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Copyright Pearson Prentice Hall
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.
Gregor Mendel’s 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 PlantsMendel’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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Mendel's F2 Generation
P GenerationF1 Generation
Tall Tall Tall Tall Tall TallShort Short
F2 Generation
Segregation
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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 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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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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A capital letter represents the dominant allele for tall.
A lowercase letter represents the recessive allele for short.
In this example,
T = tallt = short
Punnett Squares
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Gametes produced by each F1 parent are shown along the top and left side.
Punnett Squares
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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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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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Punnett Squares
The plants have different genotypes (TT and Tt), but they have the same phenotype (tall).
TTHomozygous
TtHeterozygous
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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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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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11-2
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 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.
Independent Assortment
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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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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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Independent Assortment
The Punnett square predicts a 9 : 3 : 3 :1 ratio in the F2 generation.
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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.
Independent Assortment
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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.
Independent
Assortment
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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. 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.
A Summary of Mendel's Principles
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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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A cross between red (RR) and white (WW) four o’clock plants produces pink-colored flowers (RW).
Beyond Dominant and Recessive Alleles
WW
RR
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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.
Full color: CC, Ccch, Cch, or CcChinchilla: cchch, cchcch, or cchcHimalayan: chc, or chchAIbino: cc
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
Epistasis
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
Fig. 10.13, p.161
EB Eb eB eb
EB
Eb
eB
eb EeBbblack
EeBBblack
EEBbblack
EEBBblack
EEBbblack
EeBBblack
EeBbblack
Eebbchocolate
EeBbblack
EEbbchocolate
EeBbblack
Eebbchocolate
eeBByellow
eeBbyellow
eebbyellow
eeBbyellow
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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.
LE 14-12
aabbcc Aabbcc AaBbcc AaBbCc AABbCc AABBCc AABBCC
AaBbCcAaBbCc
20/64
15/64
6/64
1/64
Fra
ctio
n of
pro
geny
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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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11–3
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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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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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.
Phases of
Meiosis
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Phases of
Meiosis
Meiosis I
Prophase I Metaphase I Anaphase I Telophase I and Cytokinesis
Interphase I
Meiosis I
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Phases of
Meiosis
Cells undergo a round of DNA replication, forming duplicate chromosomes.
Interphase I
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Phases of
Meiosis
Each chromosome pairs with its corresponding homologous chromosome to form a tetrad.
There are 4 chromatids in a tetrad.
MEIOSIS I Prophase II
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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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Phases of
Meiosis
Spindle fibers attach to the chromosomes.
MEIOSIS I Metaphase I
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Phases of
Meiosis MEIOSIS I Anaphase I
The fibers pull the homologous chromosomes toward opposite ends of the cell.
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Phases of
Meiosis
MEIOSIS I Telophase I and Cytokinesis
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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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 II and Cytokinesis
Prophase IIMetaphase II Anaphase IITelophase I and
Cytokinesis I
Meiosis II
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Phases of
Meiosis
Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original cell.
MEIOSIS IIProphase II
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Phases of
Meiosis
The chromosomes line up in the center of cell.
MEIOSIS II Metaphase II
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Phases of
Meiosis
The sister chromatids separate and move toward opposite ends of the cell.
MEIOSIS II Anaphase II
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Phases of
Meiosis
Meiosis II results in four haploid (N) daughter cells.
MEIOSIS II Telophase II and Cytokinesis
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Gamete Formati
on
Gamete Formation
In male animals, meiosis results in four equal-sized 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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Comparing
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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Comparing
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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Comparing
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.