Observing Patterns in
Inherited Traits
Chapter 10
10.1 Mendel,
Pea Plants, and Inheritance Patterns
 By experimenting with pea plants, Mendel was
the first to gather evidence of patterns by which
parents transmit genes to offspring
Mendel’s Experiments
carpel
stamen
a Garden pea flower, cut in half. Sperm form in
pollen grains, which originate in male floral parts
(stamens). Eggs develop, fertilization takes place,
and seeds mature in female floral parts (carpels).
b Pollen from a plant that breeds true for purple
flowers is brushed onto a floral bud of a plant that
breeds true for white flowers. The white flower had its
stamens snipped off. This is one way to assure
cross-fertilization of plants.
c Later, seeds develop inside pods of the crossfertilized plant. An embryo within each seed
develops into a mature pea plant.
d Each new plant’s flower color is indirect but
observable evidence that hereditary material
has been transmitted from the parent plants.
Fig. 10.3, p.154
Animation: Crossing garden pea plants
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Producing Hybrids
 Hybrids
• Offspring of a cross between two individuals that
breed true for different forms of a trait
 Each inherits nonidentical alleles for a trait being
studied
Producing Hybrid Offspring
Homozygous
dominant parent
Homozygous
recessive parent
(chromosomes
duplicated
before meiosis)
meiosis
I
meiosis II
(gametes)
(gametes)
fertilization
produces
heterozygous
offspring
Fig. 10.5, p.156
Heritable Units of Information
 Genes
• Heritable units of information about traits
• Each has its own locus on the chromosome
 Alleles
• Different molecular forms of the same gene
 Mutation
• Permanent change in a gene’s information
Heritable Units of Information
a A pair of homologous chromosomes,
both unduplicated. In most species, one
is inherited from a female parent and its
partner from a male parent.
b 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
c A pair of alleles may be identical
or not. Alleles are represented in
the text by letters such as D or d.
d Three pairs of genes (at three
loci on this pair of homologous
chromosomes); same thing as
three pairs of alleles.
Fig. 10.4, p.155
Modern Genetic Terms
 Homozygous dominant
• Has two dominant alleles for a trait (AA)
 Homozygous recessive
• Has two recessive alleles (aa)
 Heterozygote
• Has two nonidentical alleles (Aa)
Modern Genetic Terms
 Dominant allele may mask effect of recessive
allele on the homologous chromosome
 Genotype
• An individual’s alleles at any or all gene loci
 Phenotype
• An individual’s observable traits
Animation: Genetic terms
CLICK HERE TO PLAY
Key Concepts:
MODERN GENETICS
 Gregor Mendel gathered the first indirect,
experimental evidence of the genetic basis of
inheritance
 His meticulous work tracking traits in many
generations of pea plants gave him clues that
heritable traits are specified in units
 The units, distributed into gametes in predictable
patterns, were later identified as genes
10.2 Mendel’s Theory of Segregation
 Mendel’s Theory of Segregation:
• Diploid organisms have pairs of genes, on pairs
of homologous chromosomes
• Based on monohybrid experiments
 During meiosis
• Genes of each pair separate
• Each gamete gets one or the other gene
Producing Hybrid Offspring
 Crossing two true-breeding parents of different
genotypes yields hybrid offspring
 All F1 offspring are heterozygous for a gene, and
can be used in monohybrid experiments
• All F1 offspring of parental cross AA x aa are Aa
A Monohybrid Cross
 Crosses between F1 monohybrids resulted in
these allelic combinations among F2 offspring
• Phenotype ratio 3:1
• Evidence of dominant and recessive traits
F2 Offspring:
Dominant and Recessive Traits
Trait
Studied
Dominant
Form
Recessive
Form
F2 Dominantto-Recessive
Ratio
Seed
shape
5,474 round
1,850 wrinkled
2.98:1
Seed
color
6,022 yellow
2,001 green
3.01:1
Pod
shape
882 inflated
299 wrinkled
2.95:1
Pod
color
428 green
152 yellow
2.82:1
Flower
color
705 purple
224 white
3.15:1
651 long stem
207 at tip
3.14:1
Flower
position
Stem
length
787 tall
277 dwarf
2.84:1
Fig. 10.6, p.156
Predicting Probability: Punnett Squares
male gametes
female gametes
A
a
A
a
A
a
A
aa
a
A
A
Aa
aa
a
Aa
a
A
a
Aa
A
AA
Aa
aa
a
Aa
aa
a From left to right, step-by-step construction of a Punnett square. Circles
signify gametes. A stands for a dominant allele and a for a recessive allele at
the same gene locus. Offspring genotypes are indicated inside the squares.
Fig. 10.7a, p.157
female gametes
male gametes
A
a
A
a
A
a
A
aa
a
A
A
Aa
aa
a
Aa
a
A
a
Aa
A
AA
Aa
aa
a
Aa
aa
Stepped Art
Fig. 10-7a, p.157
Predicting F1 Offspring
F1 offspring
aa
True-breeding homozygous
recessive parent plant
a
a
A
Aa
Aa
A
Aa
Aa
Aa
Aa
Aa
Aa
AA
True-breeding homozygous
dominant parent plant
b Cross between two plants that breed true for different forms of a trait.
Fig. 10.7b, p.157
Predicting F2 Offspring
F2 offspring
Aa
Heterozygous
F1 offspring
A
a
A
AA
Aa
a
Aa
aa
AA
Aa
Aa
aa
Aa
Heterozygous
F1 offspring
c Cross between heterozygous F1 offspring.
Fig. 10.7c, p.157
Key Concepts:
MONOHYBRID EXPERIMENTS
 Some experiments yielded evidence of gene
segregation
 When one chromosome separates from its
homologous partner during meiosis, the pairs of
alleles on those chromosomes also separate
and end up in different gametes
Animation: Monohybrid cross
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Animation: F2 ratios interaction
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Animation: Testcross
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10.3 Mendel’s Theory of
Independent Assortment
 Mendel’s Theory of Independent Assortment:
• Meiosis assorts gene pairs of homologous
chromosomes independently of gene pairs on all
other chromosomes
• Based on dihybrid experiments
 Pairs of homologous chromosomes align
randomly at metaphase I
Independent Assortment in Meiosis I
One of two possible alignments
a Chromosome
alignments at
metaphase I:
b The resulting
alignments at
metaphase II:
B
c Possible
combinations
of alleles in
gametes:
The only other possible alignment
A
a
a
A
Aa
a
B
Bb
b
b
bB
B
A
A
a
a
A
A
a
a
B
B
b
b
b
b
B
B
A
A
a
a
A
A
a
a
AB
B b
ab
b
b
Ab
b
B
B
aB
Fig. 10.8, p.158
Dihybrid Experiments
 Start with a cross between true-breeding
heterozygous parents that differ for alleles of two
genes (AABB x aabb)
 All F1 offspring are heterozygous for both genes
(AaBb)
Mendel’s Dihybrid Experiments
 AaBb x AaBb
 Phenotypes of the F2 offspring of F1 hybrids
were close to a 9:3:3:1 ratio
•
•
•
•
9 dominant for both traits
3 dominant for A, recessive for b
3 dominant for B, recessive for a
1 recessive for both traits
Results of
Mendel’s Dihybrid Experiments
Meiosis, gamete formation
in true-breeding parent
plants
parent homozygous dominant
for purple flowers, tall stems
parent homozygous recessive
Gametes at fertilization for white flowers, short stems
meiosis,
gamete
formation
Possible genotypes resulting from a cross between two F1 plants:
meiosis,
gamete
formation
All F1 plants are AaBb
heterozygotes with purple
flowers and tall stems.
Fig. 10.9, p.159
Key Concepts:
DIHYBRID EXPERIMENTS
 Some experiments yielded evidence of
independent assortment
 During meiosis, the members of a pair of
homologous chromosomes are distributed into
gametes independently of all other pairs
Animation: Dihybrid cross
CLICK HERE TO PLAY
10.4 Beyond Simple Dominance
 Other types of gene expression
•
•
•
•
Codominant alleles
Incomplete dominance
Epistasis
Pleiotropy
Codominant Alleles
 Both expressed at the same time in heterozygotes
• Example: Multiple alleles in ABO blood typing
Genotypes:
Phenotypes
(Blood type):
AA
or
AO
A
AB
BB
or
BO
OO
AB
B
O
Fig. 10.10, p.160
Animation: Codominance: ABO blood
types
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Incomplete Dominance
 An allele is not fully dominant over its partner on
a homologous chromosome
• Both are expressed
• Produces a phenotype between the two
homozygous conditions
Incomplete Dominance
homozygous
homozygous
parent (RR) x parent (rr)
Cross two of the
F1 plants, and
the F2 offspring
will show three
phenotypes in
a 1:2:1 ratio:
RR
Rr
heterozygous
F1 offspring (Rr)
Rr
rr
Fig. 10.11, p.160
Animation: Incomplete dominance
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Epistasis
 Interacting products of one or more genes affect
the same trait
RRpp (rose comb)
F1 of spring:
F2 offspring:
RRPP, RRPp,
RrPP, or RrPp
9/16 walnut
RrPp
RRpp or Rrpp
3/16 rose
X
rrPP (pea comb)
RrPp (all walnut comb)
X
RrPp
rrPP or rrPp
3/16 pea
rrpp
1/16 single comb
Fig. 10.12, p.161
Animation: Comb shape in chickens
CLICK HERE TO PLAY
More Epistasis
EB
Eb
eB
eb
EB
EEBB
black
EEBb
black
EeBB
black
EeBb
black
Eb
EEBb
black
EEbb
chocolate
EeBb
black
Eebb
chocolate
eB
EeBB
black
EeBb
black
eeBB
yellow
eeBb
yellow
eb
EeBb
black
Eebb
chocolate
eeBb
yellow
eebb
yellow
Fig. 10.13, p.161
Animation: Coat color in Labrador
retrievers
CLICK HERE TO PLAY
Pleiotropy
 A single gene may affects two or more traits
• Example: Marfan syndrome
Animation: Pleiotropic effects of Marfan
syndrome
CLICK HERE TO PLAY
10.5 Linkage Groups
 All genes on the same chromosome are part of
one linkage group
 Crossing over between homologous
chromosomes disrupts gene linkages
Linkage Groups and Meiosis
 During meiosis, genes relatively close together
on a chromosome tend to stay together
• Few crossover events occur between them
 Genes that are relatively far apart tend to assort
independently into gametes
• Greater frequency of crossing over between them
Linkage and Crossing Over
Parental
generation
AC
ac
A
a
C
c
×
A
a
c
C
F1 offspring
All AaCc
meiosis, gamete formation
Gametes
A
C
a
c
Most gametes have
parental genotypes
A
c
a
C
A smaller number have
recombinant genotypes
Fig. 10.15, p.162
Animation: Crossover review
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10.6 Genes and Environment
 Environmental factors may affect gene
expression in individuals
• Example: Temperature and fur color
Animation: Coat color in the Himalayan
rabbit
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Elevation and Plant Height
Height (centimeters)
60
Height (centimeters)
60
Height (centimeters)
60
0
0
0
a Mature cutting
at high elevation
(3,060 meters
above sea level)
b Mature cutting
at mid-elevation
(1,400 meters
above sea level)
c Mature cutting
at low elevation
(30 meters above
sea level)
Fig. 10.17, p.163
Predation and Body Form
10.7 Complex Variations in Traits
 Polygenic Inheritance
• When products of
many genes
influence a trait,
individuals of a
population show a
range of continuous
variation for the trait
Continuous Variation
Number of individuals
with a measurable
value for the trait
This red graph line of the
range of variation for a trait
in a population plots out as
a bell-shaped curve. Such
curves indicate continuous
variation in a population.
Range of values for the trait
Fig. 10.19a, p.164
Animation: Continuous variation in
height
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Variations in Gene Expression
 Gene interactions and environmental factors
affect most phenotypes
• Gene products control metabolic pathways
• Mutations may alter or block pathways
Key Concepts:
VARIATIONS ON MENDEL’S THEME
 Not all traits have clearly dominant or recessive
forms
 One allele of a pair may be fully or partially
dominant over its nonidentical partner, or
codominant with it
VARIATIONS ON MENDEL’S THEME
(cont.)
 Two or more gene pairs often influence the
same trait, and some single genes influence
many traits
 The environment also influences variation in
gene expression
Video: One Bad Transporter and Cystic
Fibrosis
CLICK HERE TO PLAY