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Chapter 11:
Introduction to
Genetics
Section 11-1: The Work of
Gregor Mendel
• Every living thing—plant or animal,
microbe or human being– has a set of
characteristics inherited from its parent or
parents.
• Since the beginning of recorded history,
people have wanted to understand how that
inheritance is passed from generation to
generation.
• More recently, scientists have begun to
appreciate that heredity holds the key to
understanding what makes each species
unique.
• As a result, genetics, the scientific study of
heredity, is now at the core of a revolution
in understanding biology.
Gregor Mendel’s Peas
• The work of an Austrian monk named
Gregor Mendel was particularly
important to understanding biological
inheritance.
• Mendel was born in 1822 in what is now the
Czech Republic.
• After becoming a priest, Mendel spent
several years studying science and
mathematics at the University of Vienna.
• He spent the next 14 years working in the
monastery and teaching at the high school.
• In addition to his teaching duties, Mendel
was in charge of the monastery garden.
• In this ordinary garden, he was to do the
work that changed biology forever.
• Mendel worked with garden peas.
• Like many plants, pea plants use parts of
their flowers to reproduce.
• The male part of each flower produces
pollen– which contains male sex cells.
• The female part of the flower produces eggs–
female sex cells.
• When pollen fertilizes an egg cell, a seed for a
new plant is formed.
• Pea plants normally reproduce by selfpollination, in which pollen fertilizes the egg
cells in the very same flower.
• Seeds that are produced by self-pollination
inherit all of their characteristics from the single
plant that bore them.
• In effect, they have only one parent.
• When Mendel took charge of the monastery
garden, he had several different stocks of pea
plants.
• These peas were true-breeding, meaning that
if they were allowed to self-pollinate, they
would produce offspring identical to
themselves.
• One stock of seeds would produce only tall
plants, another only short plants.
• One stock produced only green seeds, another
only yellow seeds.
• These true-breeding plants were the basis of
Mendel’s experiments.
• Pea plants can also cross-pollinate.
• In cross-pollination, male sex cells in pollen
from the the flower on one plant fertilize the
egg cells of a flower on another plant.
• The seeds produced from cross-pollination
have two plants as parents.
• To perform his experiments, Mendel has to
select the pea plants that he would mate
with each other.
• Mendel would have to prevent the pea
flowers from self-pollinating and control
their cross-pollination.
• Mendel accomplished this task my first
cutting away the male parts of a flower.
• Then, he dusted that flower with pollen
from a second flower.
• The resulting seeds were crosses between
the 2 plants.
Genes & Dominance
• Mendel studied seven different pea plant
traits.
• A trait is a specific characteristic, such as
seed color or plant height, that varies
from one individual to another.
• Each of the seven traits Mendel studied
had two contrasting characters, for
example, green seed color and yellow
seed color.
• Mendel crossed plants with each of the 7
contrasting characters and studied their
offspring.
• Mendel called each original pair of plants
the P (parental) generation.
• He called the offspring the F1, or “first
filial,” generation.
• Filius is the Latin word for “son.”
• The offspring of crosses between parents
with different traits are called hybrids.
Figure 11-3 Mendel’s Seven F1
Crosses on Pea Plants
Section 11-1
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Section:
Seed Coat
Color
Pod
Shape
Seed
Shape
Seed
Color
Round
Yellow
Gray
Wrinkled
Green
White
Constricted
Round
Yellow
Gray
Smooth
Smooth
Pod
Color
Green
Flower
Position
Plant
Height
Axial
Tall
Yellow
Terminal
Short
Green
Axial
Tall
• What were the F1 hybrid plants like?
• Did the characters of the parent plants blend
in the offspring?
• To Mendel’s surprise, all of the offspring
had the character of only one of the parents.
• In each cross, the character of the other
parent seemed to have disappeared.
• From this set of experiments, Mendel drew
two conclusions.
• Mendel’s 1st conclusion was that
biological inheritance is determined by
factors that are passed from one
generation to the next.
• Today, scientists call the chemical factors
that determine traits genes.
• Each of the traits Mendel studied was
controlled by one gene that occurred in 2
contrasting forms.
• These contrasting forms produced the
different characters of each trait.
• For example, the gene for plant height
occurs in one form that produces tall plants
and in another form that produces short
plants.
• The different forms of a gene are called
alleles.
• Mendel’s 2nd conclusion is called the
principle of dominance.
• The principle of dominance states that
some alleles are dominant and others are
recessive.
• An organism with a dominant allele for a
particular form of a trait will always have
that form.
• An organism with a recessive allele for a
particular form of a trait will have that form
only when the dominant allele for a trait is
not present.
• In Mendel’s experiments, the allele for tall
plants was dominant and the allele for short
plants was recessive.
• The allele for yellow seeds was dominant,
while the allele for green seeds was
recessive.
Segregation
• Mendel wanted to answer another question:
Had the recessive alleles disappeared, or
were they still present in the F1 plants?
• To answer this question, he allowed all 7
kinds of F1 hybrid plants to produce an F2
(second filial) generation by selfpollination.
• He crossed the F1 generation with itself to
produce the F2 offspring as shown in the
next figure.
Principles of Dominance
Section 11-1
P Generation
Tall
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Section:
Short
F1 Generation
Tall
Tall
F2 Generation
Tall
Tall
Tall
Short
• The F1 cross showed that the traits
controlled by the recessive alleles
reappeared.
• Roughly ¼ of the F2 plants showed the
trait controlled by the recessive allele.
• What caused these recessive alleles to
seemingly disappear in the F1 generation
and then reappear in the F2 generation?
• Mendel assumed that a dominant allele had
masked the corresponding recessive allele
in the F1 generation.
• However, the trait controlled by the
recessive allele showed up in some of the F2
plants.
• This reappearance indicated that at some
point the allele for shortness had been
separated from the allele for tallness.
• How did this segregation occur?
• Mendel suggested that the alleles for
tallness and shortness in the F1 plants
were segregated from each other during
the formation of the sex cells, or gametes.
• Segregation– the separation of alleles
during gamete formation.
• Let’s assume that the F1 plants inherited an
allele for tallness from one parent and an
allele for shortness from the other parent.
• Because the allele for tallness is dominant,
all the F1 plants are tall.
• When each F1 plant flowers, the 2 alleles
are segregated from each other so that
each gamete carries only a single copy of
each gene.
• Each F1 plant produces two types of
gametes– those will the allele for tallness
and those with the allele for shortness.
Section 11-2: Probability &
Punnett Squares
• Whenever Mendel performed a cross with pea
plants, he carefully counted the offspring.
• Every time Mendel repeated a particular cross,
he obtained similar results.
• For example, whenever Mendel crossed 2 plants
that were hybrid for stem height (Tt), about ¾ of
the resulting plants were tall and about ¼ were
short.
• Mendel realized that the principles of probability
could be used to explain the results of genetic
crosses.
Genetics and Probability
• The likelihood that a particular event will
occur is called probability.
• As an example of probability, consider a coin
flip.
• There are 2 possible outcomes: The coin may
land heads up or tails up.
• The chances, or probabilities, of either outcome
are equal.
• The probability that a single coin flip will come
up heads is 1 chance in 2.
• This is ½ or 50%.
• If you flip a coin 3 times in a row, what is the
probability that it will land heads up every time?
• Because each flip is an independent event, the
probability of each coin’s landing heads up is ½.
• The probability of flipping 3 heads in a row is:
• ½ x ½ x ½ = 1/8
• The fact that the individual probabilities are
multiplied together illustrates an important
point– past outcomes do not affect future ones.
• The way in which alleles segregate is
completely random, like a coin flip.
• The principles of probability can be used to
predict the outcomes of genetic crosses.
Punnett Squares
• The gene combinations that might result from
a genetic cross can be determined by drawing
a diagram known as a Punnett Square.
• The types of gametes produced by each F1 parent
are shown along the top and left sides of the
square.
• The letters in the Punnett square represent alleles:
capital letters for dominant alleles and lowercase
letters for recessive alleles.
Section 11-2
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Section:
Tt X Tt Cross
• Organisms that have 2 identical alleles
for a particular trait– TT or tt in the
example– are said to be homozygous.
• Organisms that have 2 different alleles
for the same trait– Tt in the example– are
heterozygous.
• Homozygous organisms are true-breeding
for a particular trait.
• Heterozygous organisms are hybrid for a
particular trait.
• All of the tall plants have the same
phenotype, or physical characteristics.
• They do not have the same genotype, or
genetic makeup.
• Some tall plants are homozygous for the
trait, while others are heterozygous.
Probability & Segregation
• Referring back to the Punnett square we’ve
been using as an example, we see that ¼ of
the F2 plants have 2 alleles for tallness (TT),
and 2/4, or ½, of the F2 plants have one
allele for tallness and one allele for
shortness (Tt).
• Because the allele for tallness is dominant,
over the allele for shortness, ¾ of the F2
plants should be tall.
• Overall, there are 3 tall plants for every 1 short
plant in the F2 generation.
• The ratio of tall plants to short plants is 3:1.
• Mendel’s data did fit his model.
• The predicted ratio– 3 dominant to 1
recessive– showed up consistently,
indicating that Mendel’s assumptions about
segregation had been correct.
Probabilities Predict Averages
• Probabilities predict the average outcome
of a large number of events.
• Probability cannot predict the precise
outcome of an individual event.
• If you flip a coin twice, you are likely to get
one head and one tail.
• However, you might also get two heads or
two tails.
• To be sure of getting the expected 50:50
ratio, you would have to flip the coin many
times.
• The same is true of genetics.
• The larger the number of individuals, the
closer the resulting offspring numbers will
get to expected values.
• If an F1 generation contains just 3 or 4
offspring, it may not match Mendelian
predictions.
• When an F1 generation contains hundreds or
thousands of individuals, the ratios usually
come very close to matching expectations.
Section 11-3: Exploring
Mendelian Genetics
• After showing that alleles segregate during the
formation of gametes, Mendel wondered if they
did so independently.
• In other words, does the segregation of one pair
of alleles affect the segregation of another pair
of alleles?
• For example, does the gene that determines
whether a seed is round or wrinkled in shape
have anything to do with the gene for seed
color?
• Must a round seed also be yellow?
Independent Assortment
• First, Mendel crossed true-breeding plants
that produced only round yellow peas
(RRYY) with plants that produced wrinkled
green peas (rryy).
• All of the F1 offspring produced round
yellow peas.
• This shows that the alleles for yellow and
round peas are dominant over the alleles for
green and wrinkled peas.
• The genotype of each of these F1 plants
would be RrYy.
• This provides the hybrid plants needed for
the next cross– the cross of F1 plants to
produce the F2 generation.
• How would the alleles segregate when the
F1 plants were crossed to each other to
produce an F2 generation?
• Remember that each plant in the F1
generation was formed by the fusion of a
gamete carrying the dominant RY alleles
with another gamete carrying the recessive
ry alleles.
• Did this mean that the 2 dominant alleles
would always stay together?
• Or would they segregate independently, so
that any combination of alleles was
possible?
• The result of the cross was that some of the
seeds had combinations of the phenotypes–
and therefore combinations of the alleles–
not found in either parent.
• This clearly meant that the alleles for seed
shape segregated independently of those for
seed color– a principle known as
independent assortment.
• Independent assortment– independent
segregation of genes during the formation
of gametes.
Figure 11-10 Independent
Assortment in Peas
Section 11-3
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Section:
• Genes that segregate independently– such
as the genes for seed shape and seed color
in pea plants– do not influence each other’s
inheritance.
• Mendel’s experimental results were very
close to the 9: 3: 3: 1 ratio that the Punnett
square predicts.
A Summary of Mendel’s
Principles
• 1.) The inheritance of biological
characteristics is determined by individual
units known as genes. In organisms that
reproduce sexually, genes are passed from
parents to their offspring.
• 2.) In cases in which 2 or more forms of the
gene for a single trait exist, some forms of the
gene may be dominant and others may be
recessive.
• 3.) In most sexually reproducing
organisms, each adult has 2 copies of
each gene– one from each parent. These
genes are segregated from each other
when gametes are formed.
• 4.) The alleles for different genes usually
segregate independently of one another.
Beyond Dominant & Recessive
Alleles
• Despite the importance of Mendel’s work, it
would be a mistake to characterize the
principles he discovered as “laws,” because
there are important exceptions to most of them.
• Not all genes show simple patterns of dominant
and recessive alleles.
• In most organisms, genetics is more
complicated because the majority of genes have
more than two alleles.
• Some alleles are neither dominant nor
recessive, and many traits are controlled
by multiple alleles or multiple genes.
Incomplete Dominance
• A cross between 2 four o’clock (Mirabilis)
plants shows one of these complications.
• The F1 generation produced by a cross
between red-flowered (RR) and whiteflowered (WW) plants consists of pinkcolored flowers (RW).
• Which allele is dominant in this case?
Figure 11-11 Incomplete
Dominance in Four O’Clock Flowers
Section 11-3
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Section:
• Neither allele is dominant in this case.
• Cases in which one allele is not
completely dominant over another are
called incomplete dominance.
• In incomplete dominance, the
heterozygous phenotype is somewhere in
between the two homozygous phenotypes.
Codominance
• Codominance is a situation in which both
alleles contribute to the phenotype of the
organism.
• For example, in cattle the allele for red hair
is codominant with the allele for white hair.
• Cattle with both alleles are roan, or pinkish
brown, because their coats are a mixture of
both red and white hairs.
• In certain varieties of chickens, the allele
for black feathers is codominant with the
allele for white feathers.
• Heterozygous chickens appear speckled
with black and white feathers.
Multiple Alleles
• Many genes have more than two alleles
and are therefore said to have multiple
alleles.
• This does not mean that an individual
can have more than two alleles.
• It only means that more than 2 possible
alleles exist in a population.
• One of the best known examples is coat
color in rabbits.
• A rabbit’s coat color is determined by a
single gene that has at least 4 different
alleles.
• The four known alleles display a pattern of
simple dominance that can produce four
possible coat colors.
• Many other genes have multiple alleles,
including the human genes for blood type
and eye color.
Polygenic Traits
• Many traits are produced by the
interaction of several genes.
• Traits controlled by 2 or more genes are
said to be polygenic traits, which means
“having many genes.”
• For example, at least 3 genes are involved
in making the reddish-brown pigment in the
eyes of fruit flies.
• Different combinations of alleles for these
genes produce very different eye colors.
• Polygenic traits often show a wide range of
phenotypes.
• For example, the wide range of skin color in
humans comes about partly because more
than 4 different genes probably control this
trait.
Applying Mendel’s Principles
• Mendel’s principles don’t apply only to
plants.
• At the beginning of the 1900’s, the
American geneticist Thomas Hunt
Morgan decided to look for a model
organism to advance the study of
genetics.
• He wanted an animal that was small, easy
to keep in the laboratory, and able to
produce large numbers of offspring in a
short period of time.
• He decided to work on a tiny insect that
kept showing up, uninvited, in his
laboratory.
• The insect was the common fruitfly,
Drosophila melanogaster.
• Drosophila was an ideal organism for
genetics because it could produce plenty
of offspring, and it did so quickly.
• Mendel’s principles also apply to humans.
• The basic principles of Mendelian genetics
can be used to study the inheritance of
human traits and to calculate the probability
of certain traits appearing in the next
generation.
Section 11-4: Meiosis
• Gregor Mendel did not know where the
genes he had discovered were located in the
cell.
• Fortunately, his predictions of how genes
should behave were so specific that it was
not long before biologists were certain they
had found them.
• Genes are located on chromosomes in the
cell nucleus.
• Mendel’s principles of genetics require at
least two things.
• First, each organism must inherit a single
copy of every gene from both its “parents.”
• Because each pea plant has two parents, each
plant must carry two complete sets of genes.
• Second, when an organism produces its
own gametes, those two sets of genes must
be separated from each other so that each
gamete contains just one set of genes.
• This means that when gametes are formed,
there must be a process that separates the
two sets of genes so that each gamete ends
up with just one set.
• Although Mendel didn’t know it, gametes
are formed through exactly such a process.
Chromosome Number
• Let’s consider the fruit fly.
• A body cell in an adult fruit fly has 8
chromosomes.
• Four of the chromosomes came from the
fruit fly’s male parent, and 4 came from its
female parent.
• These two sets of chromosomes are
homologous, meaning that each of the 4
chromosomes that came from the male
parent has a corresponding chromosome
from the female parent.
• A cell that contains both sets of
homologous chromosomes is said to be
diploid, which means “2 sets.”
• The number of chromosomes in a diploid
cell is sometimes represented by the
symbol 2N.
• Diploid cells contain two complete sets of
chromosomes and two complete sets of
genes.
• Meiosis involves two successive divisions
of a diploid nucleus following only one
DNA replication cycle.
• By contrast, the gametes of sexually
reproducing organisms contain only a
single set of chromosomes and only a
single set of genes.
• These cells are said to be haploid.
• Haploid means “one set.”
Phases of Meiosis
• How are haploid (N) gamete cells produced
from diploid (2N) cells?
• That’s where meiosis comes into play.
• 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.
• Meiosis usually involves two distinct
stages: the first meiotic division, called
meiosis I, and the second meiotic division,
called meiosis II.
• By the end of meiosis II, the diploid cell
that entered meiosis has become 4
haploid cells.
Meiosis I
• Prior to meiosis I, each chromosome is
replicated.
• The cells then begin to divide in a way that
looks similar to mitosis.
• Remember, in mitosis, the chromosomes
line up individually in the center of the cell.
• The 2 chromatids that make up each
chromosome then separate from each other.
• In prophase of meiosis I, each
chromosome pairs with its corresponding
homologous chromosome to form a
structure called a tetrad.
• There are 4 chromatids in a tetrad.
• As homologous chromosomes pair up and
form tetrads in meiosis I, they may
exchange portions of their chromatids in
a process called crossing-over.
• Crossing over results in the exchange of
alleles between homologuous
chromosomes and produces new
combinations of alleles.
Crossing-Over
Section 11-4
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Section:
• Next, the homologous chromosomes separate,
and two new cells are formed.
• Because each pair of homologous chromosomes
was separated, neither of the daughter cells has the
2 complete sets of chromosomes that it would have
in a diploid cell.
• The result of meiosis I is two cells.
• The two cells produced by meiosis I have sets of
chromosomes that are different from each other
and from the diploid cell that entered meiosis I.
• During meiosis I, the parent cell produces two
daughter cells, each with one member of each
pair of homologous chromosomes.
• Chromosome number has been halved,
but each chromosome, having been
replicated earlier, has twice the original
amount of DNA.
Meiosis II
• The 2 cells produced by meiosis I now enter
a second meiotic division.
• Unlike the first division, neither cell goes
through a round of chromosome
replication before entering meiosis II.
• Remember, each of the cell’s
chromosomes has 2 chromatids.
• During metaphase II of meiosis, the
chromosomes line up in the center of
each cell.
• In anaphase II, the paired chromatids
separate.
• Each of the 4 daughter cells produced in
meiosis II receives a chromatid from each
chromosome lined up at the plate.
• Keep in mind, these four daughter cells
now contain the haploid (N) number of
chromosomes.
• Each of the 4 daughter cells has one
chromosome from each homologous pair of
chromosomes.
• Remember, however, that these
chromosomes are not exact copies of the
original chromosomes because of the
crossing-over that occurs between
chromosomes during meiosis I.
Gamete Formation
• In male animals, the production of the
gametes is called spermatogenesis
because the 4 mature male gametes are
referred to as sperm.
• In female animals, the production of the
gametes is called oogenesis because the 1
mature female gamete is referred to as the
ovum (egg).
• In female animals, generally only one of the
cells produced by meiosis is involved in
reproduction.
• In many females, the cell divisions at the end
of meiosis I and meiosis II are uneven, so the 1
ovum, or egg, receives most of the cytoplasm.
• The other 3 cells produced in the female
during meiosis are known as polar bodies and
usually do not participate in reproduction.
Comparing Mitosis & Meiosis
• The names of these two events are very
similar, but in fact, the processes are very
different.
• Mitosis results in the production of 2
genetically identical diploid cells.
• Meiosis results in 4 genetically different
haploid cells.