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11-1 The Work of Gregor Mendel11-1 The Work of Gregor
Mendel
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Do Now
• Turn in your meiosis worksheet.
• Write down your homework.
• Clear your desk for the quiz.
Copyright Pearson Prentice Hall
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11-1 The Work of Gregor Mendel
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Gregor Mendel’s Peas
Genetics is the scientific study of heredity.
• Gregor Mendel studied heredity using ordinary
garden peas.
Gregor Mendel’s Peas
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11-1 The Work of Gregor Mendel
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Gregor Mendel’s Peas
Mendel knew that
• the male part of each flower produces pollen, (containing sperm).
• the female part of the flower produces egg cells.
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11-1 The Work of Gregor Mendel
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Gregor Mendel’s Peas
During sexual reproduction, sperm and egg cells join
in a process called fertilization.
Fertilization produces a new cell.
Pea flowers are self-pollinating.
- offspring result from fertilization
within one plant
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11-1 The Work of Gregor Mendel
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Gregor Mendel’s Peas
Mendel had true-breeding pea plants that, if allowed
to self-pollinate, would produce offspring identical to
themselves.
Cross-pollination
- fertilizing sperm and
egg cells using two
different plants
- produced seeds that
had two different
parents.
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11-1 The Work of Gregor Mendel
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Genes and Dominance
Genes and Dominance
A trait is a specific characteristic that varies from one individual to another.
Mendel studied seven pea plant traits, each with two contrasting characteristics.
He crossed plants with each of the seven contrasting characteristics and studied their offspring.
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11-1 The Work of Gregor Mendel
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Genes and Dominance
Each original pair of plants is the P (parental)
generation.
The offspring are called the F1, or “first filial,”
generation.
The offspring of crosses between parents with
different traits are called hybrids.
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11-1 The Work of Gregor Mendel
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Genes and Dominance
Mendel's first conclusion
was that biological inheritance is determined by
factors that are passed from one generation to the
next.
Today, scientists call the factors that determine traits
genes.
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11-1 The Work of Gregor Mendel
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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.
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11-1 The Work of Gregor Mendel
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Genes and Dominance
The principle of dominance states that
some alleles are dominant and others are
recessive.
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Genes and Dominance
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11-1 The Work of Gregor Mendel
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Do Now:
• Write your homework down.
• Turn your homework into bin (Ch 11-1)
• Complete the following problem in your notebook.
A tall pea plant is crossed with a short pea plant:
The offspring (F1) is tall.
Which gene is dominant and which is recessive?
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11-1 The Work of Gregor Mendel
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Mendel's F2 Generation
P GenerationF1 Generation
Tall Tall Tall Tall Tall TallShort Short
F2 Generation
Segregation
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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.
Why do you think that happened?
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Segregation
The reappearance of the trait controlled by the
recessive allele indicated that at some point the allele
for shortness had been separated, or segregated,
from the allele for tallness.
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11-1 The Work of Gregor Mendel
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Segregation
Mendel suggested that the alleles for tallness and
shortness in the F1 plants segregated from each
other during the formation of the sex cells, or
gametes.
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11-2 Probability and Punnett Squares11-2 Probability and Punnett
Squares
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11-2 Probability and Punnett Squares
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Genetics and Probability
Genetics and Probability
The likelihood that a particular event will occur is
called probability.
The principles of probability can be used to
predict the outcomes of genetic crosses.
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11-2 Probability and Punnett Squares
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Punnett Squares
Punnett Squares
The gene combinations that might result
from a genetic cross can be determined by
drawing a diagram known as a Punnett
square.
Punnett squares can be used to predict
and compare the genetic variations that
will result from a cross.
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11-2 Probability and Punnett Squares
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A capital letter
represents the
dominant allele for tall.
A lowercase letter
represents the
recessive allele for
short.
In this example,
T = tall
t = short
Punnett Squares
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11-2 Probability and Punnett Squares
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Gametes produced by
each F1 parent are
shown along the top
and left side.
Punnett Squares
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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.
Homozygous organisms are true-breeding for a
particular trait.
Heterozygous organisms are hybrid for a particular
trait.
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Punnett Squares
All of the tall plants have the same phenotype, or
physical characteristics.
The tall plants do not have the same genotype, or
genetic makeup.
One third of the tall plants are TT, while two thirds of
the tall plants are Tt.
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Punnett Squares
The plants have
different genotypes
(TT and Tt), but they
have the same
phenotype (tall).
TT
Homozygous
Tt
Heterozygous
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Probability and
Segregation
Probability and 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).
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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.
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11-2
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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.
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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.
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11-2
Organisms that have two different alleles for a
particular trait are said to be
a. hybrid.
b. heterozygous.
c. homozygous.
d. recessive.
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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%
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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.
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11-3 Exploring Mendelian Genetics11–3 Exploring Mendelian
Genetics
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11–3 Exploring Mendelian Genetics
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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. (a
dihybrid cross)
Independent Assortment
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11–3 Exploring Mendelian Genetics
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Independent Assortment
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).
RRYY x rryy
All of the F1 offspring produced round yellow
peas (RrYy).
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Independent Assortment
The alleles for round (R) and yellow (Y) are
dominant over the alleles for wrinkled (r) and
green (y).
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Independent Assortment
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
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Independent Assortment
The Punnett square predicts a 9 : 3 : 3 :1 ratio in the
F2 generation.
Represents:
Independent Assortment
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11–3 Exploring Mendelian Genetics
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The alleles for seed shape segregated independently
of those for seed color. This principle is known as
independent assortment.
Genes that segregate independently do not influence
each other's inheritance.
Independent Assortment
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The principle of independent assortment
states that genes for different traits can
segregate independently during the
formation of gametes.
Independent assortment helps account for
the many genetic variations observed in
plants, animals, and other organisms.
Independent Assortment
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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.
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• 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
