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.

1. Genetics Chapter 2
Part 1
Dr. Tricia Hardt Smith
hardtta@vcu.edu

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

5. Mendel’s workplace
Fig. 2.5

6. Chapter 2 Opener

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

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

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”

17. Monohybrid Crosses
Female
Male
“P”
Smooth
Seeds
Wrinkled
Seeds
“F1”
Two possible Hypotheses
How could you
differentiate between
these possibilities?
Hypothesis 1: The smooth phenotype is “dominant”
to the wrinkled phenotype
Hypothesis 2: The child’s phenotype is determined by
the mother’s phenotype

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

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%

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

27. Br
A
B
C
D
E
rr

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

29. B
r
r
Br
rr
r
Br
rr
Br
A
B
C
D
E
rr

30. Red + Brown = Blond! (sometimes?)
Not all traits are dominant/reccessive!
More in upcoming lectures!

31. Punnett Square Practice
Problems!
Chapter
2: Problems 2 & 3

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?

34. Conclusion: all
Conclusion: all
F1 plants are
F1 plants are
heterozygous!
heterozygous!

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

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?

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

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

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

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

54. Questions?