Download Chapter 3 Section 1

Chapter 3 Section 1
Mendel’s Work
Gregor Mendel

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Young priest that worked in
the garden at a monastery
in Vienna.
Considered the “Father of
Genetics”
Mendel’s Work - Vocabulary
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Traits
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Heredity
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A physical characteristic that an organism can
pass on to its offspring through its genes.
Passing of traits from parent to offspring
Genetics
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Scientific study of heredity
Mendel’s Experiments
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What type of plant did Gregor Mendel use
for most of his studies?
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The Pea Plant
Mendel’s Peas
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Peas are good plants to use for
genetic studies because:
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Many traits exist only in two forms
(i.e. tall and short) And….
Peas produce many offspring in one
generation making it is easy to
collect and analyze large amounts of
data.
How did Mendel make his crosses? – Basics First..
Pea plants reproduce by the same method as most plants. The
pollen (male reproductive part of a plant) is transferred to the
pistil (female reproductive part of a plant) and the ovary is
fertilized producing a fruit. Pollen can be transferred by many
methods such as wind, water and bees.
Pollen
How did Mendel make his crosses?
1. Mendel transferred the pollen from one plant to the pistil of
another plant. For example, pollen from a tall plant was used
to fertilize the pistil of a short plant essentially making the
father plant tall, and the mother plant short.
2. In order to prevent self pollination, Mendel removed the
pollen producing parts of the short plant
Tall Plant
Short Plant
Mendel’s Experiments
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Purebred Plant
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One that always produces offspring with
the same form of a trait as the parent
For example….
Purebred pink plants will always produce pink plants
X
White flower would mean the parents are not purebred
X
Mendel’s Experiments

How did Mendel make sure he had
purebred plants to make his crosses?

By allowing pea plants of one particular
trait self pollinate for several generations.
Mendel’s Experiments

In Mendel’s first experiment he crossed
PUREBRED TALL plants with PUREBRED
SHORT plants.
 These plants were called the parental
generation or the P generation
 The offspring of this cross was called the
first filial generation or the F1 generation.
 After the F1 plants self pollinated they
produced the second filial generation or
the F2 generation
Mendel’s Experiments
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What types of plants (Short/Tall) did Mendel
observe in the F1 generation?
 All Tall plants
What types of plants (Short/Tall) did Mendel
observe in the F2 generation?
 A mix of short and tall plants.
 The ratio of tall to short was 3 to 1 or 75%
tall and 25% short.
Mendel’s Experiments

What conclusions did Mendel arrive at from
his experiments with peas?
 Some sort of “factor” controls the
inheritance of traits in pea plants.
 These “factors” exist in pairs with the
female contributing one factor and the
male contributing the other.
 One factor can “mask” the presence of the
other.
Mendel’s Experiments

What modern words do we now use for
Mendel’s discoveries.
 Mendel said “factors.” Today scientist call
these factors that control traits genes.
 Mendel concluded that these genetic
factors exist in pairs. Today we call these
two forms of a gene alleles.
 Mendel realized that one factor in the pair
can mask the other. Today we know these
as dominant alleles and recessive alleles.
Mendel’s Work - Vocabulary
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Dominant Allele
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Recessive Allele
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One whose trait always shows up in the organism
when the allele is present.
One that is masked or covered up when the
dominant allele is present. Recessive traits only
show up when two recessive alleles are present.
Hybrid

An organism that has two different alleles for a
trait.
Mendel’s Experiments
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Other “opposite” traits that Mendel studied (Figure
3 on page 83).
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Seed Shape – round (dominant) wrinkled (recessive)
Seed Color – yellow (dominant) green (recessive)
Seed Coat Color – gray (dominant) white (recessive)
Pod Shape – smooth (dominant) pinched (recessive)
Pod Color– green (dominant) yellow (recessive)
Flower Position– side (dominant) end (recessive)
Dominant and Recessive Alleles
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In modern genetics scientist represent the
dominant allele with a capitol letter and
the recessive allele with the lower case
version of the same letter.
The same letter is used for the dominant
and the recessive allele. For example….
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Tall plants would be represented by T
Short plants would be represented by t
Dominant and Recessive Alleles
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Assigning the letter to the allele is
random. However scientist often use the
dominant trait. Examples…
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Seed color:
Seed coat color:
Seed shape:
Yellow Y
Gray G
Round R
Green y
White g
Wrinkled r
Dominant and Recessive Alleles

Genes exist in pairs and are made up of two alleles. One
allele comes from mom and the other allele comes from dad.
Assign the following allele combinations.
 A plant that inherits two alleles for Yellow seed color YY
 A plant that inherits one allele for Yellow seed color and
one allele green seed color Yy
 A plant that inherits two alleles for wrinkled seeds rr
 A plant that inherits one allele for round seed shape and
one allele wrinkled seed shape Rr
 A plant that inherits two alleles gray seed coat color GG
Phenotypes and Genotypes
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Phenotype
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Genotype
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Physical appearance or visible trait
Example is flower color, stem height etc.
The genetic makeup or allele combination
Examples are TT or Tt
Genotypes determine phenotypes
Phenotypes and Genotypes
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Figure 9 on the top of page 92
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Phenotypes are Tall and Short
Genotypes for tall plants are TT and Tt
Only genotype that can produce a short plant is tt
Homozygous and Heterozygous
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Homozygous
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An organism that has two identical alleles for a trait
TT – homozygous dominant
tt – homozygous recessive
Heterozygous
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An organism that has two different alleles for a trait
Tt – heterozygous – will show the dominant trait
Homozygous and Heterozygous

What term did Mendel use that means the
same thing as heterozygous?
 Hybrid