior to beginning his
experiments to
establish purebreeding (or truebreeding) strains
• Each experiment began
with crosses between
two pure-breeding
parental generation
plants (P generation)
that produced offspring
called F1 (first filial
generation)
16. Monohybrid Crosses
Monohybrid Cross: a cross-pollination involving two
true-breeding lines that differ for only one trait
“Parental
Female
Male
generation”
Parents:
Smooth
Seeds
“P”
Wrinkled
Seeds
Progeny:
All progeny had smooth seed!
All progeny had same PHENOTYPE:
“the form that is shown”
“F1”
“First Filial
generation”
18. Mendel Made Reciprocal Crosses
Reciprocal Cross: Repeating a particular genetic cross
but with the sexes of the two parents switched
Female
Male
Wrinkled
Seeds
Smooth
Seeds
“P”
All F1 had smooth seed.
“F1”
Conclusion
- Phenotype is not determined by the mother’s phenotype
- The smooth trait is “dominant” to the wrinkled trait
19.
20. • The trait shown by the F1
offspring was called the
dominant phenotype (round
peas, e.g.)
• The other trait not apparent in
the F1 was called the
recessive phenotype
(wrinkled)
• When F1 were crossed, 75%
of the resulting F2 had the
dominant trait, but the
recessive trait reappeared in
the other 25%
21.
22. Alleles
• Mendel’s results rejected
the blending theory of
heredity
• Theorized that plants
carry two discrete
hereditary units for each
trait, alleles; a plant
receives one of these in
the egg and the second
in pollen
• Together the two alleles
for each trait determine
the phenotype of the
individual
Alleles
Phenotype
23. Homozygous and Heterozygous Individuals
Homozygous (TT & tt)
Heterozygous (Tt)
• Pure-breeding individuals, like Mendel’s parent plants, have identical copies of the
two alleles for a trait (homozygous individual)
• The F1 plants had different alleles from each parent and were heterozygous
24. Now that we have a
Now that we have a
theory, we can do
theory, we can do
some real predicting!
some real predicting!
• A 3:1
phenotypic ratio
is predicted for
the F2 produced
by a monohybrid
cross
• A 1:2:1
genotypic ratio
is also predicted
(¼ G/G, ½ G/g,
¼ g/g)
25. Punnett Square
• The alleles (in
gametes) carried by
one parent are
arranged along the
top of the square and
those of the other
parent, down the side
• The results expected
from random fusion
of the gametes are
placed within the
square
Punnett Square
R
r
RR
Rr
Rr
rr
R
r
26. Mendel’s Results Revisited: F1
♀
♂
Genotype?
Genotype?
“AA”
Smooth
Seeds
Wrinkled
Seeds
Gametes possible: “A” or “A”
“aa”
Gametes possible: “a”
or “a”
♀ Gametes
Smooth
“Aa”
A
♂ Gametes
a
a
Aa
Aa
A
Aa
Aa
Use a “Punnett Square” to determine
all possible progeny genotypes
Explains why all progeny were smooth
28. What is the predicted cross of
homozygous recessive red and
heterozygous dominant 20% 20%
brown?
20%
20%
20%
1
ed
lr
Al
re
d
br
o
w
n,
3
re
d
n,
2
br
o
w
n,
1
2
3
br
o
w
lb
Al
re
d
brown
B. 3 brown, 1 red
C. 2 brown, 2 red
D. 1 brown, 3 red
E. All red
ro
w
n
A. All
32. Mendel’s First Law
• Mendel used his theory of particulate inheritance to formulate
the law of segregation (Mendel’s first law)
• Alleles are separated into gametes. Gametes randomly
combine to create progeny in predictable proportions.
• Hypothesis!: Mendel expected that half of the gametes of
heterozygous F1 individuals would carry the dominant allele
and half the recessive
• How can we test this?
Genotype?
Genotype?
35. The Test Cross
• Allows to distinguish genotype of individual
expressing dominant phenotype by crossing it
with homozygous recessive individual
36. What other predictions
can we test?
• Mendel’s hypothesis
predicts that F2 plants with
the dominant phenotype
can be homozygous or
heterozygous
• The heterozygous state
(2/3) is twice as likely as the
homozygous state (1/3)
• HOW? Mendel used a selffertilization experiment to
test the predictions of the
hypothesis
37. F3 generation
• Hypothesis
confirmed:
• 1/3 of plants were
homozygous and
breed true
• 2/3 of heterozygous
F2 plants generated a
3:1 ratio of
dominant:recessive
phenotype among
their progeny
38.
39. WHAT HAPPENS IF WE STUDY
TWO TRAITS?
Will the presence of one charastics affect the prescent of another?
40. Dihybrid-Cross Analysis
of Two Genes
• To study the simultaneous
transmission of two traits,
Mendel made dihybrid
crosses between
organisms that differed for
two traits
• He began each cross with
pure-breeding lines
(e.g., RRGG and rrgg) and
produced F1 that were
heterozygous for both traits
(e.g., RrGg).
• If assortment is random,
four gametes should be
equally likely in the F1 (e.g.,
RG, Rg, rG, rg)
41. 2.3 Dihybrid and
Trihybrid Crosses
• How can we
calculate the
crosses of two or
more traits at the
same time?
• Dihybrid Punnet
Square
• Forked Diagram
42. An Aid to Prediction of Gamete Frequency
• The forked-line diagram is used to determine
gamete genotypes and frequencies
43. Let’s give it a try!
• Self Fertilization of a
heterozygous yellow, round
pea?
• Round (R) is dominant to
wrinkled (r)
• Yellow (G) is dominant to
green (g)
F2 ?
• What does the dihybrid
Punnet square look
like?
• What does the forked
diagram look like?
44.
45. Independent Assortment of Alleles from the
RrGg × RrGg Cross
• Mendel predicted that alleles of each locus unite at
random to produce the F2, generating
• round, yellow
R-G-
(¾)(¾) = 9/16
• round, green
R-gg
(¾)(¼) = 3/16
• wrinkled, yellow
rrG-
(¾)(¼) = 3/16
• wrinkled, green
rrgg
(¼)(¼) = 1/16
9:3:3:1 ratio!
9:3:3:1 ratio!
The dihybrid ratio: 9/16 both dominant traits, 3/16 each for two combinations
of one dominant and one recessive, and 1/16 both recessives
46. Mendel’s Second Law
• The 9:3:3:1 ratios generated in Mendel’s dihybrid crosses
illustrate Mendel’s second law, also known as Mendel’s
law of independent assortment
• The law states that during gamete formation the
segregation of alleles at one locus is independent of the
segregation of alleles at another locus.
• Within the 9:3:3:1 ratio, Mendel recognized two 3:1 ratios
for each trait
47.
48. Testing Independent Assortment by TestCross Analysis
• Mendel wants to test his hypothesis about
independent assortment. HOW?
Test Cross!
• He predicted that the F1 seeds were dihybrid, of
genotype RrGr, and that crossing them to a plant of
genotype rrgg would yield four offspring phenotypes
with equal frequency
49.
50. Testing Independent
Assortment by Trihybrid-Cross
Analysis
• To test his hypothesis about
independent assortment
further, Mendel performed
trihybrid-cross analysis
• The trihybrid cross involved
three traits: round vs.
wrinkled peas, yellow vs.
green peas, and purple vs.
white flowers
• The cross was: RRGGPP ×
rrggpp; the F1 were RrGgPp
51.
52. How many possible combinations?
• Double check yourself! Do you see all the possible
combinations of phenotypes in your answer?
• The number of possibilities can be expressed as 2 n,
where n = number of genes
• In a trihybrid cross, there are 8 possibilties 2 3 = 8!
53. Go try some problems!
• Chapter 2, problem 6