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Genetics
Patterns of Inheritance
Mendelian Genetics
Non-Mendelian Genetics
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is a field in Biology that
deals with heredity, or the
passing of traits from
parents to offspring.
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Mendel used the scientific approach to
identify the laws of inheritance
◦ Mendel discovered the basic principles of heredity by
breeding garden peas: Pisum sativum
Mendelian Genetics
◦ Advantages of using peas:
1. has distinct character variations (traits)
2. Mating can be controlled
3. Each plant can reproduce by itself (self-fertilize)
4. Can cross-fertilize via pollen transfer
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Mendel used the scientific approach to
identify the laws of inheritance
Mendel’s Experiment
Mendel mated two contrasting, true breeding
varieties (hybridization)
◦ true-breeding parents: P generation
◦ hybrid offspring: F1 generation
◦ Offspring of F1 self-pollination: F2
generation
◦ Mendel discovered the basic principles of heredity by
breeding garden peas: Pisum sativum
◦ Advantages of using peas:
1. has distinct character variations (traits)
2. Mating can be controlled
3. Each plant can reproduce by itself (self-fertilize)
4. Can cross-fertilize via pollen transfer
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True-breeding white x purple flowered peas >
F1 generation all purple
◦ Where did the white flower trait go?
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Mendel’s Experiment
Mendel’s results in the modern context
F1 x F1 hybrids --> F2 had purple,
some had white
Alternative versions of a gene is responsible for the variations in
traits or inherited characters called ALLELES
◦ Purple: DOMINANT
◦ White: RECESSIVE
Each gene is found in a specific location or locus in a chromosome
Observed in other traits
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What else did Mendel conclude?
Punnett Squares
◦ For each trait, an organism inherits
two alleles, one from each parent.
devised by Reginald C. Punnett
possible combinations of sperm and
egg can be shown, predicting results
of a genetic cross between individuals
of known genetic makeup
◦ If alleles are similar: HOMOZYGOUS
◦ If alleles are different: HETEROZYGOUS
◦ If the alleles differ, the dominant allele
determines the appearance, and the
recessive allele has no effect on the
appearance
◦ UPPERCASE LETTERS: dominant allele
◦ lowercase letters: recessive allele
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Practice Problem 1: PTC tasting
Practice Problem 1: PTC tasting
The ability to taste a bitter chemical, phenylthiocarbamide
(PTC), is due to a dominant gene. Use T and t to symbolize the
two alleles of this gene.
The ability to taste a bitter chemical, phenylthiocarbamide
(PTC), is due to a dominant gene. Use T and t to symbolize the
two alleles of this gene.
Question 1.
What is the genotype of a nontaster?
What are the possible genotypes of a
taster?
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Question 2.
If your mom is a heterozygous taster
and your dad is a homozygous
nontaster, what are the chances that
you are a taster?
Question 1.
What is the genotype of a nontaster?
What are the possible genotypes of a
taster?
Question 2.
If your mom is a heterozygous taster
and your dad is a homozygous
nontaster, what are the chances that
you are a taster?
Genotype of non-taster: tt
Genotype of tasters: TT, Tt
50% chance of you
being a taster
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Practice Problem 2: Polydactyly
Polydactyly is a dominant trait. A woman
homozygous for polydactyly, is married
to a normal man. What are the chances
of producing an offspring that has extra
fingers or toes? Use the letters P and p
to represent alleles.
Practice Problem 2: Polydactyly
Polydactyly is a dominant trait. A woman
homozygous for polydactyly, is married
to a normal man. What are the chances
of producing an offspring that has extra
fingers or toes? Use the letters P and p
to represent alleles.
A: 100% chance
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Practice Problem 3: Albinism
Practice Problem 3: Albinism
Albinism, the total lack of pigment, is due
to a recessive gene. A man and a woman
plan to marry and wish to know the
chances of them having any albino
children. What are the chances if the man
is an albino, the girl is normal, but her
father is an albino? Use the letters A and
a to represent alleles.
Albinism, the total lack of pigment, is due
to a recessive gene. A man and a woman
plan to marry and wish to know the
chances of them having any albino
children. What are the chances if the man
is an albino, the girl is normal, but her
father is an albino? Use the letters A and
a to represent alleles.
A: 50% chance of having albino children
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Dihybrid Crosses:
individuals with different alleles
of two genes are crossed
Dihybrid Crosses:
individuals with different alleles
of two genes are crossed
• ◦ 4 x 4 Punnett square
• ◦ Forked-line method
• ◦ 4 x 4 Punnett square
• ◦ Forked-line method
Curly hair in poodles (G) is dominant over straight hair
(g), and black fur color (B) is dominant over white fur (b).
Curly hair in poodles (G) is dominant over straight hair
(g), and black fur color (B) is dominant over white fur (b).
Two heterozygous black, curly haired poodles were
mated. What are the chances of the offspring to have
white, straight hair?
Two heterozygous black, curly haired poodles were
mated. What are the chances of the offspring to have
white, straight hair?
A: 1 of 16 chances (6.25%)
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Gregor Mendel established the
Principles of Inheritance
Gene: sequence of DNA that codes for a trait
Alleles: versions of one gene; usually two
Homozygous: identical alleles of a gene
Heterozygous: different alleles of a gene
Genotype: set of alleles
Phenotype: observable characteristics
Dominant: masks its accompanying allele
Recessive: is masked by its accompanying allele
Trihybrid
Crosses
Forked-line
method
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Mendel’s Postulates
• Unit factors are in pairs.
• Genetic characters (traits) are controlled by unit
factors existing in pairs (alleles) in individual
organisms.
Smooth
RR
Rough
X
Non-Mendelian Genetics
rr
the exceptions to Mendel’s Laws
• Dominance and Recessiveness
• When two unlike unit factors responsible for a single
character are present in a single individual, one unit
factor is dominant and can mask the other, which is
said to be recessive.
• Segregation
Rough
Smooth
RR
X
rr
• During the formation of gametes, the paired unit
factors separate, or segregate, randomly so that each
gamete receives one or the other with equal
likelihood.
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Incomplete Dominance
Incomplete Dominance
Occurs when the phenotype of heterozygous individuals
are a blended version of both parents
Occurs when the phenotype of heterozygous individuals
are a blended version of both parents
In plants known as “four o’clocks”, the
alleles for the dominant red flower color (F)
is incompletely dominant over the allele for
white flowers (f). Heterozygous individuals
exhibit pink coloration.
In plants known as “four o’clocks”, the
alleles for the dominant red flower color (F)
is incompletely dominant over the allele for
white flowers (f). Heterozygous individuals
exhibit pink coloration.
Identify the flower color genotypes and
phenotypes if a red four o’clock is crossed
with a pink four o’clock.
Identify the flower color genotypes and
phenotypes if a red four o’clock is crossed
with a pink four o’clock.
A: 50% red (FF), 50% pink (Ff)
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Co-dominance
Co-dominance
Occurs when the phenotype of heterozygous individuals
are both the parents’; no allele is recessive
Occurs when the phenotype of heterozygous individuals
are both the parents’; no allele is recessive
In rabbits, white coat color (CW) and black coat
color (CB) are codominant, and both of these
alleles are dominant over albino (c);
heterozygotes (CWCB) are spotted.
In rabbits, white coat color (CW) and black coat
color (CB) are codominant, and both of these
alleles are dominant over albino (c);
heterozygotes (CWCB) are spotted.
Identify the coat color genotypes and
phenotypes between a heterozygous blackcoated rabbit and a homozygous white-coated
rabbit. What are the chances that the offspring
has a black coat?
Identify the coat color genotypes and
phenotypes between a heterozygous blackcoated rabbit and a homozygous white-coated
rabbit. What are the chances that the offspring
has a black coat?
A: 0% chance (CB CB)
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PRACTICE PROBLEM:
Mendelian Genetics
PRACTICE PROBLEMS:
Incomplete Dominance
Achondroplasia (dwarfism) is due to a
dominant gene. A man with achondroplasia,
but whose mother is of normal height, and
a woman with achondroplasia, but whose
father is also of normal height, decide to
have children.
In snapdragon flowers, having a red
pigment (R) is incompletely dominant
over the allele for a white pigment (r).
Heterozygous individuals have pink
colored flowers.
If you have a red snapdragon and a
white snapdragon, what are the
chances that you’ll produce pink
flowers if you cross the two varieties?
What are the chances that they have
children with achondroplasia? Use the letter
D and d for the alleles.
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PRACTICE PROBLEMS:
Co-dominance
Blood types display codominance.
When a pure red cow (CR)is mated with a
pure white bull (CW), all the offspring is a
roan (CRCW), which has both red and
white hair. Both alleles are dominant over
c, which produces a brown color.
ABO blood group
Which type is the
universal donor?
What are the chances of getting a browncolored cow if you mated a roan with a
heterozygous white bull?
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Which type is the
universal receiver?
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Codominance in blood types
Codominance in blood types
If a person with an AB
blood type mates with
another person with an
AB blood type, what are
the possible blood types
of their offspring?
If a person with an AB
blood type mates with
another person with an
AB blood type, what are
the possible blood types
of their offspring?
A: 50% AB, 25% AA,
25% BB
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Practice Problem: Blood Types
X Chromosome-linked Disorders
A woman with an A blood type marries
a man with a B blood type. Later on,
she gives birth to an offspring with an
O blood type.
Alleles on the X chromosome are inherited and expressed
differently in males and females
• Males cannot transmit X-linked alleles to their sons (contributes Y
chromosome)
• Females pass X-linked alleles to male and female offspring
However, upon looking at the child, he
was convinced that it wasn’t his. Using
their blood types, can you determine
whether the child is his or not?
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• Male offspring: will 100% express the trait
• Female offspring: only homozygous females will express the trait;
heterozygous are carriers
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Practice Problem: Hemophilia
Practice Problem: Hemophilia
A woman who is heterozygous for
recessive X-linked hemophilia mates
with a non-hemophiliac, and bears a
child. What is the probability that:
A woman who is heterozygous for
recessive X-linked hemophilia mates
with a non-hemophiliac, and bears a
child. What is the probability that:
A. Their daughter has hemophilia?
B. Their son has hemophilia?
A. Their daughter has hemophilia?
B. Their son has hemophilia?
A: 0%, but 50% carrier, 50% normal
B: 50% normal, 50% hemophiliac
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Practice Problem: Colorblindness
Practice Problem: Colorblindness
Red-green colorblindness is a
recessive X-linked disorder. A
colorblind woman mates with a
normal man and bears a child.
What is the probability that:
A. Their daughter is colorblind?
B. Their son is colorblind?
Red-green colorblindness is a
recessive X-linked disorder. A
colorblind woman mates with a
normal man and bears a child.
What is the probability that:
A. Their daughter is colorblind?
B. Their son is colorblind?
A: 0%, but all are carriers
B: 100% colorblind
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