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Chapter 23
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1
23.1 Mendel’s Laws
• Genetics explains the process of inheritance and
why there are variations between offspring from
one generation to the next.
• Virtually every culture in history has attempted to
explain observed inheritance patterns.
• Understanding inheritance patterns is especially
important in agriculture, animal husbandry, and
medicine.
2
Gregor Mendel
• Understanding of genetics
is based on work of
Gregor Mendel
• Investigated inheritance at
the organism level (1860s)
• Concluded that plants
transmit distinct factors to
offspring
– Now called genes, found
on chromosomes
Figure 23.1
3
Gregor Mendel
• In peas and humans, chromosomes come in pairs
called homologous chromosomes.
• One member of the pair is inherited from the
mother, and the other is inherited from the father.
• Homologous pairs have certain characteristics.
– Both members have same length and centromere
location.
– Both carry similar types of genes.
– Alternate forms of a gene for a trait are called alleles.
– Alleles are always on the same spot, or locus.
4
Homologous Chromosomes
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alleles at a gene locus
Figure 23.2
G
g
R
r
S
s
t
T
5
Gregor Mendel
• The Law of Segregation
– Mendel had no knowledge of chromosomes, but he
decided, based on his observations, that the following
were true.
• Each pea plant has two factors for each trait.
• The factors segregate (separate) during the formation of
gametes.
• Each gamete contains only one factor from each pair of factors.
• Fertilization gives each new individual two factors for each trait.
6
The Inheritance of a Single Trait
• Phenotype: an individual’s actual appearance
– May include physical characteristic, microscopic or
metabolic characteristics
• Genotype: alleles carried by the chromosomes
that are responsible for a given trait
7
The Inheritance of a Single Trait
• In diploid organisms, a pair of homologous chromosomes
contains two alleles for each trait.
– One allele is on each member of the homologous pair.
• Mendel used letters to indicate each allele.
– A capital letter symbolizes a dominant allele.
– A lowercase letter symbolizes a recessive allele.
– Dominant refers to the allele that will mask the expression of the
alternate (recessive) allele, when both are present in a given
organism.
8
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Trait
Characteristics
Dominant
F2Results
Recessive
Dominant
Recessive
Ratio
Stem length
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
Flower position
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
Totals:
9
The Inheritance of a Single Trait
• Hairline characteristic
– The dominant allele of widow’s peak is assigned (W).
– The recessive allele of straight hairline is assigned (w).
– In the case of a single trait, there are three possible
combinations of the two alleles.
• WW, Ww, ww
– If the two alleles are the same (homo), the individual is
said to be homozygous.
– If the two alleles are different (hetero), the individual is
said to be heterozygous.
10
Two Human Characters
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a. Widow’s peak: WW or Ww b. Straight hair line: ww
Figure 23.3
c. Short fingers: SS or Ss
d. Long fingers: ss
a: © HFPA, 63rd Golden Globe Awards; b: © Dynamic Graphics/PictureQuest RF;
c–d(both): © The McGraw-Hill Companies, Inc. Bob Coyle, photographer
11
Gamete Formation
• The genotype has two alleles for each trait,
whereas the gamete has one allele for a trait.
• Mendel’s law of segregation states:
– During meiosis, homologous chromosomes separate so
only one member of each pair is in a gamete.
– Therefore, only one allele exists for each trait in a
haploid gamete.
• When solving genetics problems,
– No two letters in a gamete can be the same letter of the
alphabet.
– If genotype is Ww, then gametes from this individual will
contain either a W or a w.
12
One-Trait Cross
• A sample cross
– A homozygous man
with a widow’s peak
reproduces with a
woman with a straight
hairline.
• What type of hairline
will their children
have?
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
widow’s peak
WW
Parents
meiosis
gametes
Offspring

straight hairline
ww
W
w
Ww
widow’s peak
13
One-Trait Cross
• Steps used to solve a genetics cross
1. Determine the genotype of each parent.
2. List the possible gametes from each parent.
3. Combine all possible gametes.
4. Determine the genotypes and phenotypes of
all offspring.
14
One-Trait Cross
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
• Solving a cross when
two individuals are both
Ww
Ww

♀
Ww
Key
W = Widow’s peak
w = Straight hairline
oocytes
♀
W
W
w
WW
Ww
sperm
– Perform a monohybrid
cross
– A Punnett square is
useful to solve this
problem
Parents
Widow’s peak
Straight hairline
Phenotypic Ratio
w
Ww
ww
3
1
Widow’s peak
Straight hairline
Offspring
Figure 23.4
15
One-Trait Cross
• After determining the genotype and phenotypes of
the offspring from the Punnett square analysis, a
tally is performed.
– The genotypic ratio is: 1 WW: 2Ww: 1ww or 1:2:1
– The pheotypic ratio is: 3 Widow’s peak: 1 straight
hairline or 3:1
– Three individuals have a widow’s peak because they
carry at least one dominant allele
16
One-Trait Crosses and Probability
• The chance of two or more independent events
occurring together is the product of their chance of
occurring separately (product rule of probability).
• In the cross Ww X Ww, what is the chance of obtaining
either a W or a w from a parent?
– Chance of W = ½ and the chance of w = ½
• Probability of having these genotypes is as follows.
1.Chance of WW= ½ X ½ = ¼
2.Chance of Ww = ½ X ½ = ¼
3.Chance of wW= ½ X ½ = ¼
4.Chance of ww = ½ X ½ = ¼
17
One-Trait Crosses and Probability
• The sum rule of probability, states that the chance
of an event can occur in more than one way is the
sum of the individual chances.
• To calculate the chance of an offspring having
widow’s peak, add the chances of WW, Ww or
wW, or ww.
• Chance of widow’s peak: ¼ + ¼ + ¼ = ¾ or 75%
– (WW) + (Ww) + (wW) = 3 out of 4 total possibilities
• Chance of straight hairline: ¼ or 25%
– (ww) = 1 out of 4 total possibilities
18
The One-Trait Testcross
• One cannot determine by observation if an
individual expressing the dominant trait is
homzygous or heterozygous.
– In a test cross, an organism with the dominant
phenotype is crossed with a recessive individual.
– If there are any offspring produced with the
recessive phenotype, then the dominant parent
must be heterozygous.
19
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Parents
♀

ww
WW
oocytes
♀
w
Ww
Ww
sperm
W
w
W
Figure 23.5a
Ww
Ww
Key
W = Widow’s peak
w = Straight hairline
Widow’s peak
Straight hairline
Phenotypic Ratio
All
Widow’s peak
Offspring
20
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
♀
Parents

Ww
ww
oocytes
♀
w
Ww
Ww
sperm
W
w
Key
W = Widow’s peak
w = Straight hairline
Widow’s peak
Straight hairline
Phenotypic Ratio
w
ww
ww
1
1
Widow’s peak
Straight hairline
Offspring
Figure 23.5b
21
One-Trait Testcross
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
♀

ww
WW
oocytes
♀
w
Ww
Ww
sperm
W
w
a.
Ww
Offspring
Figure 23.5
Ww

Ww
Key
W = Widow’s peak
w = Straight hairline
Widow’s peak
Straight hairline
Phenotypic Ratio
W
♀
Parents
All
ww
oocytes
♀
sperm
Parents
W
b.
w
Ww
Ww
Phenotypic Ratio
w
Widow’s peak
w
Key
W = Widow’s peak
w = Straight hairline
Widow’s peak
Straight hairline
ww
ww
1
1
Widow’s peak
Straight hairline
Offspring
22
The Inheritance of Two Traits
• Homologous chromosomes are inherited as a pair,
one member from each parent.
– Each has similar length and similar genes.
• During meiosis, each gamete receives one member
of each pair of homologues.
• The homologues separate independently.
• It does not matter which member of a pair goes into
which gamete.
• We can assume that the alleles for two genes are on
these homologues in the following examples.
23
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one pair
S
Cell has
two pairs of
homologues.
W
Allele Key
s
W = Widow’s peak
w = Straight hairline
S = Short fingers
s = Long fingers
w
one pair
Figure 23.6
24
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one pair
S
Cell has
two pairs of
homologues.
W
Allele Key
s
w
either
S
Ss
W
W
w
Figure 23.6
W = Widow’s peak
w = Straight hairline
S = Short fingers
s = Long fingers
or
s
one pair
S
Ss
s
w
W
w
W
MEIOSIS I
w
25
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one pair
S
Cell has
two pairs of
homologues.
W
Allele Key
s
w
either
S
S
S
Ss
s
W
W
w
w
W = Widow’s peak
w = Straight hairline
S = Short fingers
s = Long fingers
or
one pair
S
Ss
s
w
W
w
W
MEIOSIS I
s
s
S
S
s
s
w
w
W
W
MEIOSIS II
W
Figure 23.6
W
w
w
26
The Inheritance of Two Traits
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
one pair
Figure 23.6
S
Cell has
two pairs of
homologues.
W
Allele Key
W = Widow’s peak
w = Straight hairline
S = Short fingers
s = Long fingers
s
w
either
S
S
Ss
s
W
W
w
w
or
one pair
S
Ss
s
w
W
w
W
MEIOSIS I
S
s
s
S
S
s
w
w
W
s
MEIOSIS II
W
W
S
w
s
S
W
W
SW
w
s
w
S
w
sw
S
w
s
w
Sw
W
s
W
W
sW
27
Independent Assortment
• Mendel had no knowledge of meiosis, but realized
his results were attainable only if the sperm and
egg contains every possible combination of factors.
• This gave rise to the Law of Independent
Assortment.
– Each pair of factors assorts independently
(without regard to how the others separate).
– All possible combinations of factors can occur in
the gametes.
28
Two-Trait Crosses
• A two-trait (dihybrid) cross is used to test
the law of independent assortment.
• Example cross
– A person homozygous for widow’s peak and
short fingers (WWSS)
– A person who has a straight hairline and long
fingers (wwss)
– The cross is
WWSS X wwss
29
Two-Trait Crosses
• According to the law of segregation, the
gametes for WWSS parent must be WS.
• The gametes for the wwss parent must be ws.
• The offspring will all have the genotype WwSs.
• All offspring will have same phenotype (widow’s
peak and short fingers).
• This phenotype is called a dihybrid, because the
individual is heterozygous for both hairline and
fingers.
30
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♀

P generation
WWSS
wwss
Allele Key
W = Widow’s peak
w = Straight hairline
S = Short fingers
s = Long fingers
Figure 23.7
31
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
P generation
WWSS
P gametes
♀
wwss
ws
WS
Allele Key
W = Widow’s peak
w = Straight hairline
S = Short fingers
s = Long fingers
Figure 23.7
32
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
♀
P generation
WWSS
P gametes
wwss
ws
WS
Allele Key
W = Widow’s peak
w = Straight hairline
S = Short fingers
s = Long fingers
F1 generation
WwSs
Figure 23.7
33
Two-Trait Crosses
• In the next generation, the dihybrid (WwSs)
reproduces with another dihybrid; what
gametes are possible?
– Law of segregation states that each gamete
gets only one letter of each kind.
– Law of independent assortment states that all
combinations are possible.
• WS, Ws, wS, ws
34
Two-Trait Crosses (Dihybrid Cross)
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♀

P generation
WWSS
wwss
ws
WS
P gametes
F1 generation
WwSs
oocytes
♀
F1 gametes
WS
Ws
wS
ws
WWSS
WWSs
WwSS
WwSs
WWSs
WWss
WwSs
Wwss
wwSS
wwSs
wwSs
wwSs
WS
sperm
F2 generation Ws
wS
WwSS
WwSs
ws
WwSs
Wwss
Offspring
Allele Key
Figure 23.7
W = Widow’s peak
w = Straight hairline
S = Short fingers
s = Long fingers
9
3
3
1
Phenotypic Ratio
Widow’s peak, short fingers
Widow’s peak, long fingers
Straight hairline, short fingers
Straight hairline, long fingers
35
Two-Trait Crosses
• WwSs (X) WwSs
– Phenotypic Ratio
• 9 widow’s peak, short fingers
• 3 widow’s peak, long fingers
• 3 straight hairline, short fingers
• 1 straight hairline, long fingers
• A 9:3:3:1 phenotypic ratio is always expected
for a dihybrid cross when simple dominance is
present.
36
Two-Trait Crosses and Probability
• Probability Laws
–
–
–
–
Probability of widow’s peak = ¾
Probability of short fingers= ¾
Probability of straight hairline= ¼
Probability of long fingers= ¼
• Product Rule
–
–
–
–
Probability of widow’s peak and short fingers = ¾ X ¾ = 9/16
Probability of widow’s peak and long fingers = ¾ X ¼ = 3/16
Probability of straight hairline and short fingers = ¼ X ¾ = 3/16
Probability of straight hairline and long fingers = ¼ X ¼ = 1/16
37
Two-Trait Testcross
• It is impossible to tell by inspection whether an
individual expressing the dominant allele for two
traits is homozygous dominant or heterozygous
in regard to these traits.
• A testcross with an individual homozygous
recessive for both traits is necessary.
• A recessive phenotype is used because it has a
known phenotype.
38
Two-Trait Testcross
• If a man who is homozygous dominant for widow’s
peak and short fingers reproduces with a women
who is homozygous recessive for both traits, then
all his children will have dominant phenotypes.
• If a man is heterozygous for both traits, then each
child has a 25% chance of showing either one or
both recessive traits.
– 4 phenotypes at a ratio of 1:1:1:1
39
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Parents

WwSs
wwss
Key
W = Widow’s peak
w = Straight hairline
S = Short fingers
s = Long fingers
Figure 23.8
40
Two-Trait Testcross
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Parents

WwSs
wwss
eggs
♀
ws
WS
WwSs
sperm
Ws
Wwss
wS
wwSs
ws
wwss
Offspring
Key
W = Widow’s peak
w = Straight hairline
S = Short fingers
s = Long fingers
Figure 23.8
Phenotypes
9
3
3
1
Widow’s peak, short fingers
Widow’s peak, long fingers
Straight hairline, short fingers
Straight hairline, long fingers
41
23.2 Pedigree Analysis and
Genetic Disorders
• Many traits and disorders in humans, and
other organisms, are genetic in origin.
• Most follow Mendel’s laws.
• Such traits are controlled by a single allele
pair on the autosomal chromosomes-any
chromosome other than (X or Y).
42
Patterns of Inheritance
• Pedigree
– A chart of family’s history with regard to a particular
genetic trait
– Used to determine whether inherited condition is
due to autosomal dominant or an autosomal
recessive allele
43
Pedigrees for Autosomal Disorders
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Pattern I
Key
= affected
= unaffected
• In this pattern, the child is affected, but neither parent is.
• This can happen only if the disorder is recessive and the
parents are heterozygotes.
• Notice that the parents are carriers because they are
unaffected but are capable of having a child with the
genetic disorder.
44
Autosomal Recessive Pedigree
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I
♀
II
A?
aa
A?
Aa
*
III
Aa
A?
Relatives
Aa
Aa
A?
IV
aa
aa
Key
A?
Autosomal recessive disorders
• Most affected children have unaffected
parents.
A?
aa = affected
Aa = carrier (unaffected)
A A = unaffected
A? = unaffected
(one allele unknown)
• Heterozygotes (Aa) have an unaffected phenotype.
• Two affected parents will always have affected children.
• Affected individuals with homozygous unaffected mates will have
unaffected children.
• Close relatives who reproduce are more likely to have
affected children.
• Both males and females are affected with equal frequency.
Figure 23.9
45
Autosomal Recessive Disorders
• Requires the inheritance of two recessive alleles to
display the disorder
– Tay-Sachs disease
• Usually occurs among Jewish people in the US
• Results from a lack of the enzyme hexosaminidase A (Hex
A)
• The substrate, glycosphingolipid, is stored in lysosomes
• Lysosomes build up in brain cells, resulting in progressive
deterioration of psychomotor functions
46
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normal lysosome
defective lysosomes
Malfunctioning lysosomes in Tay-Sachs disease.
Courtesy Robert D. Terry/University of California, San Diego School of Medicine
Figure 23.10a
47
Autosomal Recessive Disorders
• Cystic Fibrosis (CF)
– Most common lethal genetic disorder among
Caucasians in the United States
– Chloride ions fail to pass through a plasma
membrane channel protein in cells
– Causes abnormally thick mucus in bronchial
tubes and pancreatic ducts
48
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Cl
Cl
H2O
H2O
Cl
Cl
Cl
H2O
cytoplasm
Chloride ions and water are trapped
inside cell.
Defective chloride ion channel does
not allow chloride ions to pass through.
Lumen of respiratory tract fills with
thick, sticky mucus.
Malfunctioning channel protein in cystic fibrosis.
Figure 23.10b
49
Genetic Disorders
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
normal lysosome
defective lysosomes
Cl
Cl
H2O
H2O
Cl
Cl
Cl
H2O
cytoplasm
Chloride ions and water are trapped
inside cell.
Defective chloride ion channel does
not allow chloride ions to pass through.
Lumen of respiratory tract fills with
thick, sticky mucus.
a. Malfunctioning lysosomes in Tay-Sachs disease.
b. Malfunctioning channel protein in cystic fibrosis.
a: Courtesy Robert D. Terry/University of California, San Diego School of Medicine
Figure 23.10
50
Autosomal Recessive Disorders
• Phenylketonuria (PKU)
– Affected in dividuals lack the enzyme needed for
the normal metabolism of the amino acid
phenylalanine
– Infants will develop normally if placed on a low
phenylalanine diet
– Severe mental retardation will result otherwise
51
Autosomal Recessive Disorders
• Sickle cell disease
– Red blood cells are sickle-shaped due to abnormal
hemoglobin.
– Red blood cells differ from normal hemoglobin by one
amino acid.
– Immediate symptoms include clogging of blood vessels
and breakdown of red blood cells.
– Other symptoms include poor circulation, anemia, and
low infection resistance.
– Internal hemorrhaging leads to further complications.
52
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
normal red blood cells
sickled red blood cell
Abnormally-shaped red blood cells in
sickle cell disease.
Figure 23.10
© Eye of Science/Photo Researchers, Inc.;
53
Autosomal Dominant Disorders
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Pattern II
Key
= affected
= unaffected
• The child is unaffected, but the parents are both affected.
• This is possible if the condition is autosomal dominant and the parents
are heterozygotes.
• This also illustrates that when both parents are unaffected, all their
children are unaffected.
– Neither parent has a dominant gene that passes the condition on.
54
Autosomal Dominant Pedigree
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
♀
I
Aa
Aa
*
II
III
Aa
aa
Aa
Aa
A?
aa
Autosomal dominant disorders
Figure 23.11
aa
aa
aa
aa
aa
aa
Key
AA = affected
Aa = affected
A? = affected
(one allele unknown)
aa = unaffected
• Affected children will usually have
an 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.
55
Autosomal Dominant Disorders
• Marfan syndrome
– Caused by a defect in an elastic connective
tissue protein called fibrillin
– Fibrillin is present in the eye lens, bones of
limbs, fingers, ribs and aorta
– Symptoms-dislocated lens, long limbs, and
fingers and caved in chest
– Aorta wall is weak-risk for aneurism
56
Autosomal Dominant Disorders
• Huntington disease
– Neurological disorder that leads to progressive
degeneration of brain cells
– Caused by a mutated copy of the gene for a protein
called huntingtin
– Most patients appear normal until they are of
middle age and have already had children
– Test now exists to detect presence of gene
57
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Many neurons in normal brain.
Courtesy Dr. Hemachandra Reddy, The Neurological Science Institute, Oregon Health & Science University
Figure 23.12
58
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Loss of neurons in Huntington brain.
Figure 23.12
Courtesy Dr. Hemachandra Reddy, The Neurological Science Institute, Oregon Health & Science University
59
23.3 Beyond Simple Inheritance
Patterns
• Incomplete Dominance
– Occurs when the heterozygote has an intermediate
phenotype between the two homozygotes
– Curly-haired person reproduces with a straighthaired person
• Their children have wavy hair
– Two wavy-haired persons reproduce with an
expected phenotypic ratio of: 1:2:1
• 1 curly-hair: 2 wavy-hair: 1 straight-hair
60
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Heterozygous
Parents (H1 H2)
H1 H2

♀
H1 H2
oocytes
sperm
♀
H1
Key
Straight hair
Wavy hair
Curly hair
H2
H1 H1 H1 H1 H2
H2 H1 H2 H2 H2
Phenotypic Ratio
1
2
1
Offspring
Figure 23.13
61
Incomplete Dominance
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A person with straight hair
(H1H1)
Heterozygous
Parents (H1 H2)

H1 H2
♀
Straight hair
Wavy hair
Curly hair
H1
H2
H1 H1
H1 H2
sperm
H1
H2
Figure 23.13
H1 H2
Key
H1 H2
oocytes
♀
A person with naturally curly
hair (H2H2)
H2 H2
Phenotypic Ratio
1
2
1
Offspring
(man): © Vol. 88/PhotoDisc; (woman): © Larry Williams/Corbis
62
Incomplete Dominance and
Codominance
• Codominance
– Occurs when alleles are equally expressed in a
heterozygote
• Ex: blood type AB
– Red blood cells have both Type A and Type B surface
antigens.
– We assume both alleles code for a product, and we observe
the result of both products being present.
63
Incompletely dominant Disorders
• Symptom severity in familial hypercholesterolemia (FH)
parallels the number of LDL-cholesterol receptor proteins
in the plasma membrane.
• A heterozygote has a mild form of disorder.
Plasma cholesterol (milligrams/deciliter)
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Homozygote
1000
900
800
700
600
Heterozygote
500
400
300
Normal
cholesterol
deposits
200
100
0
© Mediscan/Medical-On-Line
Figure 23.14
64
Multiple Allele Inheritance
• When a trait is controlled by multiple
alleles, the gene exists in several allelic
forms.
– Each person has only two of the possible
alleles.
– An example is demonstrated by the three alleles
responsible for the ABO blood type group.
65
ABO Blood Types
• ABO Blood Types
–
–
–
–
IA = A antigens on red blood cells
IB = B antigens on red blood cells
i = neither A nor B antigens on red blood cells
Both IA and IB are dominant over i, IA and IB are
codominant
Phenotype
A
B
AB
O
Genotype
IAIA or IAi
IBIB or IBi
IAIB
ii
66
ABO Blood Types
• ABO Blood Types
– Both IA and IB are dominant over I; IA and IB are
codominant.
• Rh factor
– Rh factor is inherited separately from ABO blood types.
– An Rh-positive person has the Rh antigen on their red
blood cells.
– There are multiple recessive alleles for Rh-negative
phenotype.
• All recessive to the Rh-positive allele
67
Inheritance of Blood Types
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
♀
Parents

IBi
IAi
oocytes
♀
i
IAIB
IBi
sperm
IB
IA
i
IAi
ii
Key
Blood type A
Blood type B
Blood type AB
Blood type O
Phenotypic Ratio
1
1
1
1
Offspring
Figure 23.15
68
Polygenic Inheritance
– Polygenic inheritance occurs when a trait is
governed by two or more genes (sets of alleles).
– Dominant alleles have a quantitative effect on
the phenotype, and these effects are additive.
– The result is a continuous variation of
phenotypes.
– Skin color is an example.
69
Skin Color
• Skin color is the result of pigmentation produced by
melanocyte cells.
• Over 100 different genes influence skin color.
• We use three pairs of alleles as a simplified
example, (Aa, Bb, and Cc).
• Each dominant allele (capital letter) contributes to
pigmentation.
• When a very dark person reproduces with a very
light person, the children have medium-brown skin.
70
Skin Color
• When two individuals with the genotype AaBbCc
reproduce with one another their offspring may
range in skin color from very dark to very light.
• The phenotype distribution follows a bell-shaped
curve (characteristic of a polygenic trait).
– Implies that few individuals have the extreme
phenotypes
– Most have the phenotype that lies in the middle
71
Polygenic Inheritance
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
AaBbCc
aaBbCC
AAbbCc
AabbCC
AABbcc
aaBBCc
AaBBcc
aaBBCC
AAbbCC
AABBcc
AaBbCC
AaBBCc
AABbCc
0
1
2
3
4
Frequency
aabbcc
Aabbcc
aaBbcc
aabbCc
AaBbcc
AabbCc
aaBbCc
AAbbcc
aaBBcc
aabbCC
AaBBCC
AABbCC
AABBCc
5
AABBCC
6
Number of dominant alleles
Figure 23.16
72
23.4 Environmental Influences
• Environmental factors such as nutrition or
temperature can influence the expression of
genetic traits.
• Polygenic traits are especially influenced by
environment.
– Examples
• Height is influenced by nutrition.
• Primrose is influenced by temperature.
– Plants have white flowers grown above 32°C.
– Plants have red flowers grown at 24°C.
73
23.4 Environmental Influences
• Environmental factors can influence the
expression of genetic traits.
– In the case of height, differences in nutrition are one of
the factors that bring about a bell-shaped curve.
Figure 23.17
74
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Number of Men
most
are
this
height
few
62
short
64
few
66
68
Height in Inches
70
72
74
tall
75
23.4 Environmental Influences
• Siamese cats and Himalayan rabbits are darker in color
at the ears, nose, paws and tail.
• Himalayan rabbit are known to be homozygous for the ch
allele, involved in producing melanin.
• The enzyme coded for by this gene is active only at low
temperatures, and that explains why the black color is
limited to the extremities, where body heat is lost to the
environment.
76
Coat Color in Himalayan Rabbits
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
© Jane Burton/Bruce Coleman, Inc.
Temperature can also affect the phenotypes of plants and animals.
Figure 23.18
77