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Chapter 11: Observing Patterns in Inherited Traits
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Earlobe Variation
• Whether a person has attached or detached earlobes depends on a single gene
• Attached earlobes: two copies of the recessive allele for this gene
• Detached earlobes: either one or two copies of the dominant allele
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Early Ideas about Heredity
• People knew that sperm and eggs transmitted information about traits
• Blending theory
• Problem:
– Would expect variation to disappear
– Variation in traits persists
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Gregor Mendel
• Strong background in plant breeding and mathematics
• Using pea plants, found indirect but observable evidence of how parents transmit genes to offspring
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Gregor Mendel
Fig. 11-2, p.170
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Genes
• Units of information about specific traits
• Passed from parents to offspring
• Each has a specific location (locus) on a chromosome
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Alleles
• Different molecular forms of a gene
• Arise by mutation
• Dominant allele masks a recessive allele
that is paired with it
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Allele Combinations
• Homozygous
– having two identical alleles at a locus
– AA or aa
• Heterozygous
– having two different alleles at a locus
– Aa
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Genetic Terms
A pair of homologous
chromosomes
A gene locus
A pair of alleles
Three pairs of genes
Figure 11.4
Page 171
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A pair of homologous chromosomes, each in the
unduplicated state (most often, one from a male
parent and its partner from a female parent)
A gene locus (plural, loci), the location for a specific
gene on a chromosome. Alleles are at corresponding
loci on a pair of homologous chromosomes
A pair of alleles may be identical or nonidentical.
They are represented in the text by letters such as D
or d
Three pairs of genes (at three loci on this pair of
homologous chromosomes); same thing as three
pairs of alleles
Fig. 11-4, p.171
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Genotype & Phenotype
• Genotype refers to particular genes an individual carries
• Phenotype refers to an individual’s observable traits
• Cannot always determine genotype by observing phenotype
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Tracking Generations
• Parental generation P
mates to produce
• First-generation offspring F1
mate to produce
• Second-generation offspring F2
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Monohybrid Crosses
Experimental intercross between
two F1 heterozygotes
AA X aa Aa (F1 monohybrids)
Aa X Aa ?
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Mendel’s Monohybrid Cross Results
787 tall 277 dwarf
651 long stem 207 at tip
705 purple 224 white
152 yellow428 green
299 wrinkled882 inflated
6,022 yellow 2,001 green
5,474 round 1,850 wrinkled
F2 plants showed
dominant-to-
recessive ratio that
averaged 3:1
Fig. 11-6, p. 172
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Trait Studied Dominant
Form
Recessive
Form
F2 Dominant-to-
Recessive Ratio
SEED SHAPE
SEED COLOR
POD SHAPE
POD COLOR
FLOWER COLOR
FLOWER POSITION
STEM LENGTH
2.96:1
3.01:1
2.95:1
2.82:1
3.15:1
3.14:1
2.84:1787 tall 277 dwarf
651 long stem 207 at tip
705 purple 224 white
152 yellow428 green
299 wrinkled882 inflated
6,022 yellow 2,001 green
5,474 round 1,850 wrinkled
Fig. 11-6, p.172
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Probability
The chance that each outcome of a given event will occur is proportional to the number of ways that event can be reached
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Monohybrid Cross
Illustrated
True-breeding
homozygous recessive
parent plant
True-breeding
homozygous dominant
parent plant
An F1 plant
self-fertilizes
and produces
gametes:
F1
PHENOTYPES
F2
PHENOTYPES
aa
Aa
AA
aaAa
Aa
Aa Aa
Aa Aa
Aa Aa
Aa Aa
Aa
Aa
AA
aa
A
A
A
A
a a
a
a
AA
Figure 11.7
Page 173
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Mendel’s Theory of Segregation
• An individual inherits a unit of information (allele) about a trait from each parent
• During gamete formation, the alleles segregate from each other
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fertilization
produces
heterozygous
offspring
meiosis
II
meiosis
I
(chromosomes
duplicated
before meiosis)
Homozygous
dominant parent
Homozygous
recessive parent
(gametes) (gametes)
Fig. 11-5, p.172
Mendel’s
Theory of
Segregation
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Test Cross
• Individual that shows dominant phenotype is crossed with individual with recessive phenotype
• Examining offspring allows you to determine the genotype of the dominant individual
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Punnett Squares of
Test Crosses
Homozygous
recessive
a a
A
a aa
Aa Aa
aa
Homozygous
recessive
a a
A
A Aa
Aa Aa
Aa
Two phenotypes All dominant phenotype
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POSSIBLE EVENT: PROBABLE OUTCOME:
sperm A meets egg A
sperm A meets egg a
sperm a meets egg A
sperm a meets egg a
1/4 AA offspring
1/4 Aa
1/4 Aa
1/4 aa
p.173
Punnett Squares of
Test Crosses
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female gametes
male
gam
ete
s
aA
a aa
A
aA
aaAa
A
a
aA
aaAa
AaA
a
aA
aaAa
AaAAA
a
Fig. 11-7a, p.173
Punnett Squares of Test Crosses
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aA
aaAa
A
a
aA
aaAa
AaA
a
aA
aaAa
AaAAA
a
aA
a aa
A
female gametes
ma
le g
am
ete
s
Stepped Art
Fig. 11-7a, p.173
Punnett Squares of Test Crosses
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True-breeding
homozygous recessive
parent plant
True-breeding
homozygous dominant
parent plant
F1
PHENOTYPES
aa
Aa Aa
Aa Aa
Aa Aa
Aa AaA
A
AA
a a
Fig. 11-7b1, p.173
Punnett Squares of Test Crosses
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An F1 plant self-fertilizes
and produces gametes: F2
PHENOTYPES
Aa
AA Aa
Aa aa
AA Aa
Aa aaa
A
A a
Fig. 11-7b2, p.173
Punnett Squares of Test Crosses
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Dihybrid Cross
Experimental cross between individuals that
are homozygous for different versions of
two traits
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Dihybrid Cross: F1 Results
AABB aabbx
AaBb
AB AB ab ab
TRUE-
BREEDING
PARENTS:
GAMETES:
F1 HYBRID
OFFSPRING:
purple
flowers,
tall
white
flowers,
dwarf
All purple-flowered, tall
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1/16aaBB
1/16aaBb
1/16aaBb
1/16Aabb
1/16Aabb
1/16AAbb
1/16AABB
1/16AABb
1/16AaBB
1/16AaBb
1/16AABb
1/16
AaBb
1/16
AaBB
1/16AaBb
1/16AaBb
1/4 AB 1/4 Ab 1/4 aB 1/4 ab
1/16aabb
1/4 AB
1/4 Ab
1/4 aB
1/4 ab
AaBb AaBbX
1/16 white-flowered, dwarf
3/16 white-flowered, tall
3/16 purple-flowered, dwarf
9/16 purple-flowered, tall
Dihybrid Cross: F2 Results
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AB ab
AABB
purple-
flowered
tall parent
(homozygous
dominant)
aabb
white-
flowered
dwarf parent
(homozygous
recessive)
F1 OUTCOME:
All of the F1 plants are AaBb heterozygotes
(purple flowers, tall stems).
X
AaBb
Fig. 11-9a, p.175
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AB
Ab
aB
ab
1/4
1/4
1/4
AB
1/4
Ab
1/4
aB
1/4
ab
1/4meiosis,gameteformation
AABB
1/16
AABb
1/16
AaBB
1/16
AaBb
1/16
AABb
1/16
AAbb
1/16
AaBb
1/16
Aabb
1/16
AaBB
1/16
AaBb
1/16
aaBB
1/16
aaBb
1/16
AaBb
1/16
Aabb
1/16
aaBb
1/16
aabb
1/16
Fig. 11-9b, p.175
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Independent Assortment
• Mendel concluded that the two “units” for the first trait were to be assorted into gametes independently of the two “units” for the other trait
• Members of each pair of homologous chromosomes are sorted into gametes at random during meiosis
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Independent Assortment
Metaphase I:
Metaphase II:
Gametes:
1/4 AB 1/4 ab 1/4 Ab 1/4 aB
A A A A
A A A A
AAAA
B B
B B
BB
B B
BBBB
a a a a
aa aa
aaaa
bb b b
bb b b
b b b b
OR
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Nucleus of a
diploid (2n)
reproductive cell
with two pairs of
homologous
chromosomes
Possible alignmentsof the two homologouschromosomes duringmetaphase I of meiosis
The resulting alignments at metaphase II
Allelic combinationspossible in gametes
1/4 AB 1/4 ab 1/4 Ab 1/4 aB
Fig. 11-8, p.174
Independent Assortment
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Tremendous Variation
Number of genotypes possible in offspring
as a result of independent assortment and
hybrid crossing is
3n
(n is the number of gene loci
at which the parents differ)
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Impact of Mendel’s Work
• Mendel presented his results in 1865
• Paper received little notice
• Mendel discontinued his experiments in 1871
• Paper rediscovered in 1900
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Dominance Relations
Complete dominance
Incomplete dominance
Codominance
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Incomplete Dominance X
Homozygous
parent
Homozygous
parent
All F1 are
heterozygous
X
F2 shows three phenotypes in 1:2:1 ratio
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Xhomozygous parent homozygous parent
All F1 offspring
heterozygous for
flower color:
Fig. 11-11, p.176
Cross two of the F1
plants and the F2
offspring will show
three phenotypes in
a 1:2:1 ratio:
Incomplete Dominance
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Codominance: ABO Blood Types
• Gene that controls ABO type codes for enzyme that dictates structure of a glycolipid on blood cells
• Two alleles (IA and IB) are codominant when paired
• Third allele (i) is recessive to others
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Fig. 11-10a, p.176
Range of genotypes:
IAIA IBIB
IAi IAIB IBi ii
Blood
Types:
or
A AB B O
or
ABO Blood Type
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Fig. 11-10b, p.176
ABO Blood Type
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ABO and Transfusions
• Recipient’s immune system will attack blood
cells that have an unfamiliar glycolipid on
surface
• Type O is universal donor because it has
neither type A nor type B glycolipid
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Pleiotropy
• Alleles at a single locus may have effects on two or more traits
• Marfan syndrome - Mutation in gene for fibrillin affects skeleton, cardiovascular system, lungs, eyes, and skin
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Epistasis
• Interaction between the products of gene pairs
• Common among genes for hair color in mammals
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Fig. 11-13, p.177
Epistasis
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Coat Color inRetrievers bbeeBBEE X
F1 puppies
are all BbEe
BBEE
BBEe
BbEE
BbEe Bbee
BbEe
Bbee
BBEe BbEE BbEe
BBee BbEe
bbeebbEe
bbEE bbEe
BE Be bE be
BE
Be
bE
be
black
brown
yellow
F2 puppies
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rose comb pea comb walnut comb single comb
all walnutRrPp
F1
RrPp RrPp
F2
roseRRpp
pearrPP
X
9/16 walnutRRPP, RRPp,RrPP, or RrPp
3/16 roseRRpp or Rrpp
3/16 pearrPP or rrPp
1/16 singlerrpp
Fig. 11-12, p.177
Comb Shape in Poultry
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Crossing Over
•Each chromosome
becomes zippered to its
homologue
•All four chromatids are
closely aligned
•Nonsister chromosomes
exchange segments
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Effect of Crossing Over
• After crossing over, each chromosome
contains both maternal and paternal
segments
• Creates new allele combinations in offspring
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Linkage Groups
• Genes on one type of chromosome
• Fruit flies
– 4 homologous chromosomes
– 4 linkage groups
• Not all genes on chromosome are
tightly linked
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Full Linkage
x
AB ab
50% AB 50% ab
All AaBb
meiosis, gamete formation
Parents:
F1 offspring:
Equal ratios of two
types of gametes:
AB
ab
A
Ba
b
ab
AB
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Incomplete Linkage
Parents:
F1 offspring:
Unequal ratios of four
types of gametes:
All AaCc
x
meiosis, gamete formation
AC acA
C A
C
AC
a
c
ac
A
c
a
C
a
c
parental
genotypes
recombinant
genotypesFigure 11.15
Page 178
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Crossover Frequency
Proportional to the distance that
separates genes
A B C D
Crossing over will disrupt linkage between
A and B more often than C and D
In-text figure
Page 178
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Linkage Mapping in Humans
• Linkage maps based on pedigree analysis through generations
• Color blindness and hemophilia are very closely linked on X chromosome
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Environmental Effects on Plant Phenotype
• Hydrangea macrophylla
• Action of gene responsible for floral color is influenced by soil acidity
• Flower color ranges from pink to blue
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Fig. 11-17a, p.179
Environmental Effects on Plant Phenotype
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Fig. 11-17b, p.179
Environmental Effects on Plant Phenotype
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Temperature Effects on Phenotype
• Rabbit is homozygous for an allele that specifies a heat-sensitive version of an enzyme in melanin-producing pathway
• Melanin is produced in cooler areas of body
Figure 11.16
Page 179
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Fig. 11-16, p.179
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Campodactyly: Unexpected Phenotypes
• Effect of allele varies:
– Bent fingers on both hands
– Bent fingers on one hand
– No effect
• Many factors affect gene expression
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Continuous Variation
• A more or less continuous range of small differences in a given trait among individuals
• The greater the number of genes and
environmental factors that affect a trait, the
more continuous the variation in versions of
that trait
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Human Variation
• Some human traits occur as a few discrete types– Attached or detached earlobes
– Many genetic disorders
• Other traits show continuous variation– Height
– Weight
– Eye color
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Fig. 11-18, p.180
Continuous
Variation Variation in human
eye color
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Describing Continuous Variation
Range of values for the trait
Nu
mb
er
of
ind
ivid
ua
ls w
ith
so
me
va
lue
of
the
tra
it
(line of bell-shaped curve indicates
continuous variation in population)
Range of values for the trait
Nu
mb
er
of
ind
ivid
ua
ls w
ith
so
me v
alu
e o
f th
e t
rait
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The line of a bell-
shaped curve reveals
continuous variation
in the population
Range of values for the trait
Nu
mb
er
of
ind
ivid
ua
ls w
ith
so
me v
alu
e o
f th
e t
rait
Fig. 11-19a, p.180
Describing Continuous Variation
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Range of values for the trait
Nu
mb
er
of
ind
ivid
uals
wit
h
so
me v
alu
e o
f th
e t
rait
Fig. 11-19b, p.180
Describing Continuous Variation
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Fig. 11-20, p.181
Describing
Continuous
Variation
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Fig. 11-21, p.183
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Fig. 11-22, p.183
dominant recessive
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Fig. 11-22, p.183
dominant recessive
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Fig. 11-24, p.185