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Genetics
Genetics is the
scientific study of
heredity.
Chapter 11
Sections 1-3
The Work of Mendel
1822 – Gregor Mendel used
purebred pea plants to study the
inheritance of traits.
 Fertilization – the process in sexual
reproduction in which male & female
reproductive cells join to form a new
cell.
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Mendel’s Experiment with
Pea Plants
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Mendel used the pea flower because they
have both male & female parts.
Self-pollinating – sperm cell in pollen
fertilize the egg cells in the same flower.
The offspring inherit characteristics from
a single parent. Ex. Pea Plant flowers
True-breeding- the peas plant flowers
were self-pollinating and could produce
offspring identical to themselves.
Experiment I: Pea Plant
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Mendel had to prevent self-pollination so
he made a controlled cross between
plants. He cut away the male parts &
dusted pollen from another plant on the
flower.
Cross-pollination – seeds produced are
the offspring of 2 different parents.
Mendel studied a few traits, or specific
characteristics of the pea plant such as
color & height.
Mendel Studies 7 Traits /
Characters pg. 264
Seed Shape
 Seed Color
 Seed Coat Color
 Pod Shape
 Pod Color
 Flower Position
 Plant Height
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Genes and Dominance
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Mendel crossed 2 plants with different
characters, or forms, for the same trait.
Ex. 1 tall & 1 short
The plants that grew were hybrid.
Hybrid – are the offspring of crosses
between parents with different traits.
Parent plant – P generation
First set of offspring – F1 generation
Experiment I Concluded:
Traits are inherited through the passing
of factors from parents to offspring.
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The hybrid plants looked like only 1
parent and the character of the other
parent seemed to disappear.
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Each trait is controlled by 1 gene.
Alleles – controls the different forms of a
gene.
Genes – chemical factors that determine
traits.

Principle of Dominance
States that some alleles are
dominant & others are recessive.
 Whenever a living thing inherits a
dominant allele, that trait is visible.
 The effects of a recessive allele are
not seen if the dominant allele is
present.
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Experiment II: Segregation
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Mendel crossed a tall plant
(dominant) with a short plant
(recessive), the F1 plant inherited an
allele for tallness from the tall
parent & an allele for shortness from
the short parent. Pg. 265
What happened to the
recessive allele?
Mendel allowed his hybrid plants to
self-pollinate.
 Some showed recessive traits, the
recessive traits did not disappear.
 Earlier, the dominant masked the
recessive, so it was not visible.
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Experiment II Conclusion
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Alleles for the same trait can be
separated.
Segregation – When sex cells, or
gametes, are formed. Each gamete
carries only 1 copy of each gene.
Therefore, each F1 plant produces 2 types
of gametes (some with an allele for
tallness & some with an allele for
shortness). Ex. T, t, T, t = TT, Tt, Tt, tt
Probability & Punnett
Squares
Probability – the likelihood that a
particular event will occur.
 Ex. Flipping a coin. The probability
that it will land on tails is ½.

The Principles of Probability
It is used to predict the traits of the
offspring produced by genetic
crosses.
 Mendel used a Punnett Square to
show the gene combinations that
might result from a genetic cross.
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Punnett Squares:
The F1 parent is shown along the top
& left sides.
 The possible F2 offspring are in the 4
boxes that make up the square.
 Letters represent alleles.
 T is dominant allele for tallness.
 T is recessive allele for shortness.
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Pg. 268
The Punnett Square show:
All the possible results of a genetic
cross.
 The genotypes of the offspring.
 The alleles in the gametes of each
parent.
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Punnett Squares
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Homozygous – organisms that have 2
identical alleles for a particular trait; they
are true-breeding. Ex. TT or tt
Heterozygous – organisms that have 2
different alleles for the same trait; they
are hybrid. Pg. 269
Genotype – genetic make up.
Phenotype – Physical characteristics;
don’t have the same genotype.
Probability of Segregation
Pg. 268-269
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¼ (25%) of F2 plants have 2 alleles for
tallness (TT).
½ (50%) of F2 plants have 1 allele for
tallness & 1 allele for shortness (Tt).
¾ (75%) of F2 plants are tall because the
allele for tallness is dominant over allele
for shortness.
Overall, there are 3 tall for every 1 short
plant in F2 generation; Ratio of 3:1
Independent Assortment
states that during gamete
formation, genes for different traits
separate with out influencing each
other’s inheritance.
 Helps account for genetic variations.
 Ex. Pea shape & pea color.
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Experiment III
The Two-Factor Cross: F1
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Mendel wanted to see if genes that
determine 1 trait have anything to do
with genes that determine another.
He followed 2 different genes as they
passed from one generation to the next.
Mendel crossed true-breeding plants round yellow peas (RRYY) with wrinkled
green peas (rryy).
The F1 offspring were all round & yellow
showing that both were dominant alleles.
The genotype is RrYy. Pg. 270
Experiment III
The Two-Factor Cross: F2
The F1 plants were all heterozygous
with a genotype of RrYy.
 The F1 plants were segregated to
each other to produce an F2
generation.
 Mendel compared the variations in
the seeds.
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Experiment III Conclusion
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Mendel found some offspring that
were:
• Round / yellow (P phenotype)
• Wrinkled / green (P phenotype)
• Many with combinations of alleles (not
found in either parent)
• This showed that the alleles for seed shape
segregated independently of those for seed
color (independent assortment). Pg. 271
Summary of Mendel’s
Principles
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Genes are passed from parents to their
offspring.
Some forms of the gene may be
dominant and others may be recessive.
Each adult has 2 copies of each gene –
one from each parent. The genes are
segregated from each other when
gametes are formed.
Alleles for different genes usually
segregate independently of one another.
Different Patterns of Dominant &
Recessive
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Incomplete Dominance – 1 allele
is not completely dominant over
another. Ex. Red flower (RR) & white flower (WW), F1
is a pink flower (RW). Pg. 272
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Codominance – both alleles
contribute to the phenotype.
Ex. A cross of a black chicken (BB) with a white chicken
(WW) will produce all speckled offspring (BBWW); colors
appear separately.
Different Patterns of Dominant &
Recessive
Multiple Alleles – have more than
2 alleles. Ex. Coat color in rabbits.
 Polygenic Traits – traits controlled
by 2 or more genes; “having many
genes”. Ex. Variation in human skin color.
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Thomas Hunt Morgan
Geneticist who looked for a model
organism to use in studying genetics
 He chose the fruit fly because it
produces large numbers of offspring.
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Genes & the Environment
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Mendel’s principles apply to all organisms not
just plants.
Characteristics of any organism are not
determined solely by genes it inherits.
Characteristics are determined by interaction
between genes and the environment.
Ex. Genes affect the height and color of a sunflower but the same
characteristics are influenced by climate, soil, conditions, and
availability of water.
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Genes provide a plan for development, but how
the plan unfolds depends on the environment.