Download Chapter 11 Section 11_1 The Work of Gregor Mendel

Chapter 11: Introduction to Genetics
11.1 – The Work of Gregor Mendel
Mr. M. Varco
Saint Joseph High School
The Experiments of Mendel
• What is inheritance?
The Experiments of Mendel
• Every living thing has a set of characteristics inherited from its parent or
parents.
• The delivery of characteristics from parent to offspring is called heredity.
• The scientific study of heredity, known as genetics, is the key to
understanding what makes each organism unique.
The Experiments of Mendel
• The modern science of genetics was founded by an Austrian monk named
Gregor Mendel.
• Mandel carried out his work using garden peas because a single pea plant
could produce hundreds of offspring in a short period of time.
The Experiments of Mendel
The Role of Fertilization
• Mendel knew that the male part of each flower makes pollen, which
contains sperm - the plant’s male reproductive cell.
The Experiments of Mendel
The Role of Fertilization
• Mendel knew that the female portion of each flower produces reproductive
cells called eggs.
The Experiments of Mendel
The Role of Fertilization
• During sexual reproduction, male and female reproductive cells join in a
process called fertilization, to produce a new cell.
The Experiments of Mendel
The Role of Fertilization
• Pea flowers are normally self-pollinating, meaning that sperm cells fertilize
egg cells from within the same flower.
• A plant grown from seed produced by self-pollination inherits all of its
characteristics from a single parent.
The Experiments of Mendel
The Role of Fertilization
• Mendel’s garden had several “true-breeding” pea plants, meaning they
were self-pollinating and would produce genetically identical offspring.
• A trait is a specific characteristic of an individual, and may vary from one
individual to another.
The Experiments of Mendel
The Role of Fertilization
• Mendel decided to “cross” his stocks of true-breeding plants - he caused
one plant to reproduce with another plant.
The Experiments of Mendel
The Role of Fertilization
• He prevented self-pollination by cutting the pollen-bearing male parts of
each flower, and dusting pollen from another plant onto the female part of
that flower.
The Experiments of Mendel
The Role of Fertilization
• This process, known as cross-pollination, produced a plant that has two
different parents.
• Cross pollination allowed Mendel to breed plants with traits different from
those of their parents and then study the results.
The Experiments of Mendel
The Role of Fertilization
• Mendel studied 7 different traits of pea plants, each of which had two
contrasting characteristics.
• The offspring of crosses between parents with different traits are called
hybrids
The Experiments of Mendel
• When doing genetic crosses, we called each original pair of plants the
parental, or P generation.
• Their offspring are called the F1, or filial generation.
The Experiments of Mendel
Conclusion #1
• An individual’s characteristics are determined by factors that are passed
down from one parental generation to the next.
• These “factors” are known as genes
The Experiments of Mendel
Conclusion #1
• Each trait is controlled by a single gene that occurred in two different forms.
These forms are called alleles.
• The expression of a trait is dictated by the combination of alleles for a
particular gene.
The Experiments of Mendel
Conclusion #2
• The Principle of Dominance states that some alleles are dominant and
others are recessive.
• An organism with at least one dominant allele for a particular trait will
express that form of the trait.
The Experiments of Mendel
Conclusion #2
• An organism with a recessive allele for a particular form of a trait will exhibit
that form only when the dominant allele for the trait is absent.
Segregation
• What happened to the recessive alleles from the F1 generation?
• To determine what happened to the recessive alleles, Mendel allowed all 7
kinds of F1 hybrids to self-pollinate.
• The offspring of an F1 cross are called
the F2 generation.
Segregation
• The traits controlled by the recessive allele reappeared in the second
generation.
• Approximately 25% of the F2 plants showed the trait controlled by the
recessive allele
Segregation
• Mendel assumed that a dominant allele had masked the recessive allele in
the F1 generation.
• The reappearance of the recessive allele in the F2 generation indicated
that at some point, the allele for shortness separated from the allele for
tallness.
• How did this segregation occur?
Segregation
• Mendel assumed that the two alleles segregated from each other during
the formation of the sex cells, or gametes.
• During gamete formation, the two alleles for each gene segregated from
each other, so that each gamete carries only one allele for each gene.
Segregation
• A capital letter represents a dominant allele; a lower-case letter represents
a recessive allele.
• Whenever a gamete carrying the “t” allele
paired with the other gamete that carried
the “t” allele to produce the F2 plant, that
plant was short.