Chapter Eleven- Intro to Genetics

Copyright Pearson Prentice Hall
biology

11-1 The 11-1 Th eWo rWk of oGrergkor Moenfd eGl regor Mendel

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.

Mendel knew that
•the male part of
each flower
produces pollen,
(containing
sperm).
•the female part
of the flower
produces egg
cells.

During sexual reproduction, sperm and egg
cells join in a process called fertilization.
Fertilization- sperm and egg join to
produce a new cell.

Pea flowers are self-pollinating.
Sperm cells in pollen fertilize the egg cells
in the same flower.
The seeds that are produced by self-pollination
inherit all of their characteristics
from the single plant that bore them.

Mendel had true-breeding pea plants.
True-breeding plants, if allowed to
self-pollinate, produce offspring
identical to themselves.

Mendel wanted to produce seeds by joining
male and female reproductive cells from
two different plants.
He cut away the pollen-bearing male parts
of the plant and dusted the plant’s flower
with pollen from another plant.

This process is called
cross-pollination.
Cross-pollination
produces
seeds that
have two
different
parents.

Genes and Dominance
A trait is a specific characteristic
that varies from one individual to
another.

Genes and Dominance
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.

Genes and Dominance
Each original
pair of plants is
the P (parental)
generation.
The offspring are
called the F1, or
“first filial,”
generation.

Genes and Dominance
The offspring of crosses between
parents with different traits are
called hybrids.
The F1 hybrid plants all had the
character of only one of the
parents.

Mendel’s F1 Crosses on Pea Plants

Mendel’s F1 Crosses on Pea Plants
Mendel’s Seven F1 Crosses on Pea Plants

Genes and Dominance
Mendel's first conclusion was that
biological inheritance is determined
by factors that are passed from one
generation to the next. = genes!

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.

Genes and Dominance
The principle of dominance
states that some alleles are
dominant and others are
recessive.

Genes and Dominance
An organism with a dominant allele for a
trait will always exhibit that form of the trait.
An organism with the recessive allele for a
trait will exhibit that form only when the
dominant allele for that trait is not present.

Genes and Dominance
Pink x white = all pink
Pink is dominant allele
White is recessive allele

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.

Mendel's F2 Generation
P Generation F1 Generation
Segregation
Tall Short Tall Tall Tall Tall Tall Short
F2 Generation

Segregation
Mendel assumed that the dominant
allele masks the recessive allele in
the F1 generation.
The trait controlled by the recessive allele
showed up in some of the F2 plants.

Segregation
The reappearance of the recessive trait
showed that the allele for shortness had
been separated, or segregated, from the
allele for tallness.

Segregation
Mendel suggested that the alleles
segregate from each other during
the formation of the sex cells, or
gametes.

Segregation When each F1 plant flowers and
produces gametes, the two alleles
segregate from each other so that
each gamete carries only a
single copy of each gene.
Therefore, each F1 plant produces two
types of gametes—those with the
allele for tallness, and those with the
allele for shortness.

Segregation
Alleles separate during gamete formation.

11-2 Probability and Punnett Squares
11-2 Probability and Punnett Squares

Genetics and Probability
The likelihood that a particular
event will occur is called
probability.
Probability can be used to
predict the outcomes of
genetic crosses.

Punnett Squares
Punnett squares are
diagrams used to
predict and compare
the genetic variations
that will result from a
cross.

A capital letter represents
the dominant allele.
A lowercase letter
represents the
recessive allele.
In this example,
T = tall
t = short
Punnett Squares

Punnett Squares
Gametes
produced by each
parent are shown
along the top and
left side.

Punnett Squares
Possible gene
combinations for
the offspring
appear in the four
boxes.

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.

Punnett Squares
Homozygous organisms are true-breeding
for a particular trait.
Heterozygous organisms are hybrid
for a particular trait.

Punnett Squares
Offspring have the
same phenotype if
they show the
same trait.
Offspring have
the same
genotype if they
have the same
genetic makeup
(alleles).

Punnett Squares
The plants have
different
genotypes (TT
and Tt), but they
have the same
phenotype (tall).
TT
Homozygous
Tt
Active art Heterozygous

Probability and
Probability and Segregation
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).

Probability and
Segregation
The ratio of plants showing the
dominant phenotype (TT or Tt
genotype) to those showing the
recessive phenotype(tt genotype)
plants is 3:1.
The predicted ratio showed up in Mendel’s
experiments indicating that segregation did
occur.

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.

11-3 Exploring Mendelian Genetics
11–3 Exploring Mendelian Genetics

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.

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).
•All of the F1 offspring produced round
yellow peas (RrYy).

The alleles for round (R) and yellow (Y)
are dominant over the alleles for
wrinkled (r) and green (y).

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

For a two-factor F1 hybrid cross,
the Punnett square predicts a
9 : 3 : 3 :1 ratio in the F2
generation.

In Mendel’s experiment, the F2 generation
produced the following:
• 9 seeds that were round and yellow
• 3 seeds that were round and green
• 3 seeds that were wrinkled and yellow
• 1 seed that was wrinkled and green

The principle of independent
assortment states that alleles for
different traits can segregate
independently during the
formation of gametes.
Genes that segregate independently do not
influence each other's inheritance.

Mendel's experimental results were very
close to the 9 : 3 : 3 : 1 ratio predicted by
the Punnett square.
Mendel had discovered the principle of
independent assortment.
Independent assortment helps account
for the many genetic variations observed
in plants, animals, and other organisms.

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.

• 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.
A Summary of Mendel's
Principles

Beyond Dominant and
Recessive Alleles
Some alleles are neither dominant nor
recessive, and many traits are controlled
by multiple alleles or multiple genes.

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.

A cross between
red (RR) and
white (WW) four
o’clock plants
produces pink-colored
flowers
(RW).
Beyond Dominant and
Recessive Alleles
WW
RR

Beyond Dominant and
Recessive Alleles
•In codominance, both alleles
contribute to the phenotype
(like spots!).
• 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.

Beyond Dominant and
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.

Beyond Dominant and
Recessive Alleles
Different combinations of alleles result in
the colors shown here.
Full color: ACHIihmbininacolah:yi lclaacn : :Cc ccChhc,ch C, ccoccrhh c,c cChhc,c hoh ,r ocrc hCcc
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

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.

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.

Applying Mendel's
Principles
Mendel’s principles can be used to study
inheritance of human traits and to calculate
the probability of certain traits appearing in
the next generation.

Genetics and the
Environment
Genetics and the Environment
•Characteristics of any organism are
determined by the interaction between
genes and the environment.

11-4 Me1i1o-4s Miesiosis

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.

Chromosome Number
Chromosome Number
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.

Chromosome Number
Chromosomes come in
homologous pairs.
Homologous chromosomes
contain genes for the same traits.
Each of the 4 chromosomes that came from
the male parent has a homologous
chromosome from the female parent.

Chromosome Number
A cell that contains both sets of
homologous chromosomes is said
to be diploid (2N).
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.

Chromosome Number
The gametes of sexually reproducing
organisms contain only a single set of
chromosomes, and therefore only a single set
of genes.
Gametes are haploid (N), with only
one set of the homologous
chromosomes.
Haploid cells are represented by the symbol N.
For Drosophila, the haploid number is 4,
which can be written as N=4.

Phases of Meiosis
Meiosis is a process of division in which
the number of chromosomes per cell is
cut in half.
Diploid = 2 sets of each gene
Haploid = half = one of each
gene

•Meiosis involves two divisions,
meiosis I and meiosis II.
•The diploid cell that enters
meiosis becomes 4 haploid cells.
Phases of Meiosis
Movie

Phases of Meiosis
Meiosis I
Prophase I Metaphase I Anaphase I Telophase I
and
Cytokinesis
Interphase I
Meiosis I
Movie

Phases of Meiosis
Cells undergo a
round of DNA
replication, forming
duplicate
chromosomes.
Interphase I

Phases of Meiosis
During prophase of
meiosis 1, each
chromosome pairs
with its
corresponding
homologous
chromosome to
form a tetrad.
There are 4 chromatids
in a tetrad.
MEIOSIS I
Prophase I

Phases of Meiosis
Movie
•When tetrads form in prophase I,
they exchange portions of their
chromatids in a process called
crossing over.
• Crossing-over produces new combinations of alleles.

Phases of Meiosis
Spindle fibers attach
to the
chromosomes.
MEIOSIS I
Metaphase I

Phases of Meiosis
MEIOSIS I
Anaphase I The fibers pull the
homologous
chromosomes
toward opposite
ends of the cell.

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 parent cell.

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.

Meiosis II
Telophase I and Metaphase II Anaphase II
Cytokinesis I
Phases of Meiosis
Meiosis II
Telophase II
and
Cytokinesis
Prophase II
Movie

Phases of Meiosis
Meiosis I results in
two haploid (N)
daughter cells,
each with half the
number of
chromosomes as
the original cell.
MEIOSIS II
Prophase II

Phases of Meiosis
The chromosomes
line up in the center
of cell.
MEIOSIS II
Metaphase II

Phases of Meiosis
The sister
chromatids separate
and move toward
opposite ends of the
cell.
MEIOSIS II
Anaphase II

Phases of Meiosis
Meiosis II results in
four haploid (N)
daughter cells.
MEIOSIS II
Telophase II and Cytokinesis
Active art

Gamete Formation
Gamete Formation
In male animals, meiosis results
in four equal-sized gametes called
sperm.

Gamete Formation
In many female animals, one egg
and 3 polar bodies result from
meiosis. The three cells, called polar bodies, are
usually not involved in reproduction.

Comparing Mitosis and Meiosis
Mitosis results in the production of
two genetically identical diploid cells.
Meiosis produces four genetically
different haploid cells.

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.

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.

Gene Linkage
Thomas Hunt Morgan
researched the genetics of fruit
flies because they had many
offspring in a short time.

11-5 Linkage and Gene Maps
11-5 Linkage and Gene Maps

Gene Linkage
Gene Linkage
•Thomas Hunt Morgan’s research on fruit
flies led him to the principle of linkage.
•Morgan discovered that many of the
more than 50 Drosophila genes he had
identified appeared to be “linked”
together.
•They seemed to violate the principle of
independent assortment.

Morgan and his associates grouped the
linked genes into four linkage groups.
Each linkage group assorted independently
but all the genes in one group were
inherited together.
Each chromosome is actually a
group of linked genes ( a linkage
group).
Gene Linkage

Gene Linkage
Morgan concluded that Mendel’s principle of
independent assortment still holds true.
Chromosomes assort
independently, not individual genes.
Mendel did not observe gene
linkage because the genes he
studied happened to be on different
chromosomes.

Gene Maps
Gene Maps
•Crossing-over during meiosis
sometimes separates genes that had
been on the same chromosomes onto
homologous chromosomes.
•Crossover events can separate
linked genes and produce new
combinations of alleles.

Alfred Sturtevant, a student of Morgan,
reasoned that the farther apart two
linked genes are, the more likely
they are to be separated due to
crossover.
Recombination frequencies can be used to
determine the distance between genes.
Gene Maps

Sturtevant created a gene map of a
Drosophila chromosome. A gene map
shows the relative locations of each
known gene on a chromosome.
Gene Maps

If two genes are close together, the
recombination frequency between them
should be low, since crossovers are rare.
If they are far apart, recombination rates
between them should be high.
Gene Maps

Exact location on chromosome Chromosome 2
0.0
Gene Maps
Aristaless (no bristles on antenna)
13.0 Dumpy wing
48.5 Black body
54.5 Purple eye
67.0 Vestigial (small) wing
99.2 Arc (bent wings)
107.0 Speck wing

Exact location on chromosome Chromosome 2
1.3 Star eye
31.0 Dachs (short legs)
51.0 Reduced bristles
55.0 Light eye
75.5 Curved wing
104.5 Brown eye
Gene Maps

11-1
Gametes are also known as
a. genes.
b. sex cells.
c. alleles.
d. hybrids.

11-1
The offspring of crosses between parents with
different traits are called
a. alleles.
b. hybrids.
c. gametes.
d. dominant.

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.

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.

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.

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.

11-2
Organisms that have two different alleles for a
particular trait are said to be
a. hybrid.
b. heterozygous.
c. homozygous.
d. recessive.

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%

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.

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.

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.

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.

11–3
Mendel's principles apply to
a. pea plants only.
b. fruit flies only.
c. all organisms.
d. only plants and animals.

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.

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.

11-4
The formation of a tetrad occurs during
a. anaphase I.
b. metaphase II.
c. prophase I.
d. prophase II.

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.

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.

11-5
According to Mendel's principle of independent
assortment, the factors that assort independently
are the
a. genes.
b. chromosomes.
c. chromatids.
d. gametes.

11-5
Linkage maps can be produced because the
farther apart two genes are on a
chromosome,
a. the less likely they are to assort
independently.
b. the more likely they are to be linked.
c. the more likely they are to be separated by a
crossover.
d. the less likely they are to be separated by a
crossover.

11-5
If two genes are close together on the same
chromosome, they are more likely to
a. behave as though they are linked.
b. behave independently.
c. move to different chromosomes.
d. belong to different linkage groups.

Chapter Eleven- Intro to Genetics

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Chapter Eleven- Intro to Genetics