LECTURE 8
Mendel’s
Experiments
(Chapter 2)
2.1 MENDEL’S EXPERIMENTS
• Gregor Johann Mendel (1822-1884) is
considered the father of genetics
• His success can be attributed, in part, to
– His boyhood experience in grafting trees
• This taught him the importance of precision and attention to
detail
– His university experience in physics and natural
history
• This taught him to view the world as an orderly place
governed by natural laws
– These laws can be stated mathematically
• Mendel was an Austrian monk
• He conducted his landmark studies in a
small 115- by 23-foot plot in the garden of
his monastery
• From 1856-1864, he performed thousands
of crosses
• He kept meticulously accurate records that
included quantitative analysis
• His work, entitled “Experiments on Plant
Hybrids” was published in 1866
• It was ignored for 34 years
• Probably because
– The title did not capture the importance of the
work
– Chromosome behavior had not yet been
observed by light microscopy
• In 1900, Mendel’s work was rediscovered
by three botanists working independently
– Hugo de Vries of Holland
– Carl Correns of Germany
– Erich von Tschermak of Austria
Mendel Chose Pea Plants as His
Experimental Organism
• Hybridization
– The mating or crossing between two individuals
that are pure-breeding for specific phenotypes
• Purple-flowered plant X white-flowered plant
• We now know that “pure-breeding” = homozygous,
e.g. PP x pp
• Hybrids
– The offspring that result from such a mating
• These are heterozygous – e.g. Pp
Mendel Chose Pea Plants as His
Experimental Organism
• Mendel chose the garden pea (Pisum sativum)
to study the natural laws governing plants
hybrids
• The garden pea was advantageous because
– 1. It existed in several varieties with distinct
characteristics
– 2. Its structure allowed for easy crosses where
the choice of parental plants could be controlled
Figure 2.2
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Petals
Keel
Sepal
Stigma
Anther
Ovule
Style
Ovary
(a) Structure of a pea flower
Contain the female gametes (eggs)
Contain the pollen grains,
where the male gametes
(sperm) are produced
Mendel Chose Pea Plants as His
Experimental Organism
• Mendel carried out two types of crosses
– 1. Self-fertilization
• Pollen and egg are derived from the same plant
• Naturally occurs in peas because a modified petal
isolates the reproductive structures
– 2. Cross-fertilization
• Pollen and egg are derived from different plants
• Required removing and manipulating anthers
• Refer to Figure 2.3
Figure 2.3
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White
Remove anthers
from purple flower.
Anthers
Parental
generation
Purple
Transfer pollen
from anthers of
white flower to
the stigma of a
purple flower.
Cross-pollinated flower
produces seeds.
Plant the seeds.
Firstgeneration
offspring
Mendel Studied Seven
Characters That Bred True
• The morphological characteristics of an
organism are termed characters
• The term trait describes the specific
properties of a character
– eye color is a character, blue eyes is a trait
• A variety that produces the same trait over
several generations is termed a true-breeder
• The seven characters that Mendel studied are
illustrated in Figure 2.4
Figure 2.4
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CHARACTER
VARIANTS
CHARACTER
VARIANTS
Seed color
Yellow
Green
Round
Wrinkled
Green
Yellow
Smooth
Constricted
Height
Seed shape
Tall
Dwarf
Pod color
Flower color
Purple
White
Pod shape
Flower position
Axial
Terminal
Mendel’s Experiments
• Mendel did not have a hypothesis to explain
the formation of hybrids
– Rather, he believed that a quantitative analysis of
crosses may provide mathematical relationships
that govern hereditary traits
• This is called an empirical approach
– This approach is used to deduce empirical laws
Mendel’s Experiments
• Mendel studied seven characteristics
• Each characteristic showed two variants found
in the same species
– plant height variants were tall and dwarf
• His first experiments involved crossing two
variants of the same characteristic
– This is termed a monohybrid cross
– A single characteristic is being observed
• The experimental procedure is shown in
Figure 2.5
Figure 2.5
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Experimental level
P plants
1. For each of seven characters, Mendel
cross-fertilized two different
true-breeding lines. Keep in mind
that each cross involved two plants
that differed in regard to only one of
the seven characters studied. The
illustration at the right shows one
cross between a tall and dwarf plant.
This is called a P (parental) cross.
2. Collect many seeds. The following
spring, plant the seeds and allow the
plants to grow. These are the plants of
the F1 generation.
Conceptual level
x
Tall
Dwarf
Note: The P
cross produces
seeds that are
part of the F1
generation.
F1 seeds
All Tt
F1 plants
Tt
All
tall
Selffertilization
3. Allow the F1 generation plants to
self-fertilize. This produces seeds that
are part of the F2 generation.
Selffertilization
F2 seeds
4. Collect the seeds and plant them the
following spring to obtain the F2
generation plants.
5. Analyze the characteristics found
in each generation.
TT x tt
F2 plants
Tall
Tall
Dwarf
Tall
TT + 2 Tt + tt
DATA FROM MONOHYBRID CROSSES
P Cross
F1 generation
F2 generation
Ratio
Tall X
dwarf stem
All tall
787 tall,
277 dwarf
2.84:1
Round X
wrinkled seeds
All round
5,474 round,
1,850 wrinkled
2.96:1
Yellow X
Green seeds
All yellow
6,022 yellow,
2,001 green
3.01:1
Purple X
white flowers
All purple
705 purple,
224 white
3.15:1
Axial X
terminal flowers
All axial
651 axial,
207 terminal
3.14:1
Smooth X
constricted pods
All smooth
882 smooth,
229 constricted
2.95:1
Green X
yellow pods
All green
428 green,
152 yellow
2.82:1
Interpreting the Data
• For all seven characteristics studied
– 1. The F1 generation showed only one of the two
parental traits
– 2. The F2 generation showed an ~ 3:1 ratio of the
two parental traits
• These results refuted a “blending mechanism”
of heredity previously proposed
Interpreting the Data
• Indeed, the data suggested a particulate
theory of inheritance
• Mendel postulated the following:
• 1. A pea plant contains two discrete hereditary
factors for a given character, one from each parent
• 2. The two factors may be identical or different
• 3. When the two factors of a single character are
different and present in the same plant
– One variant is dominant and its effect can be seen
– The other variant is recessive and is not seen
• 4. During gamete formation, the paired factors for a
given character segregate randomly so that half of
the gametes receive one factor and half of the
gametes receive the other
– This is Mendel’s Law of Segregation
– Refer to Figure 2.6
• But first, let’s introduce a few terms
– Mendelian factors are now called genes
– Alleles are different versions of the same gene
– An individual with two identical alleles is termed
homozygous
– An individual with two different alleles, is termed
heterozygous
– Genotype refers to the specific allelic composition
of an individual
– Phenotype refers to the outward appearance of an
individual
Figure 2.6
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Tall
Dwarf
x
P generation
TT
tt
Segregation
Gametes
T
t
T
t
Cross-fertilization
Tall
F1 generation
(all tall)
Tt
Segregation
Gametes
F2 generation
Genotypes:
(1 : 2 : 1)
Phenotypes:
(3 : 1)
T
t
T
t
Selffertilization
TT
Tt
Tt
tt
Tall
Tall
Tall
Dwarf
Punnett Squares
• A Punnett square is a grid that enables one to
predict the outcome of simple genetic crosses
– It was proposed by the English geneticist,
Reginald Punnett
• We will illustrate the Punnett square approach
using the cross of heterozygous tall plants as
an example
Punnett Squares
• 1. Write down the genotypes of both parents
– Male parent = Tt
– Female parent = Tt
• 2. Write down the possible gametes each
parent can make.
– Male gametes: T or t
– Female gametes: T or t
• 3. Create an empty Punnett square
Female gametes
Male gametes
T
t
T
t
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4. Fill in the Punnett square with the possible
genotypes of the offspring by combining the
alleles of the gametes
Male gametes
T
Female gametes

T
t
TT
Tt
Tt
tt
t
• 5. Determine the relative proportions of
genotypes and phenotypes of the offspring
– Genotypic ratio
• TT : Tt : tt
• 1 : 2 : 1
– Phenotypic ratio
• Tall : dwarf
•
3 :
1
Mendel’s Experiments
• Mendel also performed dihybrid crosses
– Crossing individual plants that differ in two
characters
• For example
– Character 1 = Seed texture (round vs. wrinkled)
– Character 2 = Seed color (yellow vs. green)
• There are two possible patterns of inheritance
for these characters
– Refer to Figure 2.7
Figure 2.7
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P generation
RRYY
Haploid gametes
rryy
RY
ry
x
rryy
RY
ry
x
RrYy
F1 generation
Haploid gametes
RRYY
1/
2
RY
(a) HYPOTHESIS: Linked assortment
1/
2
ry
RrYy
Haploid gametes
1/
4
RY
1/
4
Ry
1/
4
rY
1/
(b) HYPOTHESIS: Independent assortment
4
ry
Mendel’s Experiments
• The experimental procedure for the dihybrid
cross is shown in Figure 2-8
Figure 2.8
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Experimental level
Conceptual level
True-breeding
True-breeding
round, yellow seed wrinkled, green seed
1. Cross the two true-breeding plants to
each other. This produces F1 generation
seeds.
rryy
RRYY
Seeds are planted
Gametes
formed
RY
ry
x
Crosspollination
2. Collect many seeds and record their
phenotype.
F1 generation
seeds
All RrYy
3. F1 seeds are planted and grown, and the
F1 plants are allowed to self-fertilize.
This produces seeds that are part of the
F2 generation.
RrYy
x
RrYy
RrYy
rY
ry
RRYY RRYy
RrYY
RrYy
RRYy RRyy
RrYy
Rryy
RrYY
RrYy
rrYY
rrYy
RrYy
Rryy
rrYy
rryy
RY
Ry
RY
F2 generation
seeds
RrYy
4. Analyze the characteristics found in the
F2 generation seeds.
Ry
rY
ry
DATA FROM DIHYBRID CROSSES
P Cross
F1 generation F2 generation
Round,
Yellow seeds
X wrinkled,
green seeds
All round,
yellow
315 round, yellow seeds
101 wrinkled, yellow seeds
108 round, green seeds
32 green, wrinkled seeds
Interpreting the Data
• The F2 generation contains seeds with novel
combinations (i.e.: not found in the parentals)
– Round and Green
– Wrinkled and Yellow
• These are called nonparentals
• Their occurrence contradicts the linkage
model
– Refer to Figure 2.7a
• If the genes, on the other hand, assort independently
– Then the predicted phenotypic ratio in the F2 generation
would be 9:3:3:1


P Cross
F1 generation
F2 generation
Round,
Yellow seeds
X wrinkled,
green seeds
All round, yellow 315 round, yellow seeds
101 wrinkled, yellow seeds
108 round, green seeds
32 green, wrinkled seeds
Ratio
9.8
3.2
3.4
1.0
Mendel’s data was very close to segregation expectations
Thus, he proposed the law of Independent assortment
 During gamete formation, the segregation of any pair of
hereditary determinants is independent of the segregation
of other pairs
Figure 2.9
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RY
Ry
rY
ry
Four possible female
RY
gametes:
Ry
rY
ry
Four possible male
gametes:
RRYY RRYy RrYY RrYy RRYy RRyy RrYy
Rryy RrYY RrYy
rrYY rrYy
RrYy
Rryy
rrYy
By randomly combining male and female gametes, 16 combinations are possible.
Totals: 1 RRYY : 2 RRYy : 4 RrYy : 2 RrYY : 1 RRyy : 2 Rryy
Phenotypes:
9 round,
yellow seeds
3 round,
green seeds
: 1 rrYY :
2 rrYy : 1 rryy
3 wrinkled,
yellow seeds
1 wrinkled,
green seed
rryy
• Independent assortment is also revealed by a dihybrid testcross
– TtYy X ttyy
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TY
Ty
tY
ty
TtYy
Ttyy
ttYy
ttyy
ty
Tall, yellow

Tall, green
Dwarf, yellow
Dwarf, green
Thus, if the genes assort independently, the
expected phenotypic ratio among the offspring is
1:1:1:1
Figure 2.10
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Cross: TtYy x TtYy
TY
Ty
tY
ty
TTYY
TTYy
TtYY
TtYy
Tall, yellow
Tall, yellow
Tall, yellow
Tall, yellow
TTYy
TTyy
TtYy
Ttyy
Tall, yellow
Tall, green
Tall, yellow
Tall, green
TtYY
TtYy
ttYY
ttYy
TY
Ty
tY
Tall, yellow
TtYy
Tall, yellow Dwarf, yellow Dwarf, yellow
Ttyy
ttYy
ttyy
ty
Tall, yellow
Tall, green Dwarf, yellow Dwarf, green
Genotypes: 1 TTYY : 2 TTYy : 4 TtYy : 2 TtYY
:
Phenotypes:
9 tall
plants with
yellow seeds
1 TTyy : 2 Ttyy
3 tall
plants with
green seeds
1 ttYY : 2 ttYy
1 ttyy
3 dwarf
1 dwarf
plants with
plant with
yellow seeds green seeds
Linkage Problem
• In unicorns, horn length is under the control
of a single gene where L =long, l = short; horn
color is under the control of a second gene
where G = gold, g = silver
• LlGg is test-crossed to determine whether the
genes assort independently or are linked
• Data 1: 34 long silver horns (Lg)
31 short silver horns (lg)
33 short gold horns (lG
29 long gold horns (LG)
• Data 2: 22 long silver horns (Lg)
62 short silver horns (lg)
18 short gold horns (lG)
58 long gold horns (LG)
• Data 3: 45 long silver horns (Lg)
14 short silver horns (lg)
54 short gold horns (lG)
12 long gold horns (LG)
• Data 4: 5 long silver horns (Lg)
92 short silver horns (lg)
89 short gold horns (lG)
9 long gold horns (LG)