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Chapter 4
Section 1 Mendel and His Peas
Objectives
Content
Explain the relationship between traits and heredity.
Explain the difference between dominant and
recessive traits.
Language
Describe the experiments of Gregor Mendel.
Chapter 4
Section 1 Mendel and His Peas
Who Was Gregor Mendel?
• Gregor Mendel was born in 1822 in
Heinzendorf, Austria.
• At age 21, Mendel entered a monastery.
He performed many scientific
experiments in the monastery garden.
• Mendel discovered the principles of
heredity, the passing of traits from
parents to offspring.
Chapter 4
Section 1 Mendel and His Peas
Unraveling the Mystery
• Mendel used garden pea plants for
his experiments.
• Self-Pollinating Peas have both male
and female reproductive structures. So,
pollen from one flower can fertilize the
ovule of the same flower.
• When a true-breeding plant self
pollinates, all of the offspring will have
the same trait as the parent.
Chapter 4
Section 1 Mendel and His Peas
• Pea plants can also cross-pollinate.
Pollen from one plant fertilizes the ovule
of a flower on a different plant.
• The image below shows crosspollination and self-pollination.
Chapter 4
Section 1 Mendel and His Peas
• Characteristics Mendel
studied only one pea
characteristic at a time. A
characteristic is a feature
that has different forms in a
population.
• Different forms
of a characteristic
are called traits.
Chapter 4
Section 1 Mendel and His Peas
• Mix and Match Mendel was
careful to use plants that were
true breeding for each of the traits
he was studying. By doing so, he
would know what to expect if his
plants were to self-pollinate.
Chapter 4
Section 1 Mendel and His Peas
Mendel’s First Experiments
• Mendel crossed pea plants to study
seven different characteristics.
• Mendel got similar results for each
cross. One trait was always present
in the first generation, and the other
trait seemed to disappear.
• Mendel called the trait that
appeared the dominant trait. The
trait that seemed to fade into the
background was called the
recessive trait.
Chapter 4
Section 1 Mendel and His Peas
Mendel’s Second
Experiments
• To find out more about
recessive traits, Mendel
allowed the first-generation
plants to self-pollinate.
• In each case some of the
second-generation plants
had the recessive trait.
Chapter 4
Section 1 Mendel and His Peas
• Ratios in Mendel’s Experiments The recessive
trait did not show up as often as the dominant trait.
• Mendel decided to figure out the ratio of dominant
traits to recessive traits.
Chapter 4
Section 1 Mendel and His Peas
In all cases the ratio
was about 3:1
dominant : recessive.
Chapter 4
Section 1 Mendel and His Peas
Mendel’s Second Experiments
• Gregor Mendel – Gone But Not Forgotten
Mendel realized that his results could be explained
only if each plant had two sets of instructions for
each characteristic.
• Mendel’s work opened the door to modern
genetics.
Chapter 4
Section 2 Traits and Inheritance
Bellringer
If you flip a coin, what are the chances that it will land
on heads? tails? Suppose that you flipped the coin
and got heads. What are the chances that you will get
heads again?
Record your answers in your science journal.
Chapter 4
Section 2 Traits and Inheritance
Objectives
• Explain how genes and alleles are related to
genotype and phenotype.
• Use the information in a Punnett square.
• Explain how probability can be used to predict
possible genotypes in offspring.
• Describe three exceptions to Mendel’s
observations.
Chapter 4
Section 2 Traits and Inheritance
A Great Idea
• Mendel knew that there must be two sets of
instructions for each characteristic.
• The instructions for an inherited trait are called
genes.
• The different forms (often dominant and recessive)
of a gene are alleles.
• Phenotype An organism’s appearance is known as
its phenotype. Genes affect the phenotype.
Chapter 4
Section 2 Traits and Inheritance
A Great Idea, continued
• Genotype The combination of inherited alleles
together form an organism’s genotype.
• Punnett Squares are used to organize all the
possible genotype combinations of offspring from
particular parents.
Chapter 4
Section 2 Traits and Inheritance
Chapter 4
Section 2 Traits and Inheritance
What Are the Chances?
• Probability is the mathematical chance that
something will happen.
• Probability is most often written as a fraction of
percentage.
Chapter 4
Section 2 Traits and Inheritance
Calculating Probability
Chapter 4
Section 2 Traits and Inheritance
What Are the Chances?, continued
• Genotype Probability To have white flowers, a pea
plant must receive a p allele from each parent. Each
offspring of a Pp Pp cross has a 50% chance of
receiving either allele from either parent. So, the
probability of inheriting two p alleles is 1/2  1/2,
which equals 1/4, or 25%.
Chapter 4
Section 2 Traits and Inheritance
More About Traits
• Incomplete Dominance Researchers have found
that sometimes one trait is not completely dominant
over another.
• One Gene, Many Traits Sometimes one gene
influences more than one trait.
• Many Genes, One Trait Some traits, such as the
color of your skin, hair, and eyes, are the result of
several genes acting together.
Chapter 4
Section 2 Traits and Inheritance
More About Traits, continued
• The Importance of Environment Genes aren’t
the only influences on traits. A combination of things
determine an individual’s characteristics.
• Your environment also influences how you grow.
• Lifestyle choices can also affect a person’s traits.
Chapter 4
Section 3 Meiosis
Bellringer
Write a sentence to describe each of the following
terms: heredity, genotype, and phenotype. Note
how genotype and phenotype are related, and
how they are different. Is heredity necessarily a
factor in both genotype and phenotype? Why or
why not?
Record your answers in your science journal.
Chapter 4
Section 3 Meiosis
Objectives
• Explain the difference between mitosis and meiosis.
• Describe how chromosomes determine sex.
• Explain why sex-linked disorders occur in one sex
more often than in the other.
• Interpret a pedigree.
Chapter 4
Section 3 Meiosis
Asexual Reproduction
• In asexual reproduction, only one parent cell is
needed. The structures inside the cell are copied, and
then the parent cell divides, making two exact copies.
• This type of cell reproduction is called mitosis. Most
of the cells in your body and most single-celled
organisms reproduce this way.
Chapter 4
Mitosis
Section 3 Meiosis
Chapter 4
Section 3 Meiosis
Sexual Reproduction
• In sexual reproduction, two parent cells (sex cells)
join together to form offspring that are different from
both parents.
• Chromosomes that carry the same sets of genes
are called homologous chromosomes.
• Each sex cell has only one of the chromosomes
from the homologous pair.
Chapter 4
Section 3 Meiosis
Sexual Reproduction, continued
• Meiosis Sex cells are made during meiosis.
• Meiosis is a copying process that produces
cells with half the usual number of
chromosomes.
Chapter 4
Section 3 Meiosis
Meiosis
Click below to watch the Visual Concept.
Visual Concept
Chapter 4
Section 3 Meiosis
Sexual Reproduction, continued
• Genes and Chromosomes Walter Sutton
studied meiosis in sperm cells in grasshoppers.
• Using his observations and his knowledge of
Mendel’s work, Sutton proposed that:
Genes are located on chromosomes.
Chapter 4
Section 3 Meiosis
The Steps of Meiosis
• During meiosis, chromosomes are copied once, and
then the nucleus divides twice.
• The resulting sex cells (sperm and eggs) have half
the number of chromosomes of a normal body cell.
Chapter 4
Section 3 Meiosis
Chapter 4
Section 3 Meiosis
Chapter 4
Section 3 Meiosis
Meiosis and Mendel
• The steps of meiosis explain Mendel’s results.
The following slide shows what happens to a pair
of homologous chromosomes during meiosis and
fertilization.
Chapter 4
Section 3 Meiosis
Chapter 4
Section 3 Meiosis
Meiosis and Mendel, continued
• Sex Chromosomes
carry genes that
determine sex.
• Human females have
two X chromosomes.
• Human males have
one X chromosome and
one Y chromosome.
Chapter 4
Section 3 Meiosis
Meiosis and Mendel, continued
• Sex-Linked Disorders The genes for certain
disorders, such as colorblindness, are carried on the
X chromosome.
• Genetic Counseling Genetic counselors use
pedigrees to trace traits through generations of a
family. These diagrams can often predict if a person
is a carrier of a hereditary disease.
• Selective Breeding In selective breeding,
organisms with desirable characteristics are mated.
Chapter 4
Heredity
Concept Mapping
Use the terms below to complete the concept map on
the next slide.
alleles
parents
phenotype
genes
offspring
genotype
characteristics
dominant
Chapter 4
Heredity
Chapter 4
Heredity