Section 2: Mendelian Genetics
Mendel explained how a dominant allele can mask the presence
of a recessive allele.
Essential Questions
•
•
•
What is the significance of Mendel’s experiments to the study of genetics?
What is the law of segregation and the law of independent assortment?
What are the possible offspring from a cross using a Punnett square?
Vocabulary
Review
New
•
1. genetics
2. allele
3. dominant
4. recessive
5. homozygous
6. heterozygous
7. genotype
8. phenotype
9. law of segregation
10. hybrid
11. law of independent assortment
segregation
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Mendelian Genetics
Genetic traits
Main idea
•
Do all dogs look alike?
•
What type of features indicate a
particular breed?
•
Are these feature inherited?
•
Can you tell individuals within a
breed apart?
•
What does this tell you about the
inheritance of this feature?
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Mendelian Genetics
How Genetics Began
•
The passing of traits to the next generation is called
inheritance, or heredity.
•
Gregor Mendel published his findings on the method of
inheritance in garden pea plants:
• Pea plants are true-breeding- they consistently
produced offspring with only one form of trait
• Cross-pollinated pea plants (transferred a male
gamete from one flower to the female reproductive
organ of another flower), which normally self-fertilize
(male gamete fertilizes female gamete in the same
flower)
• He rigorously followed various traits in the pea plants he
bred, forming a hypothesis on the traits that were
inherited.
•
Mendel began the study of genetics, the science of heredity.
•
Reading check: Infer why is it important that Mendel’s
experiments used a true breeding plant.
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Mendelian Genetics
Correct
book info
The Inheritance of Traits
•
One trait Mendel noticed was seed color – some plants always produced
green seeds, and others always produced yellow seeds.
•
Mendel cross-bred the green and yellow seed plants.
•
–
Transferring male gametes of the yellow seed to the female of the green seed
–
Removed the male organ of the green seed
Mendel called the green-seed and yellow-seed plants the parent, or P,
generation.
Green seed
yellow
seed
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Mendelian Genetics
The Inheritance of Traits
F1 and F2 generations
•
The offspring of this P cross are called
the first filial (F1) generation.
•
The second filial (F2) generation is the
offspring from the F1 cross.
– Mendel allowed the plants to grow and
self-fertilize
– The F2 generation showed a 3:1 ratio of
yellow: green seeds
•
In Mendel’s peas, the green-seed trait
disappeared in the F1 generation, but
reappeared in the F2 generation.
Why the seeds in the F1
generation were all yellow?
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Mendelian Genetics
Match:
______inheritance, or heredity
______true-breeding
______Cross-pollinated
1. male gamete fertilizes female gamete in the
same flower
2. The passing of traits to the next generation
______self-fertilize
3. pea plants that consistently produced
offspring with only one form of trait
______genetics
4. the science of heredity.
5. transferred of a male gamete from one
flower to the female reproductive organ of
another flower
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Mendelian Genetics
Allele Frequencies
Go to your ConnectEd resources to play Animation: Allele
Frequencies.
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Mendelian Genetics
Heredity
Go to your ConnectEd resources to play BrainPOP: Heredity.
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Mendelian Genetics
The Inheritance of Traits
F1 and F2 generations
•
Mendel studied seven different traits.
• Seed or pea color
• Flower color
• Seed pod color
• Seed pod shape
• Seed shape or texture
• Stem length
• Flower position
•
In all cases, Mendel found the F2 generation
plants showed a 3:1 ratio of traits.
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Mendelian Genetics
The Inheritance of Traits
Genes in Pairs
•
Mendel concluded that there must be two forms of the seed trait in the pea
plants, and that each was controlled by a factor.
•
An allele is an alternative form of a single gene.
• The gene for yellow seeds and the gene for green seeds are different alleles
for the same gene.
• Mendel concluded that the 3:1 ratio could be explained if the alleles were
paired in each of the plants.
• Dominant alleles controlled the traits that appeared in the F1 generation.
• Recessive alleles were masked in the F1 generation.
•
Which do you think are better or more desirable- dominant genes or
recessive genes?
Copyright © McGraw-Hill Education
Mendelian Genetics
• Polydactyly. Polydactyly
is an inherited condition in
which a person has extra
fingers or toes. It is caused
by a dominant allele of a
gene. This means it can be
passed on by just one
allele from one parent if
they have the disorder.
• It is uncommon because
most people are
homozygous for the
recessive gene
polydactyly genetics
The Inheritance of Traits
Read first paragraph page 279
Dominance
•
When modeling inheritance, the dominant allele is represented by a capital
letter (Y), and a recessive allele is represented with a lowercase letter (y).
•
An organism with two of the same alleles for a particular trait is homozygous
for that trait (YY or yy).
•
An organism with two different alleles for a particular trait is heterozygous for
that trait (Yy).
•
In heterozygous individuals, the dominant trait will be observed.
Copyright © McGraw-Hill Education
Mendelian Genetics
The Inheritance of Traits
Genotype and phenotype
•
The appearance of an organism does not always indicate which pair of
alleles it possesses.
•
An organism’s allele pairs are called its genotype.
•
The observable characteristic or outward expression of an allele pair is
called the phenotype.
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Mendelian Genetics
The Inheritance of Traits
Mendel’s law of segregation
•
The law of segregation states that the two alleles for each trait separate
during meiosis.
•
During fertilization, two alleles for that trait unite.
•
Heterozygous organisms are called hybrids.
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Mendelian Genetics
The Inheritance of Traits
Dihybrid
cross
YYRR yyrr
Monohybrid cross
•
A cross that involves hybrids for
a single trait is called a
monohybrid cross.
YR
yr
YyRr
Dihybrid cross
•
The simultaneous inheritance of
two or more traits in the same
plant is a dihybrid cross.
Yellow: (Y)
Green seeds: (y)
Round seeds: (R)
Wrinkled seeds: (r)
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YY, Yy
yy
RR, Rr
rr
Genotype:
1:2:1
Phenotype:
3:1
Mendelian Genetics
The Inheritance of Traits
Law of independent assortment
•
The law of independent assortment states that random distribution
of alleles occurs during gamete formation.
•
Genes on separate chromosomes sort independently during
meiosis.
•
Each allele combination is equally likely to occur.
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Mendelian Genetics
Punnett Squares
•
Punnett squares predict the possible offspring of a cross between two
known genotypes.
•
Given data:
– Trait• dominant
• Recessive
– Genotype
• Tt x Tt
• Tt x TT
• Tt x tt
• TT x TT
• tt x tt
•
Dominant trait is always written first
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Mendelian Genetics
Punnett Squares
Punnett square—monohybrid cross
•
The number of squares is determined by the number of different
types of alleles produced by each parent.
•
Male gametes are written horizontal.
•
Female gametes are written vertically.
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Mendelian Genetics
Punnett Squares
Go to your ConnectEd resources to play Virtual Lab: Punnett
Squares.
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Mendelian Genetics
Punnett Squares
Punnett square—dihybrid cross
•
Four types of alleles from the male
gametes and four types of alleles from the
female gametes can be produced.
•
The resulting phenotypic ratio is 9:3:3:1.
–
9 yellow round
–
3 green round
–
3 yellow wrinkled
–
1 green wrinkled
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Mendelian Genetics
Genotype
Phenotype
Gg
_______body color
ee
_______eye
cc
_______claw
Ff
_______fingers
1. Which of Mike’s traits are homozygous?
2. Which of Mike’s traits are heterozygous?
3. Which of Mike’s traits are dominant?
4. Which are recessive?
Alleles:
G- green body
g- yellow body
E- two eyes
e- one eye
C- long claw
c- short claw
F- four fingers
f- five fingers
Color
eyes claws fingers
Mike’s father genotype:
P
GG
ee
CC
ff
Mike’s mother genotype:
P
gg
EE
cc
FF
Mike’s genotype
F1
Alleles:
G- green body
g- yellow body
E- two eyes
e- one eye
C-long claw
c- short claw
F- four fingers
f-five fingers
Color
eyes claws fingers
Mike’s father genotype:
P
GG
ee
CC
ff
Mike’s mother genotype:
P
gg
EE
cc
FF
Mike’s genotype
F1
F2
Alleles:
G- green body
g- yellow body
E- two eyes
e- one eye
C- long claw
c- short claw
F- four fingers
f- five fingers
Color
eyes
claws
fingers
GG
ee
CC
ff
Mike’s mother genotype: gg
EE
cc
FF
Mike’s genotype
Ee
Cc
Ff
Mike’s father genotype:
F2
Gg
Alleles:
G- green body
g- yellow body
E- two eyes
e- one eye
C- long claw
c- short claw
F- four fingers
f- five fingers
Color
eyes
claws
fingers
Mike’s father genotype:
GG
ee
CC
ff
Mike’s mother genotype:
gg
EE
cc
FF
Mike’s genotype
Gg
Ee
Cc
Ff
F2
Alleles:
G- green body
g- yellow body
E- two eyes
e- one eye
C- long claw
c- short claw
F- four fingers
f- five fingers
GgBb
GgBb
GgBb
GgBb
Ggbb
Ggbb
Ggbb
Ggbb
GgBb
GgBb
GgBb
GgBb
Ggbb
Ggbb
Ggbb
Ggbb
Probability
•
The inheritance of genes can be
compared to the probability of flipping a
coin.
•
Actual data might not perfectly match the
predicted ratios.
•
Mendel’s results were not exactly a
9:3:3:1 ratio, but the larger the number of
offspring involved, the more likely it will
match the results predicted by Punnett
squares.
9:3:3:1
9 yellow round
3 green round
3 yellow
wrinkled
1 green wrinkled
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Mendelian Genetics
Review
Essential Questions
•
•
•
What is the significance of Mendel’s experiments to the study of genetics?
What is the law of segregation and the law of independent assortment?
What are the possible offspring from a cross using a Punnett square?
Vocabulary
•
•
•
•
genetics
allele
dominant
recessive
Copyright © McGraw-Hill Education
•
•
•
•
homozygous
heterozygous
genotype
phenotype
•
•
•
law of segregation
hybrid
law of independent
assortment
Mendelian Genetics