Chapter 8- Mendel And Heredity
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I. The origins of Genetics
A. The passing of traits from parents to
offspring is called heredity.
1. Mendel was a mathematician and
looked at genetics differently than
anyone in the past ever did.
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2. Genetics is the branch of biology that
focuses on heredity.
3. Mendel counted the number of each kind
of offspring and analyzed the data.
4. Quantitative approaches to science-those
that include measuring and counting.
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B. Useful features in peas
1. The garden pea is a good subject for
studying heredity for several reasons.
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a. The garden pea has many traits that
have two clearly different forms that are easy
to tell apart.
b. The mating of the garden pea flowers
can be easily controlled.
c. The garden pea is small, grows easily,
matures quickly and produces many
offspring.
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C. Mendel observed that traits are expressed
as simple ratios
1. Mendel’s initial experiments were
monohybrid crosses. A cross with ONE pair
of contrasting traits.
2. Mendel allowed true breeding or selfpollination to occur.
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3. These true-breeding plants served
as the parental generation in Mendel’s
experiments.
4. The parental generation, or P
generation are the first two individuals
that are crossed.
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5. Mendel crossed the P generation to get
the F1 generation. First generation. He then
examined each F1 plant and recorded the
number of F1 plants and their traits.
6. Mendel then mated the F1 generation to
get a F2 generation. The second generation
and then again examined each of their traits.
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D. Mendel’s results
P generation
(true-breeding
parents)
Purple flowers White flowers
F1 generation
Mendel’s
Results
All plants have
purple flowers
Fertilization among
F1 plants (F1 ´ F1)
F2 generation
3
–
4
of plants
have purple flowers
1
–
4
of plants
have white flowers
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II. Mendel’s theory
A. Mendel’s work became a theory of
heredity.
1. The four hypotheses Mendel
developed were based directly on the
result of his experiments.
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a. For each inherited trait, an
individual has two copies of genes- one
from each parent.
b. There are alternative versions of
genes. These are called alleles.
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c. When two different alleles occur
together, one of them bay be completely
expressed, while the other may have no
affect on the appearance. Mendel explained
these to be dominant and recessive.
d. When gametes are formed, the alleles
for each gene in an individual separate
independently of one another.
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B. Mendel’s findings in modern terms
1. If two traits are the same they
are called homozygous.
2. If two traits are different they are
called heterozygous.
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3. Dominant genes are expressed with
capital letters.
4. Recessive genes are expressed with lower
caps.
5. Example Brown is dominant= B, blue is
recessive =b.
6. A set of alleles that an individual has is
called the genotype.
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7. What the individual expresses is called the
phenotype.
8. Example: BB, Bb and bb are genotypes.
9. BB, and Bb would have brown eyes, and
bb would have blue eyes. The colors are the
phenotypes.
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Dominant Traits
Recessive Traits
Freckles
No freckles
Widow’s peak
Straight hairline
Free earlobe
Attached earlobe
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Dihybrid
Cross
Phenotype
Mendel’s dihybrid crosses:
Mendel also performed crosses involving
two pairs of traits, e.g., seed shape
(smooth vs. wrinkled) and color
(yellow vs. green).
Pea Traits:
Seed Coat
Color
Pod
Shape
Pod
Color
Smooth
Green
Seed
Shape
Seed
Color
Round
Yellow
Gray
Wrinkled
Green
White
Constricted
Round
Yellow
Gray
Smooth
Flower
Position
Plant
Height
Axial
Tall
Yellow
Terminal
Short
Green
Axial
Tall
Dihybrid Cross Example:
Dihybrid crosses:
SsYy
SsYy
Dihybrid
Crosses:
B. Mendel’s Ideas gave rise to the laws
of heredity.
1. The first law, the law of
segregation, states that the two alleles
for a trait segregate or separate when
gametes are formed.
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Genetic makeup (alleles)
PP
pp
P plants
Gametes
All P
F1 plants
(hybrids)
The Law of
Segregation
p
All
All Pp
1–
2
Gametes
1–
2
P
Sperm
P
F2 plants
Phenotypic ratio
3 purple : 1 white
p
P
PP
Pp
p
Pp
pp
Eggs
Genotypic ratio
1 PP : 2 Pp : 1 pp
p
2. The law of independent assortment
states that the alleles of different genes
separate independently of one another
during gamete formation.
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The law of independent assortment
Hypothesis: Independent assortment
Hypothesis: Dependent assortment
P
generation
rryy
RRYY
ry
Gametes RY
F1
generation
rryy
RRYY
ry
Gametes RY
RrYy
RrYy
Sperm
Sperm
1–
2
F2
generation
1–
2
RY
1–
2
1
–
4
1
–
4
rY
1
–
4
Ry
1
–
4
ry
ry
1
–
4
RY
Eggs
1–
2
RY
1
–
4
ry
RY
Hypothesized
(not actually seen)
1
–
4
RrYY
RRYy
RrYy
RrYY
rrYY
RrYy
rrYy
rY
Eggs
1
–
4
RRYY
9
––
16
Ry
RRYy
RrYy
RRyy
Rryy
RrYy
rrYy
Rryy
rryy
ry
Actual results
(support hypothesis)
3
––
16
3
––
16
1
––
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Yellow
round
Green
round
Yellow
wrinkled
Green
wrinkled
II. Studying heredity
A. Punnet squares
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F1 genotypes
Bb male
Formation of sperm
Bb female
Formation of eggs
B. Probability
1
–
2
1–
2
1–
2
B
B
B
b
B
B
1–
4
1–
4
1
–
2
b
b
B
1–
4
F2 genotypes
b
b
b
1–
4
C. Pedigrees
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Pedigree Sample
Symbols
Pedigree analysis
• Rather than manipulate mating patterns of people,
geneticists analyze the results of matings that
have already occurred.
• In a pedigree analysis, information about the
presence/absence of a particular phenotypic trait
is collected from as many individuals in a family
as possible and across generations.
• The distribution of these characters is then
mapped on the family tree.
Pedigree analysis
 For
example, the occurrence of widows
peak (W) is dominant to a straight
hairline (w).
 The relationship among alleles can be
integrated with the phenotypic
appearance of these traits to predict the
genotypes of members of this family.
Examples: Widow’s Peak &
Earlobes
Is widow’s peak a dominant or recessive trait?
Widow’s Peak
No Widow’s Peak
Is an attached earlobe a dominant or recessive trait?
Attached Earlobe
Free Earlobe
D. Sex-Linked
Traits
Sex-Linked
Traits
Color-Blindness
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What type of inheritance pattern?
Color blindness Red-Green
Hemophilia in the royal family of
Russia
Queen
Victoria
Albert
Alice
Louis
Alexandra
Czar
Nicholas II
of Russia
Alexis
Hemophilia Example
XN
Y
XN
XNXN
XNY
Xn
XNXn
XnY
N = Normal Blood
n = Hemophilia
III. Patterns of Heredity can be
complex
A. Traits influenced by several genes
1. When several genes influence a
trait, the trait is said to be polygenetic.
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Polygenic Inheritance Example
Polygenic (multiple Genes) for a given
phenotypic character.
Example: Skin Color, Eye Color, Hair Color, &
Rh Factor.
Polygenic Inheritance Example: Skin Color
The melanin factor is located on three loci of
each chromosome. (total of six locations)
Polygenic Inheritance: Human skin color is a good
example of polygenic (multiple gene) inheritance.
Assume that three "dominant" capital letter genes (A, B
and C) control dark pigmentation because more
melanin is produced. The "recessive"alleles of these
three genes (a, b & c) control light pigmentation
because lower amounts of melanin are produced.
AABBCC
Most Melanin
aabbcc
Least Melanin
Polygenic Inheritance Example
Let’s do a cross.
A
B C
a
b c
a
b c
X
A
B C
Polygenic Inheritance Example
Result would be the F1 generation would be AaBbCc.
abc
abc
ABC
ABC
F1
abc
ABC
abc
ABC
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Polygenic Inheritance Example
Let’s cross the F1 with another F1.
A
B C
A
B C
a
b
X
a
b
c
c
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Polygenic Inheritance Example F2
F1
F2
Polygenic Inheritance Example: Skin Color
Gametes
ABC
ABC
ABC/
6
ABC
ABc
ABC/
5
ABc
AbC
ABc
AbC
Abc
aBC
aBc
abC
abc
ABC/
5
ABc
5
4
5
4
4
3
ABc/
4
ABc
4
3
4
3
3
2
5
4
4
3
4
3
3
2
Abc
4
3
3
2
3
2
2
1
aBC
5
4
4
3
4
3
3
2
aBc
4
3
3
2
3
2
2
1
abC
4
3
3
2
3
2
2
1
abc
3
2
2
1
2
1
1
0
2. In some traits there is neither
dominant nor recessive genes. These
are called incomplete dominance.
Example: Red flower RR and a White
WW flower both with incomplete
dominance will have pink flowers.
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Incomplete Dominance Example
X
Red Carnation
X
=
White
Carnation
=
Pink
Carnation
P Generation

Red
CRCR
CR
Incomplete Dominance with
Snapdragons Flower Color
Gametes
White
CWCW
CW
Pink
CRCW
F1 Generation
Gametes
1⁄
2
CR
CR
Sperm
1⁄
1⁄ CR
R
2 C
2
F2 Generation
1⁄
2
CR
2
Cw
Eggs
1⁄
CR CR
CR CW
CR CW CW CW
1⁄
2
3. When two dominant alleles are expressed
at the same time, both forms of the trait are
displayed this is called co dominance.
Example: Coat colors in a horse.
4. Genes with three or more alleles are said
to have multiple alleles. Example blood
groups ABO.
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Example of Codominance:
In chickens, black feather color (BB) is codominant to
white feather color (WW). Both feather colors show
up in a checkered pattern in the heterozygous
individual (BW). Cross a checkered chicken with a
checkered chicken.
WW X BB
checkered chicken
B
B
W
BW
BW
W
BW
BW
BW
Blood Types
Multiple Alleles
Multiple Alleles
1.
Genotypes
AA
Ao
BB
Bo
AB
oo
Phenotypes
A
A
B
B
AB
O
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A
B
Mendelian Inheritance In Humans
See page
261
Mendelian Inheritance In Humans
See
page
261
5. An individual’s phenotype often depends
on conditions in the environment.
6. Some traits are affected by the
temperature, sunlight or seasons.
7. In humans the environment influences
height. Height is influenced by nutrition, an
internal environment conditions.
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Multiple Alleles
Full coat
color:
CC
CCch
Cch
Cc
Himalayan
coat color:
c hc h
c hc
**temp
Sensitive!
Chinchilla
coat color:
cchcch
cchch
cchc
Albino
coat color:
cc
B. Some traits are caused
by mutations
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Sickle Cell Anemia
** NN is Not afflicted; Nn is a carrier; nn is afflicted
**Sickle cell anemia is caused by an abnormal type of hemoglobin
and cells become sickle shaped when oxygen is low
**Sickle cells clot give
rise to recurrent painful
episodes called a “sickle
cell pain crisis”
** “n” makes a person
immune to malaria
• Results in the absence of an enzyme
that normally breaks down a lipid
(fat) produced and stored in nervous
tissues and brain
A child with
Tay-Sachs
Syndrome are
mentally
challenged
• Phenylketonuria (PKU)lacks an enzyme that
converts one amino acid, phenylalanine, to a different
amino acid, tyrosine.
* Results in severe damage to the central nervous
system.
• When detected in infants, dietary adjustments
(about 1 in 15k babies born with it)
Phenylketonurics: Contains Phenylalanine
Genetic Methemoglobinemia
“Blue People” who have too much methhemoglobin in their
blood and oxygen cannot bind due to an enzyme deficiency
*Although oxygen-poor blood is NOT blue, but appears bluish
because of the distortion of skin
**The Fugates of Hazard,
Kentucky. Mom was a carrier
while Dad had the disease
Picture taken in the 1950’s
• It results in a breakdown of certain areas
of the brain.
~Causes dementia
due to progressive
deterioration of the brain, increase in
involuntary movements, eventually
bedridden
Achondroplasia - type of dwarfism.
AA genotypes are
lethal and result in
spontaneous
abortion, Aa have
dwarfism. 99% of
Americans are aa
Polydactyl (extra fingers and/or toes):
PP or Pp = extra digits. 98% of all people in
the world are homozygous recessive (pp).
Progeria (very premature aging):
Spontaneous
mutation of one gene
creates a dominant
mutation that rapidly
accelerates aging.
* Most die before
age 13
5. There is genetic testing to detect
any bad genes.
6. Gene technology may soon by
replacing defected genes with copies of
healthy ones this is called gene therapy.
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