11-1 The Work of Gregor
Mendel
11-1 The Work of Gregor Mendel
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11-1 The Work of Gregor Mendel
Gregor Mendel’s Peas
Gregor Mendel’s Peas
Genetics is the scientific study of heredity.
Gregor Mendel was an Austrian monk. His work
was important to the understanding of heredity.
Mendel carried out his work with ordinary garden
peas.
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11-1 The Work of Gregor Mendel
Gregor Mendel’s Peas
Mendel knew that
• the male part of
each flower
produces pollen,
(containing
sperm).
• the female part
of the flower
produces egg
cells.
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11-1 The Work of Gregor Mendel
Gregor Mendel’s Peas
During sexual reproduction, sperm and egg cells join
in a process called fertilization.
Fertilization produces a new cell.
Pea flowers are self-pollinating.
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11-1 The Work of Gregor Mendel
Gregor Mendel’s Peas
Mendel had true-breeding pea plants that, if allowed
to self-pollinate, would produce offspring identical to
themselves.
Cross-pollination
Mendel was able to
produce seeds that
had two different
parents.
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11-1 The Work of Gregor Mendel
Genes and Dominance
Genes and Dominance
A trait is a specific characteristic that varies from
one individual to another.
Mendel studied seven pea plant traits, each with two
contrasting characters.
He crossed plants with each of the seven
contrasting characters and studied their offspring.
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11-1 The Work of Gregor Mendel
Genes and Dominance
Each original pair of plants is
the P (parental) generation.
The offspring are called the F1,
or “first filial,” generation.
The offspring of crosses
between parents with different
traits are called hybrids.
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11-1 The Work of Gregor Mendel
Genes and Dominance
Mendel’s F1 Crosses on Pea Plants
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11-1 The Work of Gregor Mendel
Genes and Dominance
Mendel’s Seven F1 Crosses on Pea Plants
Mendel’s F1 Crosses on Pea Plants
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11-1 The Work of Gregor Mendel
Genes and Dominance
Mendel's first conclusion
was that biological inheritance is determined by
factors that are passed from one generation to the
next.
Today, scientists call the factors that determine traits
genes.
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11-1 The Work of Gregor Mendel
Genes and Dominance
Each of the traits Mendel studied was controlled by
one gene that occurred in two contrasting forms that
produced different characters for each trait.
The different forms of a gene are called alleles.
Mendel’s second conclusion
is called the principle of dominance.
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11-1 The Work of Gregor Mendel
Genes and Dominance
The principle of dominance states that
some alleles are dominant and others are
recessive.
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11-1 The Work of Gregor Mendel
Genes and Dominance
Mendel’s F1 Crosses on Pea Plants
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11-1 The Work of Gregor Mendel
Segregation
Segregation
Mendel crossed the F1 generation with itself to
produce the F2 (second filial) generation.
The traits controlled by recessive alleles reappeared
in one fourth of the F2 plants.
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11-1 The Work of Gregor Mendel
Segregation
Mendel's F2 Generation
P Generation
Tall
Short
F2 Generation
F1 Generation
Tall
Tall
Tall
Tall
Tall
Short
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11-1 The Work of Gregor Mendel
Segregation
The reappearance of the trait controlled by the
recessive allele indicated that at some point the allele
for shortness had been separated, or segregated,
from the allele for tallness.
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11-1 The Work of Gregor Mendel
Segregation
Mendel suggested that the alleles for tallness and
shortness in the F1 plants segregated from each
other during the formation of the sex cells, or
gametes.
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11-1 The Work of Gregor Mendel
Segregation
Alleles separate during gamete formation.
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11-1 The Work of Gregor Mendel
11-2 Probability and Punnett Squares
11-2 Probability and Punnett Squares
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11-1 The Work of Gregor Mendel
Genetics and Probability
Genetics and Probability
The likelihood that a particular event will occur is
called probability.
The principles of probability can be used to
predict the outcomes of genetic crosses.
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11-1 The Work of Gregor Mendel
Punnett Squares
Punnett Squares
The gene combinations that might result from a
genetic cross can be determined by drawing a
diagram known as a Punnett square.
Punnett squares can be used to predict and
compare the genetic variations that will result
from a cross.
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11-1 The Work of Gregor Mendel
Punnett Squares
A capital letter
represents the
dominant allele for tall.
A lowercase letter
represents the
recessive allele for
short.
In this example,
T = tall
t = short
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11-1 The Work of Gregor Mendel
Punnett Squares
Gametes produced by
each F1 parent are
shown along the top
and left side.
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11-1 The Work of Gregor Mendel
Punnett Squares
Organisms that have two identical alleles for a
particular trait are said to be homozygous.
Organisms that have two different alleles for the
same trait are heterozygous.
Homozygous organisms are true-breeding for a
particular trait.
Heterozygous organisms are hybrid for a particular
trait.
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11-1 The Work of Gregor Mendel
Punnett Squares
All of the tall plants have the same phenotype, or
physical characteristics.
The tall plants do not have the same genotype, or
genetic makeup.
One third of the tall plants are TT, while two thirds of
the tall plants are Tt.
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11-1 The Work of Gregor Mendel
Punnett Squares
The plants have
different genotypes
(TT and Tt), but they
have the same
phenotype (tall).
TT
Homozygous
Tt
Heterozygous
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11-1 The Work of Gregor Mendel
Probability and
Segregation
Probability and Segregation
One fourth (1/4) of the F2
plants have two alleles for
tallness (TT).
2/4 or 1/2 have one allele
for tall (T), and one for
short (t).
One fourth (1/4) of the F2
have two alleles for short (tt).
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11-1 The Work of Gregor Mendel
Probabilities Predict
Averages
Probabilities Predict Averages
Probabilities predict the average outcome of a
large number of events.
Probability cannot predict the precise outcome
of an individual event.
In genetics, the larger the number of offspring, the
closer the resulting numbers will get to expected
values.
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11-1 The Work of Gregor Mendel
11-3 Exploring Mendelian Genetics
11–3 Exploring Mendelian Genetics
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11-1 The Work of Gregor Mendel
Independent Assortment
Independent Assortment
To determine if the segregation of one pair of
alleles affects the segregation of another pair of
alleles, Mendel performed a two-factor cross.
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11-1 The Work of Gregor Mendel
Independent Assortment
The Two-Factor Cross: F1
Mendel crossed true-breeding plants that
produced round yellow peas (genotype RRYY)
with true-breeding plants that produced wrinkled
green peas (genotype rryy).
RRYY
x
rryy
All of the F1 offspring produced round yellow
peas (RrYy).
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11-1 The Work of Gregor Mendel
Independent Assortment
The alleles for round (R) and yellow (Y) are
dominant over the alleles for wrinkled (r) and
green (y).
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Independent Assortment
The Two-Factor Cross: F2
Mendel crossed the heterozygous F1 plants (RrYy)
with each other to determine if the alleles would
segregate from each other in the F2 generation.
RrYy × RrYy
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11-1 The Work of Gregor Mendel
Independent Assortment
The Punnett square predicts a 9 : 3 : 3 :1 ratio in the
F2 generation.
Represents:
Independent Assortment
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11-1 The Work of Gregor Mendel
Independent Assortment
The alleles for seed shape segregated independently
of those for seed color. This principle is known as
independent assortment.
Genes that segregate independently do not influence
each other's inheritance.
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11-1 The Work of Gregor Mendel
Independent Assortment
The principle of independent assortment states that
genes for different traits can segregate
independently during the formation of gametes.
Independent assortment helps account for the
many genetic variations observed in plants,
animals, and other organisms.
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11-1 The Work of Gregor Mendel
A Summary of Mendel's
Principles
A Summary of Mendel's Principles
• Genes are passed from parents to their
offspring.
• If two or more forms (alleles) of the gene for a
single trait exist, some forms of the gene may
be dominant and others may be recessive.
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A Summary of Mendel's
Principles
• In most sexually reproducing organisms, each
adult has two copies of each gene. These genes
are segregated from each other when gametes
are formed.
• The alleles for different genes usually segregate
independently of one another.
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11-1 The Work of Gregor Mendel
Beyond Dominant and
Recessive Alleles
Some alleles are neither dominant nor recessive,
and many traits are controlled by multiple alleles or
multiple genes.
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Beyond Dominant and
Recessive Alleles
Incomplete Dominance
When one allele is not completely dominant over
another it is called incomplete dominance.
In incomplete dominance, the heterozygous
phenotype is between the two homozygous
phenotypes.
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Beyond Dominant and
Recessive Alleles
RR
A cross
between red
(RR) and white
(WW) four
o’clock plants
produces pinkcolored flowers
(RW).
WW
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11-1 The Work of Gregor Mendel
Beyond Dominant and
Recessive Alleles
Codominance
In codominance, both alleles contribute to the
phenotype.
In certain varieties of chicken, the allele for black
feathers is codominant with the allele for white
feathers.
Heterozygous chickens are speckled with both
black and white feathers. The black and white
colors do not blend to form a new color, but
appear separately.
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Beyond Dominant and
Recessive Alleles
Multiple Alleles
Genes that are controlled by more than two alleles
are said to have multiple alleles.
An individual can’t have more than two alleles.
However, more than two possible alleles can exist
in a population.
A rabbit's coat color is determined by a single gene
that has at least four different alleles.
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Beyond Dominant and
Recessive Alleles
Different combinations of alleles result in the colors
shown here.
KEY
C=
full color; dominant
to all other alleles
cch = chinchilla; partial
defect in pigmentation;
dominant to
ch and c alleles
ch = Himalayan; color in
certain parts of the
body; dominant to
c allele
ch
h, cor
hc
chc
h, or
AIbino:
Chinchilla:
Himalayan:
cc cCC,
chcc,
, hor
cchCc
c
Full color:
Ccch
,cch
Cc
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c = albino; no color;
recessive to all other
alleles
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11-1 The Work of Gregor Mendel
Beyond Dominant and
Recessive Alleles
Polygenic Traits
Traits controlled by two or more genes are said to
be polygenic traits.
Skin color in humans is a polygenic trait controlled
by more than four different genes.
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11-1 The Work of Gregor Mendel
AaBbCc
aabbcc
Aabbcc
AaBbcc
AaBbCc
AaBbCc AABbCc
AABBCc AABBCC
20/64
15/64
Fraction of progeny
LE 14-12
6/64
1/64
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Applying Mendel's
Principles
Applying Mendel's Principles
Thomas Hunt Morgan used fruit flies to advance
the study of genetics.
Morgan and others tested Mendel’s principles and
learned that they applied to other organisms as
well as plants.
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11-1 The Work of Gregor Mendel
11-4 Meiosis
11-4 Meiosis
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Each organism must inherit a single copy of
every gene from each of its “parents.”
Gametes are formed by a process that separates the
two sets of genes so that each gamete ends up with
just one set.
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Chromosome Number
Chromosome Number
All organisms have
different numbers of
chromosomes.
A body cell in an adult fruit
fly has 8 chromosomes: 4
from the fruit fly's male
parent, and 4 from its
female parent.
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Chromosome Number
These sets of chromosomes are homologous.
Each of the 4 chromosomes that came from the male
parent has a corresponding chromosome from the
female parent.
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Chromosome Number
A cell that contains both sets of homologous
chromosomes is said to be diploid.
The number of chromosomes in a diploid cell is
sometimes represented by the symbol 2N.
For Drosophila, the diploid number is 8, which can be
written as 2N=8.
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Chromosome Number
The gametes of sexually reproducing organisms
contain only a single set of chromosomes, and
therefore only a single set of genes.
These cells are haploid. Haploid cells are
represented by the symbol N.
For Drosophila, the haploid number is 4, which can
be written as N=4.
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Phases of Meiosis
Phases of Meiosis
Meiosis is a process of reduction division in which
the number of chromosomes per cell is cut in half
through the separation of homologous
chromosomes in a diploid cell.
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Phases of Meiosis
Meiosis involves two divisions, meiosis I and
meiosis II.
By the end of meiosis II, the diploid cell that
entered meiosis has become 4 haploid cells.
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Phases of Meiosis
Meiosis I
Interphase I
Meiosis I
Prophase I
Metaphase I
Anaphase I
Telophase I
and
Cytokinesis
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Cells undergo a round of
DNA replication, forming
duplicate chromosomes.
Phases of Meiosis
Interphase I
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Each chromosome pairs
with its corresponding
homologous
chromosome to form a
tetrad.
Phases of Meiosis
MEIOSIS I
Prophase I
There are 4 chromatids in
a tetrad.
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Phases of Meiosis
When homologous chromosomes form tetrads in
meiosis I, they exchange portions of their
chromatids in a process called crossing over.
Crossing-over produces new combinations of
alleles.
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Spindle fibers attach to
the chromosomes.
Phases of Meiosis
MEIOSIS I
Metaphase I
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The fibers pull the
homologous
chromosomes toward
opposite ends of the
cell.
Phases of Meiosis
MEIOSIS I
Anaphase I
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Nuclear membranes form.
Phases of Meiosis
MEIOSIS I
Telophase I and
Cytokinesis
The cell separates into two
cells.
The two cells produced by
meiosis I have
chromosomes and alleles
that are different from each
other and from the diploid
cell that entered meiosis I.
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Phases of Meiosis
Meiosis II
The two cells produced by meiosis I now enter a
second meiotic division.
Unlike meiosis I, neither cell goes through
chromosome replication.
Each of the cell’s chromosomes has 2 chromatids.
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Phases of Meiosis
Meiosis II
Telophase I and
Cytokinesis I
Meiosis II
Prophase II
Metaphase II
Anaphase II Telophase II
and
Cytokinesis
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Meiosis I results in two
haploid (N) daughter
cells, each with half the
number of chromosomes
as the original cell.
Phases of Meiosis
MEIOSIS II
Prophase II
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The chromosomes line
up in the center of cell.
Phases of Meiosis
MEIOSIS II
Metaphase II
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The sister chromatids
separate and move
toward opposite ends of
the cell.
Phases of Meiosis
MEIOSIS II
Anaphase II
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Meiosis II results in four
haploid (N) daughter
cells.
Phases of Meiosis
MEIOSIS II
Telophase II and Cytokinesis
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Gamete Formation
Gamete Formation
In male animals, meiosis results in four equal-sized
gametes called sperm.
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Gamete Formation
In many female animals, only one egg results from
meiosis. The other three cells, called polar bodies,
are usually not involved in reproduction.
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Comparing Mitosis and Meiosi
Comparing Mitosis and Meiosis
Mitosis results in the production of two genetically
identical diploid cells. Meiosis produces four
genetically different haploid cells.
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Comparing Mitosis and Meiosi
Mitosis
• Cells produced by mitosis have the same number
of chromosomes and alleles as the original cell.
• Mitosis allows an organism to grow and replace
cells.
• Some organisms reproduce asexually by mitosis.
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Comparing Mitosis and Meiosi
Meiosis
• Cells produced by meiosis have half the number
of chromosomes as the parent cell.
• These cells are genetically different from the
diploid cell and from each other.
• Meiosis is how sexually-reproducing organisms
produce gametes.
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