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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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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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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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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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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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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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Genes and Dominance
Mendel’s F1 Crosses on Pea Plants
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Genes and Dominance
Mendel’s Seven F1 Crosses on Pea Plants
Mendel’s F1 Crosses on Pea Plants
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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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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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Genes and Dominance
The principle of dominance states that
some alleles are dominant and others are
recessive.
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Genes and Dominance
Mendel’s F1 Crosses on Pea Plants
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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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Segregation
Mendel's F2 Generation
P Generation
Tall
Short
F2 Generation
F1 Generation
Tall
Tall
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Tall
Tall
Tall
Short
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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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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Segregation
Alleles separate during gamete formation.
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11-1
Gametes are also known as
a. genes.
b. sex cells.
c. alleles.
d. hybrids.
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11-1
The offspring of crosses between parents with
different traits are called
a. alleles.
b. hybrids.
c. gametes.
d. dominant.
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11-1
In Mendel’s pea experiments, the male gametes
are the
a. eggs.
b. seeds.
c. pollen.
d. sperm.
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11-1
In a cross of a true-breeding tall pea plant with a
true-breeding short pea plant, the F1 generation
consists of
a. all short plants.
b. all tall plants.
c. half tall plants and half short plants.
d. all plants of intermediate height.
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11-1
If a particular form of a trait is always present
when the allele controlling it is present, then the
allele must be
a. mixed.
b. recessive.
c. hybrid.
d. dominant.
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Genetics and P
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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Punnet
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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Punnet
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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Punnet
Gametes
produced by each
F1 parent are
shown along the
top and left side.
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Punnet
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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Punnet
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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Punnet
The plants have
different
genotypes (TT
and Tt), but they
have the same
phenotype (tall).
TT
Homozygous
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Tt
Heterozygous
Proba
Se
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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Probabilitie
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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11-2
Probability can be used to predict
a. average outcome of many events.
b. precise outcome of any event.
c. how many offspring a cross will produce.
d. which organisms will mate with each other.
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11-2
Compared to 4 flips of a coin, 400 flips of the
coin is
a. more likely to produce about 50% heads and
50% tails.
b. less likely to produce about 50% heads and
50% tails.
c. guaranteed to produce exactly 50% heads
and 50% tails.
d. equally likely to produce about 50% heads
and 50% tails.
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11-2
Organisms that have two different alleles for a
particular trait are said to be
a. hybrid.
b. heterozygous.
c. homozygous.
d. recessive.
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11-2
Two F1 plants that are homozygous for
shortness are crossed. What percentage of the
offspring will be tall?
a. 100%
b. 50%
c. 0%
d. 25%
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11-2
The Punnett square allows you to predict
a. only the phenotypes of the offspring from a
cross.
b. only the genotypes of the offspring from a
cross.
c. both the genotypes and the phenotypes
from a cross.
d. neither the genotypes nor the phenotypes
from a cross.
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Independent Ass
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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Independent Ass
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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Independent Ass
The alleles for round (R) and yellow (Y)
are dominant over the alleles for
wrinkled (r) and green (y).
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Independent Ass
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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Independent Ass
The Punnett square predicts a 9 : 3 : 3 :1
ratio in the F2 generation.
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Independent Ass
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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Indepen
dent
Assort
ment
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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A Summary of M
Pr
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 M
Pr
a. In most sexually reproducing organisms,
each adult has two copies of each gene.
These genes are segregated from each
other when gametes are formed.
b. The alleles for different genes usually
segregate independently of one another.
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Beyond Dominant a
Recessive Alle
Some alleles are neither dominant nor
recessive, and many traits are controlled by
multiple alleles or multiple genes.
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Beyond Dominant a
Recessive Allel
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 a
Recessive Allel
RR
A cross
between red
(RR) and
white (WW)
four o’clock
plants
produces
pink-colored
flowers
(RW).
WW
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Beyond Dominant a
Recessive Allel
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 a
Recessive Allel
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
c = albino; no color;
recessive to all other
alleles
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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Epista
sis
Epistasis
• A trait involves 2 genes
• The expression of one gene influences the expression of
the other
• For example, in Labrador Retrievers one gene
determines which color. A different gene determines if
you have color.
• E=have color in hair, e=no color in hair
• B=black pigment, b=brown pigment
EB
Eb
eB
eb
EB
EEBB
black
EEBb
black
EeBB
black
EeBb
black
Eb
EEBb
black
EEbb
chocolate
EeBb
black
Eebb
chocolate
eB
EeBB
black
EeBb
black
eeBB
yellow
eeBb
yellow
eb
EeBb
black
Eebb
chocolate
eeBb
yellow
eebb
yellow
Fig. 10.13, p.161
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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LE 14-12
AaBbCc
aabbcc
20/64
Fraction of progeny
15/64
6/64
1/64
Aabbcc
AaBbcc
AaBbCc
AaBbCc AABbCc
AABBCc AABBCC
Applying M
Pr
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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11–3
In a cross involving two pea plant traits,
observation of a 9 : 3 : 3 : 1 ratio in the F2
generation is evidence for
a. the two traits being inherited together.
b. an outcome that depends on the sex of the
parent plants.
c. the two traits being inherited independently
of each other.
d. multiple genes being responsible for each
trait.
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11–3
Traits controlled by two or more genes are
called
a. multiple-allele traits.
b. polygenic traits.
c. codominant traits.
d. hybrid traits.
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11–3
In four o'clock flowers, the alleles for red flowers
and white flowers show incomplete dominance.
Heterozygous four o'clock plants have
a. pink flowers.
b. white flowers.
c. half white flowers and half red flowers.
d. red flowers.
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11–3
A white male horse and a tan female horse
produce an offspring that has large areas of
white coat and large areas of tan coat. This is
an example of
a. incomplete dominance.
b. multiple alleles.
c. codominance.
d. a polygenic trait.
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11–3
Mendel's principles apply to
a. pea plants only.
b. fruit flies only.
c. all organisms.
d. only plants and animals.
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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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Chromo
some
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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Chromo
some
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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Chromo
some
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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Chromo
some
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
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Anaphase I
Telophase I
and
Cytokinesis
Phases
of
Meiosis
Cells undergo a
round of DNA
replication, forming
duplicate
chromosomes.
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Interphase I
Phases
of
Meiosis
Each chromosome
pairs with its
corresponding
homologous
chromosome to form
a tetrad.
There are 4
chromatids in a
tetrad.
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MEIOSIS I
Prophase I
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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Phases
of
Meiosis
Spindle fibers attach
to the
chromosomes.
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MEIOSIS I
Metaphase I
Phases
of
Meiosis
The fibers pull the
homologous
chromosomes
toward opposite
ends of the cell.
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MEIOSIS I
Anaphase I
Phases
of
Meiosis
Nuclear membranes form.
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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MEIOSIS I
Telophase I and
Cytokinesis
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
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Anaphase II Telophase II
and
Cytokinesis
Phases
of
Meiosis
Meiosis I results in
two haploid (N)
daughter cells, each
with half the number
of chromosomes as
the original cell.
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MEIOSIS II
Prophase II
Phases
of
Meiosis
The chromosomes
line up in the center of
cell.
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MEIOSIS II
Metaphase II
Phases
of
Meiosis
The sister
chromatids separate
and move toward
opposite ends of the
cell.
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MEIOSIS II
Anaphase II
Phases
of
Meiosis
Meiosis II results in
four haploid (N)
daughter cells.
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MEIOSIS II
Telophase II and Cytokinesis
Gamete
Formati
on
Gamete Formation
In male animals, meiosis results in four equalsized gametes called sperm.
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Gamete
Formati
on
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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ing
Mitosis
and
Meiosis
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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ing
Mitosis
and
Meiosis
Mitosis
a. Cells produced by mitosis have the same
number of chromosomes and alleles as the
original cell.
b. Mitosis allows an organism to grow and
replace cells.
c. Some organisms reproduce asexually by
mitosis.
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ing
Mitosis
and
Meiosis
Meiosis
a. Cells produced by meiosis have half the
number of chromosomes as the parent cell.
b. These cells are genetically different from the
diploid cell and from each other.
c. Meiosis is how sexually-reproducing
organisms produce gametes.
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11-4
If the body cells of humans contain 46
chromosomes, a single sperm cell should have
a. 46 chromosomes.
b. 23 chromosomes.
c. 92 chromosomes.
d. between 23 and 46 chromosomes.
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11-4
During meiosis, the number of chromosomes
per cell is cut in half through the separation of
a. daughter cells.
b. homologous chromosomes.
c. gametes.
d. chromatids.
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11-4
The formation of a tetrad occurs during
a. anaphase I.
b. metaphase II.
c. prophase I.
d. prophase II.
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11-4
In many female animals, meiosis results in the
production of
a. only 1 egg.
b. 1 egg and 3 polar bodies.
c. 4 eggs.
d. 1 egg and 2 polar bodies.
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11-4
Compared to egg cells formed during meiosis,
daughter cells formed during mitosis are
a. genetically different, while eggs are
genetically identical.
b. genetically different, just as egg cells are.
c. genetically identical, just as egg cells are.
d. genetically identical, while egg cells are
genetically different.
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