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Mendel and the Gene Idea
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What genetic principles account for the
transmission of traits from parents to offspring?
One possible explanation of heredity is a
“blending” hypothesis - The idea that genetic
material contributed by two parents mixes in a
manner analogous to the way blue and yellow
paints blend to make green.
An alternative to the blending model is the
“particulate” hypothesis of inheritance: the gene
idea - Parents pass on discrete heritable units,
genes.
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Documented a particulate mechanism of inheritance
through his experiments with garden peas.
Gregor Mendel’s
monastery garden.
Fig. 2.2
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Variation is widespread in nature
Observable variation is essential for
following genes
 Variation is inherited according to
genetic laws and not solely by chance
 Mendel’s laws apply to all sexually
reproducing organisms
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Mendel used the scientific approach to identify two laws of
inheritance
Mendel discovered the basic principles of heredity by breeding
garden peas in carefully planned experiments
Mendel chose to work with the garden pea (Pisum sativum)
 Because they are available in many varieties,
 easy to grow,
 easy to get large numbers
 Because he could strictly control which plants mated with
which
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1
Removed stamens
from purple flower
2
Transferred spermbearing pollen from
stamens of white
flower to eggbearing carpel of
purple flower
Parental
generation
(P)
3
Pollinated carpel
matured into pod
Carpel
(female)
Stamens
(male)
4
Planted seeds
from pod
5 Examined
First
generation
offspring
(F1)
offspring:
all purple
flowers
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Statistical analyses:
 Worked with large numbers of plants
 counted all offspring
 made predictions and tested them
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Excellent experimentalist
 controlled growth conditions
 focused on traits that were easy to score
 chose to track only those characters that varied in
an “either-or” manner
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Character: a heritable feature, such as
flower color
 Trait: a variant of a character, such as purple
or white flowers
 Each trait carries two copies of a unit of
inheritance, one inherited from the mother
and the other from the father
 Alternative forms of traits are called alleles.
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Dominant
Recessive
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Mendel also made sure that he started his experiments with
varieties that were “true-breeding”.
X
X
X
X
X
X
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Phenotype – observable characteristic of an organism
Genotype – pair of alleles present in and individual
Homozygous – two alleles of trait are the same (YY or
yy)
Heterozygous – two alleles of trait are different (Yy)
Capitalized traits = dominant phenotypes
Lowercase traits = recessive phenotypes
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Generations:
 P = parental generation
 F1 = 1st filial generation, progeny of the P generation
 F2 = 2nd filial generation, progeny of the F1 generation
(F3 and so on)
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Crosses:
 Monohybrid cross = cross of two different true-
breeding strains (homozygotes) that differ in a single
trait.
 Dihybrid cross = cross of two different true-breeding
strains (homozygotes) that differ in two traits.
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Phenotype
Purple
3
Purple
Genotype
PP
(homozygous)
1
Pp
(heterozygous)
2
Pp
(heterozygous)
Purple
White
1
Figure 14.6
Ratio 3:1
pp
(homozygous)
1
Ratio 1:2:1
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Dominant & recessive alleles (Fig. 10.7):
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In a typical breeding experiment Mendel mated
two contrasting, 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
When F1 individuals self-pollinate the F2
generation is produced
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When Mendel crossed contrasting, truebreeding white and purple flowered pea
plants all of the offspring were purple
 When Mendel crossed the F1 plants, many
of the plants had purple flowers, but some
had white flowers
 A ratio of about three to one, purple to
white flowers, in the F2 generation
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P Generation
(true-breeding
parents)
Purple
flowers
White
flowers
F1 Generation
(hybrids)
All plants had
purple flowers
F2 Generation
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In the F1 plants, only the purple trait was
affecting flower color in these hybrids
Purple flower color was dominant, and white
flower color was recessive
Mendel developed a hypothesis to explain the
3:1 inheritance pattern that he observed among
the F2 offspring
There are four related concepts that are integral
to this hypothesis
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1. 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
2. For each character an organism inherits two alleles, one from each parent, A
genetic locus is actually represented twice
3. If the two alleles at a locus differ, the dominant allele determines the
organism’s appearance
4. The law of segregation - the two alleles for a heritable character separate
(segregate) during gamete formation and end up in different gametes
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Mechanism of gene transmission
Gametogenesis:
alleles segregate
Fertilization:
alleles unite
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 Classical Punett's Square is a way to determine ways traits can
segregate
Parental P0 cross
P
P
p
p
F1 cross
P
p
P
p
Determine the genotype and phenotype
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 Classical Punett's Square is a way to determine ways traits can
segregate
Parental P cross
0
P
Pp
Pp
p
p
P
Pp
Pp
F1 cross
P
p
P
p
Determine the genotype and phenotype
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 Classical Punett's Square is a way to determine ways traits can segregate
Parental P0 cross
p
p
P
p
P
Pp
Pp
F1 cross
P
PP
Pp
P
Pp
Pp
p
Pp
pp
Determine the genotype and phenotype
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
P Generation
Appearance:
Genetic makeup:
Purple flowers
PP
Gametes:
White flowers
pp
p
P
F1 Generation
Appearance:
Genetic makeup:
Gametes:
Purple flowers
Pp
1/
2
1/
P
2
P
p
PP
Pp
p
F2 Generation
P
F1 eggs
•p
pp
Pp
3
:1
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White
(pp)
Purple
(Pp)
p
Gametes
p
Purple
(PP)
P
Gametes
p
Purple
(Pp)
P
Gametes
P
Pp
Pp
Pp
Pp
P
F1 generation
All purple
Gametes
p
PP
Pp
Pp
pp
F2 generation
¾ purple, ¼ white
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Punnett square
 F1 genotypes: 4/4
Ss
 F1 phenotypes:
4/4 smooth
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In pea plants with purple flowers the genotype is
not immediately obvious
A testcross
1. Allows us to determine the genotype of an
organism with the dominant phenotype, but
unknown genotype
2. Crosses an individual with the dominant
phenotype with an individual that is
homozygous recessive for a trait
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To determine whether an individual with a dominant phenotype is homozygous
for the dominant allele or heterozygous, Mendel crossed the individual in
question with an individual that had the recessive phenotype:
Alternative 1 – Plant with
dominant phenotype is
homozygous
Dominant
Phenotype
PP
?
Alternative 2 – Plant with
dominant phenotype is
heterozygous
?
Dominant
Phenotype
Pp
Gametes
Recessive
phenotype
P
p
pp
Gametes
P
P
Recessive
phenotype
p
Gametes
p
p
pp
Gametes
p
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To determine whether an individual with a dominant phenotype is
homozygous for the dominant allele or heterozygous, Mendel
crossed the individual in question with an individual that had the
recessive phenotype:
Alternative 1 – Plant with
dominant phenotype is
homozygous
Dominant
Phenotype
PP
?
Alternative 2 – Plant with
dominant phenotype is
heterozygous
?
Dominant
Phenotype
Pp
Gametes
Recessive
phenotype
pp
P
P
p
Pp
Pp
p
Pp
Pp
Gametes
Recessive
phenotype
Gametes
If all offspring are purple;
unknown plant is
homozygous.
pp
Gametes
P
p
p
Pp
pp
p
Pp
pp
If half of offspring are
white; unknown plant
is heterozygous.
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APPLICATION An organism that exhibits a dominant trait,
such as purple flowers in pea plants, can be either homozygous for
the dominant allele or heterozygous. To determine the organism’s
genotype, geneticists can perform a testcross.
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TECHNIQUE In a testcross, the individual with the
unknown genotype is crossed with a homozygous individual
expressing the recessive trait (white flowers in this example).
By observing the phenotypes of the offspring resulting from this
cross, we can deduce the genotype of the purple-flowered
parent.
Dominant phenotype,
unknown genotype:
PP or Pp?
Recessive phenotype,
known genotype:
pp
If PP,
then all offspring
purple:
If Pp,
then 2 offspring purple
and 1⁄2 offspring white:
p
1⁄
p
p
p
Pp
Pp
pp
pp
RESULTS
P
P
Pp
Pp
P
p
Pp
Pp
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Mendel derived the law of segregation by following a single trait
 2 alleles at a single gene locus segregate when the gametes are
formed
 The F1 offspring produced in this cross were monohybrids,
heterozygous for one character
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Mendel identified his second law of inheritance by following two
characters at the same time
 Mendel was interested in determining whether alleles at 2 different
gene loci segregate dependently or independently
 Crossing two, true-breeding parents differing in two characters
produces dihybrids in the F1 generation, heterozygous for both
characters
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With his monohybrid crosses, Mendel determined
that the 2 alleles at a single gene locus segregate
when the gametes are formed.
With his dihybrid crosses, Mendel was interested
in determining whether alleles at 2 different gene
loci segregate dependently or independently.
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For example, in pea plants seed shape is
controlled by one gene locus where round (R) is
dominant to wrinkled (r) while seed color is
controlled by a different gene locus where
yellow (Y) is dominant to green (y).
Mendel crossed 2 pure-breeding plants: one with
round yellow seeds and the other with green
wrinkled seeds.
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Dihybrid Cross
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For example, in pea plants seed shape is
controlled by one gene locus where round (R) is
dominant to wrinkled (r) while seed color is
controlled by a different gene locus where
yellow (Y) is dominant to green (y).
Mendel crossed 2 pure-breeding plants: one
with round yellow seeds and the other with
green wrinkled seeds.
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If dependent segregation (assortment) occurs:
 Alleles at the 2 gene loci segregate together, and are transmitted as a unit.
 Therefore, each plant would only produce gametes with the same
combinations of alleles present in the gametes inherited from its parents:
Parents
Parental Gametes
r
y
R
Y
Rr
Yy
F1 Offspring
F1 Offspring’s Gametes
rr
yy
RR
YY
R
Y
r
y
What is the expected phenotypic ratio for the F2?
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F2 With Dependent Assortment:
R
Y
r
y
R
Y
RR
YY
Rr
Yy
r
y
Rr
Yy
rr
yy
Ratio is 3 round, yellow : 1 wrinkled, green
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Alleles at the 2 gene loci segregate (separate) independently, and are NOT
transmitted as a unit. Therefore, each plant would produce gametes with
allele combinations that were not present in the gametes inherited from its
parents:
Parents
Parental Gametes
F1 Offspring
F1 Offspring’s Gametes
R
Y
R
y
RR
YY
rr
yy
R
Y
r
y
Rr
Yy
r
Y
r
y
What is the expected phenotypic ratio for the F2?
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Mendelian Genetics
Dihybrid cross - parental generation differs in two traits
example-- cross round/yellow peas with wrinkled/green ones
Round/yellow is dominant
RY
Ry
rY
ry
RY
Ry
rY
ry
What are the expected phenotype ratios in the F2 generation?
round, yellow =
round, green =
wrinkled, yellow =
wrinkled, green =
F2 with independent assortment:
RY Ry
RY
Ry
rY
ry
rY
ry
RR
YY
RR
Yy
Rr
YY
Rr
Yy
RR
Yy
RR
yy
Rr
Yy
Rr
yy
Rr
YY
Rr
Yy
rr
YY
rr
Yy
Rr
Yy
Rr
yy
rr
Yy
rr
yy
Phenotypic ratio is 9 : 3 : 3 : 1
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How are two characters transmitted from parents to
offspring?
 As a package?
 Independently?
 A dihybrid cross
 Illustrates the inheritance of two characters
 Produces four phenotypes in the F2 generation
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P Generation
EXPERIMENT Two true-breeding pea plants—
one with yellow-round seeds and the other with green-wrinkled
seeds—were crossed, producing dihybrid F1 plants. Selfpollination of the F1 dihybrids, which are heterozygous for both
characters, produced the F2 generation. The two hypotheses
predict different phenotypic ratios. Note that yellow color (Y)
and round shape (R) are dominant.
YYRR
yyrr
Gametes
YR
F1 Generation

yr
YyRr
Hypothesis of
independent
assortment
Hypothesis of
dependent
assortment
Sperm
1⁄
4
YR
Sperm
1⁄
2
YR
1⁄
2
yR
1⁄
4
yr
YR
YYRR
YYRr
YyRR
YyRr
YYrr
YYrr
YyRr
Yyrr
YyRR
YyRr
yyRR
yyRr
YyRr
Yyrr
yyRr
yyrr
YR
F2 Generation
(predicted
offspring)
YYRR
YyRr
1⁄
4
Yr
yr
YyRr
3⁄
4
CONCLUSION The results support the hypothesis
ofindependent assortment. The alleles for seed color and seed
shape sort into gametes independently of each other.
1⁄
4
Eggs
1⁄
4
1⁄
2
Yr
yr
Eggs
1⁄
2
1⁄
4
yyrr
1⁄
4
yR
1⁄
4
Phenotypic ratio 3:1
1⁄
4
yr
9⁄
16
3⁄
16
3⁄
16
1⁄
16
Phenotypic ratio 9:3:3:1
315
108
101
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Phenotypic ratio approximately 9:3:3:1
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F2 generation ratio:
9:3:3:1
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Mendel’s dihybrid crosses showed a
9:3:3:1 phenotypic ratio for the F2
generation.
 Based on these data, he proposed the
Law of Independent Assortment, which
states that when gametes form, each
pair of hereditary factors (alleles)
segregates independently of the other
pairs.
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Mendel’s laws of segregation and independent
assortment reflect the rules of probability
 The multiplication rule
▪ States that the probability that two or more independent
events will occur together is the product of their individual
probabilities
 The rule of addition
▪ States that the probability that any one of two or more
exclusive events will occur is calculated by adding together
their individual probabilities
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The probability of two or more independent events
occurring together is the product of the
probabilities that each event will occur by itself
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Following the self-hybridization of a heterozygous
purple pea plants (Pp), what is the probability that a
given offspring will be homozygous for the
production of white flowers (pp)?
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Probability that a pollen seed will carry p: ½
Probability that an egg will carry p: ½
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Probability that the offspring will be pp:
1/2 X 1/2 = 1/4
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The probability of either of two mutually exclusive events
occurring is the sum of their individual probabilities
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Following the self-hybridization of a heterozygous purple
pea plant (Pp), what is the probability that a given offspring
will be purple?
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Probability of maternal P uniting with paternal P: 1/4
Probability of maternal p uniting with paternal P: 1/4
Probability of maternal P uniting with paternal p: 1/4
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Probability that the offspring will be purple:
1/4 + 1/4 + 1/4 = 3/4
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Can be determined using these rules

Rr
Rr
Segregation of
alleles into eggs
Segregation of
alleles into sperm
Sperm
1⁄
R
2
1⁄
Eggs
1⁄
r
2
r
r
R
R
2
r
2
R
R
1⁄
1⁄
1⁄
4
R
1⁄
4
r
4
r
1⁄
4
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Genes are distinct entities that remain
unchanged during crosses
Each plant has two alleles of a gene
Alleles segregated into gametes in equal
proportions, each gamete got only one allele
During gamete fusion, the number of alleles
was restored to two
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Mendel’s Principle of Uniformity in F1:
 F1 offspring of a monohybrid cross of
true-breeding strains resemble only one
of the parents.
 Why? Smooth seeds (allele S) are
completely dominant to wrinkled seeds
(alleles).
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Mendel’s Law of Segregation:
 Recessive characters masked in the F1
progeny of two true-breeding strains,
reappear in a specific proportion of the F2
progeny.
 Two members of a gene pair segregate
(separate) from each other during the
formation of gametes.
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Mendel’s Law of Independent Assortment:
 Alleles
for different traits assort
independently of one another.
 Genes on different chromosomes
behave independently in gamete
production.
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
Joe has a white cat named Sam. When Joe
crosses Sam with a black cat, he obtains
1/2 white kittens and 1/2 black kittens.
When the black kittens are interbred, all
the kittens that they produce are black.
On the basis of these results, would you
conclude that white or black coat color in
cats is a recessive trait? Explain your
reasoning.
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Dihybrid cross
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In watermelons, bitter fruit (B) is dominant over sweet fruit (b),
and yellow spots (S) are dominant over no spots (s). The genes for
these two characteristics assort independently. A homozygous
plant that has bitter fruit and yellow spots is crossed with a
homozygous plant that has sweet fruit and no spots. The F1 are
intercrossed to produce the F2.
a. What are the phenotypic ratios in the F2?
b. If an F1 plant is backcrossed with the bitter, yellow-spotted
parent, what phenotypes and proportions are expected in the
offspring?
c. If an F1 plant is backcrossed with the sweet, non spotted parent,
what phenotypes and proportions are expected in the offspring?
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