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Biology
Sylvia S. Mader
Michael Windelspecht
Chapter 11
Mendelian
Patterns of
Inheritance
Lecture Outline
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1
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Outline
• 11.1 Gregor Mendel
• 11.2 Mendel’s Laws
• 11.3 Extending the Range of
Mendelian Genetics
2
11.1 Gregor Mendel
• Concept of Blending Inheritance:
 Parents of contrasting appearance produce
offspring of intermediate appearance
 Popular concept during Mendel’s time
• Mendel’s findings were in contrast with this
 He formulated the Particulate Theory of
Inheritance
• Inheritance involves reshuffling of genes from
generation to generation
3
Gregor Mendel
• Austrian monk
 Studied science and mathematics at the University of
Vienna
 Conducted breeding experiments with the garden pea
Pisum sativum
 Carefully gathered and documented mathematical
data from his experiments
• Formulated fundamental laws of heredity in the
early 1860s
 Had no knowledge of cells or chromosomes
 Did not have a microscope
4
Gregor Mendel
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© Ned M. Seidler/Nationa1 Geographic Image Collection
5
Gregor Mendel
• The garden pea:
 Organism used in Mendel’s experiments
 A good choice for several reasons:
• Easy to cultivate
• Short generation
• Normally self-pollinating, but can be crosspollinated by hand
• True-breeding varieties were available
• Simple, objective traits
6
Garden Pea Anatomy
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Flower Structure
stamen
anther
filament
stigma
style
ovules in
ovary
carpel
a.
7
Garden Pea Anatomy
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Cutting away
anthers
Brushing
on pollen
from another
plant
All peas are yellow when
one parent produces yellow
seeds and the other parent
produces green seeds.
8
Garden Pea Anatomy
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Trait
Stem length
Characteristics
*Dominant
*Recessive
Tall
Short
Pod shape
Inflated
Constricted
Seed shape
Round
Wrinkled
Seed color
Yellow
Green
Flower position
Axial
Terminal
Flower color
Purple
White
Pod color
Green
Yellow
b.
11.2 Mendel’s Laws
• Mendel performed cross-breeding
experiments
 Used “true-breeding” (homozygous) plants
 Chose varieties that differed in only one trait
(monohybrid cross)
 Performed reciprocal crosses
• Parental generation = P
• First filial generation offspring = F1
• Second filial generation offspring = F2
 Formulated the Law of Segregation
10
Monohybrid Cross done by Mendel
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Allele Key
T = tall plant
t = short plant
Phenotypic Ratio
3
1
tall
short
TT
tt
11
Monohybrid Cross done by Mendel
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Allele Key
T = tall plant
t = short plant
Phenotypic Ratio
3
1
tall
short
P generation
TT
tt
12
Monohybrid Cross done by Mendel
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Allele Key
T = tall plant
t = short plant
Phenotypic Ratio
3
tall
1
short
P generation
TT
P gametes
T
tt
t
13
Monohybrid Cross done by Mendel
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Allele Key
T = tall plant
t = short plant
Phenotypic Ratio
3
tall
1
short
P generation
TT
P gametes
tt
T
tt
t
F1 generation
Tt
14
Monohybrid Cross done by Mendel
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
T = tall plant
t = short plant
Phenotypic Ratio
3
1
P generation
TT
tall
short
P gametes
tt
t
T
F1 generation
Tt
eggs
F1 gametes
T
t
T
sperm
Allele Key
t
15
Monohybrid Cross done by Mendel
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
P generation
T = tall plant
t = short plant
Phenotypic Ratio
3
1
TT
tt
tall
short
P gametes
t
T
F1 generation
Tt
eggs
F1 gametes
T
t
T
sperm
Allele Key
TT
t
16
Monohybrid Cross done by Mendel
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
F1 generation
T = tall plant
t = short plant
Phenotypic Ratio
3
1
TT
tt
tall
short
t
T
P gametes
F1 generation
Tt
eggs
F1 gametes
T
t
T
sperm
Allele key
TT
Tt
t
17
Monohybrid Cross done by Mendel
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
P generation
T = tall plant
t = short plant
Phenotypic Ratio
3
tall
1
short
TT
P gametes
tt
t
T
F1 generation
Tt
eggs
T
F1 gametes
t
T
sperm
Allele Key
TT
Tt
t
Tt
18
Monohybrid Cross done by Mendel
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
P generation
TT
tt
t
T
P gametes
F1 generation
Tt
eggs
F1 gametes
T
t
F2 generation
sperm
T
TT
Tt
Tt
tt
t
Offspring
Allele Key
T = tall plant
t = short plant
Phenotypic Ratio
3
1
tall
short
19
Mendel’s Laws
• Law of Segregation:
 Each individual has a pair of factors (alleles) for each
trait
 The factors (alleles) segregate (separate) during
gamete (sperm & egg) formation
 Each gamete contains only one factor (allele) from
each pair of factors
 Fertilization gives the offspring two factors for each
trait
20
Mendel’s Laws
• Classical Genetics and Mendel’s Cross:
 Each trait in a pea plant is controlled by two alleles
(alternate forms of a gene)
 Dominant allele (capital letter) masks the expression
of the recessive allele (lower-case)
 Alleles occur on a homologous pair of chromosomes
at a particular gene locus
• Homozygous = identical alleles
• Heterozygous = different alleles
21
Classical View of
Homologous Chromosomes
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sister chromatids
alleles at a
gene locus
a. Homologous
chromosomes
have alleles for
same genes at
specific loci.
G
g
R
r
S
s
t
T
G
Replication
b. Sister chromatids
of duplicated
chromosomes
have same alleles
for each gene.
R
G
g
g
R
r
r
S
S
s
s
t
t
T
T
22
Relationship Between Observed
Phenotype and F2 Offspring
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Trait
F2Results
Characteristics
Dominant
Recessive
Dominant
Recessive
Ratio
Tall
Short
787
277
2.84:1
Pod shape
Inflated
Constricted
882
299
2.95:1
Seed shape
Round
Wrinkled
5,474
1,850
2.96:1
Seed color
Yellow
Green
6,022
2,001
3.01:1
Axial
Terminal
651
207
3.14:1
Flower color
Purple
White
705
224
3.15:1
Pod color
Green
Yellow
428
152
2.82:1
14,949
5,010
2.98:1
Stem length
Flower position
Totals:
Mendel’s Laws
• Genotype
 Refers to the two alleles an individual has for a
specific trait
 If identical, genotype is homozygous
 If different, genotype is heterozygous
• Phenotype
 Refers to the physical appearance of the individual
24
Mendel’s Laws
• A dihybrid cross uses true-breeding plants differing in two traits
• Mendel tracked each trait through two generations.




Started with true-breeding plants differing in two traits
The F1 plants showed both dominant characteristics
F1 plants self-pollinated
Observed phenotypes among F2 plants
• Mendel formulated the Law of Independent Assortment
 The pair of factors for one trait segregate independently of the factors
for other traits
 All possible combinations of factors can occur in the gametes
• P generation is the parental generation in a breeding experiment.
• F1 generation is the first-generation offspring in a breeding
experiment.
• F2 generation is the second-generation offspring in a breeding
experiment
25
Dihybrid Cross Done by Mendel
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×
P generation
TTGG
P gametes
ttgg
tg
TG
F1 generation
TtGg
eggs
TG
F1 gametes
Tg
tG
tg
TG
TTGg
TtGG
TtGg
TTGg
TTgg
TtGg
Ttgg
TtGG
TtGg
ttGG
ttGg
TtGg
Ttgg
ttGg
ttgg
Tg
sperm
F2 generation
TTGG
tG
tg
Offspring
Allele Key
T
t
G
g
=
=
=
=
tall plant
short plant
green pod
Yellow pod
9
3
3
1
Phenotypic Ratio
tall plant, green pod
tall plant, yellow pod
short plant, green pod
short plant, yellow pod
Independent Assortment and
Segregation during Meiosis
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A
a
B
b
Parent cell has two
pairs of homologous
chromosomes.
27
Independent Assortment and
Segregation during Meiosis
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A
A
Aa
a
B
Bb
b
a
B
b
Parent cell has two
pairs of homologous
chromosomes.
28
Independent Assortment and
Segregation during Meiosis
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A
A
Aa
a
B
Bb
b
A
Aa
a
b
bB
B
a
B
b
Parent cell has two
pairs of homologous
chromosomes.
29
Independent Assortment and
Segregation during Meiosis
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A
A
Aa
a
B
Bb
b
A
Aa
a
b
bB
B
a
B
b
Parent cell has two
pairs of homologous
chromosomes.
All orientations of homologous chromosomes
are possible at metaphase I in keeping with
the law of independent
assortment.
30
Independent Assortment and
Segregation during Meiosis
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
A
A
Aa
a
B
Bb
b
A
A
B
B
a
a
b
b
a
B
b
Parent cell has two
pairs of homologous
chromosomes.
A
Aa
a
b
bB B
All orientations of homologous chromosomes
are possible at metaphase
I in keeping with the
law of independent
assortment.
31
Independent Assortment and
Segregation during Meiosis
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
A
A
Aa
a
B
Bb
b
A
A
B
B
a
a
b
b
A
A
b
b
a
B
b
Parent cell has two
pairs of homologous
chromosomes.
A
Aa
a
b
bB
B
All orientations of homologous chromosomes
are possible at metaphase
I in keeping with
the law of independent
assortment.
a
a
B
B
32
Independent Assortment and
Segregation during Meiosis
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A A
B B
A
Aa a
B
Bb b
a a
b b
A
a
B
b
A A
b b
Parent cell has two
pairs of homologous
chromosomes.
A Aa a
b
bB B
a
a
B B
All orientations of homologous chromosomes
are possible at metaphase I in keeping with
the law of independent
assortment.
At metaphase II, each
daughter cell has only
one member of each
homologous pair in
keeping with the law of
segregation
33
Independent Assortment and
Segregation during Meiosis
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
A
B
A
A
B
B
AB
A
B
A
Aa
a
B
Bb
b
a
a
a
b
b
b
ab
a
b
A
a
B
b
A
A
A
b
b
b
Ab
A
Parent cell has two
pairs of homologous
chromosomes.
A
Aa
a
b
bB
B
a
a
B
B
a
b
B
aB
a
B
All orientations of homologous chromosomes
are possible at
metaphase I in keeping
with the law of
Independent assortment.
At metaphase II, each
daughter cell has only
one member of each
homologous pair in
keeping with the law of
segregation
All possible combi tions of chromosomes
and alleles occur in
the gametes as
suggested by Mendel's
two laws.
34
Mendel’s Laws
• Punnett Square
 Table listing all possible genotypes resulting
from a cross
• All possible sperm genotypes are lined up on one
side
• All possible egg genotypes are lined up on the
other side
• Every possible zygote genotypes are placed within
35
the squares
Mendel’s Laws
• Punnett Square
 Allows us to easily calculate probability, of genotypes
and phenotypes among the offspring
 Punnett square in next slide shows a 50% (or ½)
chance
• The chance of E = ½
• The chance of e = ½
 An offspring will inherit:
•
•
•
•
The chance of EE =½  ½=¼
The chance of Ee =½  ½=¼
The chance of eE =½  ½=¼
The chance of ee =½  ½=¼
36
Punnett Square
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Parents
Ee
Ee
eggs
e
spem
E
EE
Ee
Ee
ee
e
Punnett square
E
Offspring
Allele key
E = unattached earlobes
e = attached earlobes
Phenotypic Ratio
3
1
unattached earlobes
attached earlobes
37
Mendel’s Laws
• Testcrosses
 Individuals with recessive phenotype always have the
homozygous recessive genotype
 However, individuals with dominant phenotype have
indeterminate genotype
• May be homozygous dominant, or
• Heterozygous
 A testcross determines the genotype of an individual
having the dominant phenotype
38
Mendel’s Laws
• Two-trait testcross:
 An individual with both dominant phenotypes is
crossed with an individual with both recessive
phenotypes.
 If the individual with the dominant phenotypes
is heterozygous for both traits, the expected
phenotypic ration is 1:1:1:1.
39
Mendel’s Laws
• Genetic disorders are medical conditions caused by
alleles inherited from parents
• Autosome - Any chromosome other than a sex
chromosome (X or Y)
• Genetic disorders caused by genes on autosomes are
called autosomal disorders
 Some genetic disorders are autosomal dominant
• An individual with AA has the disorder
• An individual with Aa has the disorder
• An individual with aa does NOT have the disorder
 Other genetic disorders are autosomal recessive
• An individual with AA does NOT have the disorder
• An individual with Aa does NOT have the disorder, but is a carrier
• An individual with aa DOES have the disorder
40
Autosomal Recessive Pedigree
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Generations
I
aa
II
III
IV
A?
Aa
A?
Aa
A?
*
Aa
Aa
aa
aa
A?
A?
A?
Key
aa = affected
Aa = carrier (unaffected)
AA = unaffected
A? = unaffected
(one allele unknown)
Autosomal recessive disorders
• Most affected children have unaffected
parents.
• Heterozygotes (Aa) have an unaffected phenotype.
• Two affected parents will always have affected children.
• Close relatives who reproduce are more likely to have
affected children.
• Both males and females are affected with equal frequency.
41
Autosomal Dominant Pedigree
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Generations
Aa
Aa
I
*
II
III
Aa
aa
Aa
Aa
aa
A?
aa
aa
aa
aa
aa
aa
Key
AA = affected
Aa = affected
Autosomal dominant disorders
A? = affected
(one allele unknown)
• Affected children will usually have an
aa = unaffected
affected parent.
• Heterozygotes (Aa) are affected.
• Two affected parents can produce an unaffected child.
• Two unaffected parents will not have affected children.
• Both males and females are affected with equal frequency.
42
11.3 Extending the Range of
Mendelian Genetics
• Some traits are controlled by multiple alleles (multiple
allelic traits)
• The gene exists in several allelic forms (but each
individual only has two alleles)
• ABO blood types
 The alleles:
• IA = A antigen on red blood cells, anti-B antibody in plasma
• IB = B antigen on red blood cells, anti-A antibody in plasma
• i = Neither A nor B antigens on red blood cells, both anti-A and antiB antibodies in plasma
• The ABO blood type is also an example of codominance
 More than one allele is fully expressed
 Both IA and IB are expressed in the presence of the other
43
ABO Blood Type
Phenotype
A
B
AB
O
Genotype
IAIA, IAi
B
B
B
I I ,I i
IAIB
ii
44
Extending the Range of
Mendelian Genetics
• Incomplete Dominance:
 Heterozygote has a phenotype intermediate between
that of either homozygote
• Homozygous red has red phenotype
• Homozygous white has white phenotype
• Heterozygote has pink (intermediate) phenotype
 Phenotype reveals genotype without a test cross
45
Incomplete Dominance
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R1R2
R1R2
eggs
R1
R2
sperm
R1
R1R1
R1R2
R2
R1R2
Offspring
R2R2
Key
1 R1R1
2 R1R2
1 R2R2
red
pink
white
46
Extending the Range of
Mendelian Genetics
• Human examples of incomplete dominance:
 Incomplete penetrance
• The dominant allele may not always lead to the
dominant phenotype in a heterozygote
• Many dominant alleles exhibit varying degrees of
penetrance
• Example: polydactyly
– Extra digits on hands, feet, or both
– Not all individuals who inherit the dominant polydactyly
allele will exhibit the trait
47
Extending the Range of
Mendelian Genetics
• Pleiotropy occurs when a single mutant
gene affects two or more distinct and
seemingly unrelated traits.
• Marfan syndrome is pleiotropic and results
in the following phenotypes:
 disproportionately long arms, legs, hands, and
feet
 a weakened aorta
 poor eyesight
48
Extending the Range of
Mendelian Genetics
• Polygenic Inheritance:
 Occurs when a trait is governed by two or more
genes having different alleles
 Each dominant allele has a quantitative effect on the
phenotype
 These effects are additive
 Results in continuous variation of phenotypes within a
population
 The traits may also be affected by the environment
 Examples
• Human skin color
• Height
• Eye color
49
Polygenic Inheritance
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P generation
F1 generation
F2 generation
Proportion of Population
20
—
64
15
—
64
6
—
64
1
—
64
50
Genotype Examples
Extending the Range of
Mendelian Genetics
• X-Linked Inheritance
 In mammals
• The X and Y chromosomes determine
gender
• Females are XX
• Males are XY
51
Extending the Range of
Mendelian Genetics
• X-Linked Inheritance
 The term X-linked is used for genes that have
nothing to do with gender
• X-linked genes are carried on the X chromosome.
• The Y chromosome does not carry these genes
• Discovered in the early 1900s by a group at
Columbia University, headed by Thomas Hunt
Morgan.
– Performed experiments with fruit flies
» They can be easily and inexpensively raised in
simple laboratory glassware
» Fruit flies have the same sex chromosome pattern as
humans
52
X-Linked Recessive Pedigree
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
XbY
XBXB
XBY
XBXb
daughter
grandfather
XBY
XbXb
XbY
XBY
XBXB
XBXb
XbY
grandson
XBXB
XBXb
XbXb
XbY
XbY
Key
= Unaffected female
= Carrier female
= Color-blind female
= Unaffected male
= Color-blind male
X-Linked Recessive
Disorders
• More males than females are affected.
• An affected son can have parents who have the
normal phenotype.
• For a female to have the characteristic, her father must
also have it. Her mother must have it or be a carrier.
• The characteristic often skips a generation from the
grandfather to the grandson.
• If a woman has the characteristic, all of her sons will
have it.
53
Muscular Dystrophy
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
fibrous
tissue
abnormal
muscle
normal
tissue
(Abnormal): Courtesy Dr. Rabi Tawil, Director, Neuromuscular Pathology Laboratory, University of Rochester Medical Center;
(Boy): Courtesy Muscular Dystrophy Association; (Normal): Courtesy Dr. Rabi Tawil, Director, Neuromuscular Pathology Laboratory,
University of Rochester Medical Center.
54