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Cecie Starr
Christine Evers
Lisa Starr
www.cengage.com/chemistry/starr
Chapter 13
Observing Patterns in
Inherited Traits
(Sections 13.4 - 13.6)
Albia Dugger • Miami Dade College
13.4 Mendel’s
Theory of Independent Assortment
• When homologous chromosomes separate during meiosis,
either one of the pair can end up in a particular nucleus
• Thus, gene pairs on one chromosome get sorted into
gametes independently of gene pairs on other chromosomes
• Punnett squares can be used to predict inheritance patterns
of two or more genes simultaneously
Dihybrid Cross
• In a dihybrid cross, individuals identically heterozygous for
alleles of two genes (dihybrids) are crossed, and the traits of
the offspring are observed
• dihybrid cross
• Breeding experiment in which individuals identically
heterozygous for two genes are crossed
• The frequency of traits among the offspring offers
information about the dominance relationships between
the paired alleles
A Dihybrid Cross
• Start with one parent plant that breeds true for purple flowers
and tall stems (PPTT ) and one that breeds true for white
flowers and short stems (pptt)
• Each plant makes only one type of gamete (PT or pt)
• All F1 offspring will be dihybrids (PpTt) and have purple
flowers and tall stems
A Dihybrid Cross (cont.)
• Then cross two F1 plants: a dihybrid cross (PpTt X PpTt)
• Four types of gametes can combine in sixteen possible ways
• In F2 plants, four phenotypes result in a ratio of 9:3:3:1
• 9 tall with purple flowers
• 3 short with purple flowers
• 3 tall with white flowers
• 1 short with white flowers
Law of Independent Assortment
• Mendel discovered the 9:3:3:1 ratio in his dihybrid
experiments – and noted that each trait still kept its individual
3:1 ratio
• Each trait (gene pair) sorted into gametes independently of
other traits (gene pairs)
• law of independent assortment
• During meiosis, members of a pair of genes on
homologous chromosomes get distributed into gametes
independently of other gene pairs
Independent Assortment
Independent Assortment
A This example shows just two pairs of
homologous chromosomes in the nucleus of a
diploid (2n) reproductive cell. Maternal and
paternal chromosomes, shown in pink and
blue, have already been duplicated.
B Either chromosome of a pair may get attached to
either spindle pole during meiosis I. With two pairs
of homologous chromosomes, there are two
different ways that the maternal and paternal
chromosomes can get attached to opposite
spindle poles.
or
meiosis I
meiosis I
C Two nuclei form with each scenario, so
there are a total of four possible
combinations of parental chromosomes in
the nuclei that form after meiosis I.
meiosis II
meiosis II
D Thus, when sister chromatids separate
during meiosis II, the gametes that result
have one of four possible combinations
of maternal and paternal chromosomes.
gamete genotype:
pt
PT
pT
Pt
Fig. 13.7, p. 194
Independent Assortment
A This example shows just two pairs of
homologous chromosomes in the nucleus of a
diploid (2n) reproductive cell. Maternal and
paternal chromosomes, shown in pink and
blue, have already been duplicated.
B Either chromosome of a pair may get attached
to either spindle pole during meiosis I. With two
pairs of homologous chromosomes, there are two
different ways that the maternal and paternal
chromosomes can get attached to opposite
spindle poles.
or
meiosis I
meiosis I
C Two nuclei form with each scenario, so
there are a total of four possible
combinations of parental chromosomes in
the nuclei that form after meiosis I.
meiosis II
meiosis II
D Thus, when sister chromatids separate
during meiosis II, the gametes that result
have one of four possible combinations
of maternal and paternal chromosomes.
gamete genotype:
pt
PT
pT
Pt
Stepped Art
Fig. 13.7, p. 194
A Dihybrid Cross
A Dihybrid Cross
parent plant parent plant
homozygous homozygous
for purple for white flowers
flowers and and short stems
long stems
pptt
PPTT
1
pt
PT
2
PT
4
PT
Pt
pT
pt
PP
PPTT
PPTt
PpTT
PpTt
Pt
PPTt
PPtt
PpTt
Pptt
pT
PpTT
PpTt
ppTT
ppTt
pt
PpTt
Pptt
ppTt
pptt
PpTt
dihybrid
Pt
pT
pt
3 four types of gametes
Fig. 13.8, p. 195
A Dihybrid
Cross
Fig. 13.8.1-3, p. 195
A Dihybrid Cross
Fig. 13.8.4, p. 195
A Dihybrid Cross
parent plant parent plant
homozygous homozygous
for purple for white flowers
flowers and and short stems
longPPTT
stems
pptt
1
PT
2
PT
3
pt
4
PT
Pt
pT
pt
PP
PPTT
PPTt
PpTT
PpTt
Pt
PPTt
PPtt
PpTt
Pptt
pT
PpTT
PpTt
ppTT
ppTt
pt
PpTt
Pptt
ppTt
pptt
PpTt
dihybrid
Pt
pT
pt
four types of gametes
Stepped Art
Fig. 13.8, p. 195
ANIMATION: Dihybrid cross
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The Contribution of Crossovers
• Genes that are far apart on a chromosome tend to assort into
gametes independently because crossing over occurs
between them very frequently
• Genes that are very close together on a chromosome are
linked, they do not assort independently because crossing
over rarely happens between them
• linkage group
• All genes on a chromosome
Key Concepts
• Insights From Dihybrid Crosses
• Pairs of genes on different chromosomes are typically
distributed into gametes independently of how other gene
pairs are distributed
• Breeding experiments with alternative forms of two
unrelated traits can be used as evidence of such
independent assortment
ANIMATION: Crossover review
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13.5 Beyond Simple Dominance
• Mendel studied inheritance patterns that are examples of
simple dominance, in which a dominant allele fully masks the
expression of a recessive one
• Other patterns of inheritance are not so simple:
• Codominance
• Incomplete dominance
• Epistasis
• Pleiotropy
Codominance
• Codominant alleles are both expressed at the same time in
heterozygotes, as in multiple allele systems such as the one
underlying ABO blood typing
• codominant
• Refers to two alleles that are both fully expressed in
heterozygous individuals
• multiple allele system
• Gene for which three or more alleles persist in a
population
Codominance: ABO Blood Types
• Which two of the three alleles of the ABO gene you have
determines your blood type
• The A and the B allele are codominant when paired
• Genotype AB = blood type AB
• The O allele is recessive when paired with either A or B
• Genotype AA or AO = blood type A
• Genotype BB or BO= type B
• Genotype OO = type O
Codominance: ABO Blood Types
Codominance: ABO Blood Types
Genotypes:
Phenotypes
(blood type):
AA
or
AO
A
AB
BB
or
BO
OO
AB
B
O
Fig. 13.9, p. 196
ANIMATION: Codominance: ABO blood
types
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Incomplete Dominance
• With incomplete dominance, the heterozygous phenotype is
between the two homozygous phenotypes
• incomplete dominance
• Condition in which one allele is not fully dominant over
another, so the heterozygous phenotype is between the
two homozygous phenotypes
Incomplete Dominance: Snapdragons
• In snapdragons, one allele (R) encodes an enzyme that
makes a red pigment, and allele (r) makes no pigment
• RR = red; Rr = pink; rr = white
• A cross between red and white (RR X rr) yields pink (Rr)
• A cross between two pink (Rr X Rr) yields red, pink, and white
in a 1:2:1 ratio
Incomplete Dominance: Snapdragons
Incomplete
Dominance:
Snapdragons
homozygous (RR) x homozygous (rr)
heterozygous (Rr)
A Cross a red-flowered with a white-flowered plant,
and all of the offspring will be pink heterozygotes.
B If two of the pink heterozygotes
are crossed, the phenotypes
of the resulting offspring will
occur in a 1:2:1 ratio.
Fig. 13.10, p. 196
Incomplete Dominance: Snapdragons
Fig. 13.10a, p. 196
Incomplete Dominance: Snapdragons
homozygous (RR) x homozygous (rr)
heterozygous (Rr)
A Cross a red-flowered with a white-flowered plant, and
all of the offspring will be pink heterozygotes.
Fig. 13.10a, p. 196
Incomplete Dominance: Snapdragons
Fig. 13.10b, p. 196
Incomplete Dominance: Snapdragons
B If two of the pink
heterozygotes are
crossed, the phenotypes
of the resulting offspring
will occur in a 1:2:1 ratio.
Fig. 13.10b, p. 196
ANIMATION: Incomplete dominance
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Epistasis
• Some traits are affected by multiple gene products, an effect
called polygenic inheritance or epistasis
• epistasis
• Effect in which a trait is influenced by the products of
multiple genes
Epistasis: Labrador Retriever
• Labrador retriever coat color, can be black, brown, or yellow
Epistasis: Labrador Retriever
• A dominant allele (B) specifies black fur, and its recessive
partner (b) specifies brown fur
• A dominant allele of a different gene (E ) causes color to be
deposited in fur, and its recessive partner (e) reduces color
• A dog with an E and a B allele has black fur
• A dog with an E allele and homozygous for b is brown
• A dog homozygous for the e allele has yellow fur
regardless of its B or b alleles
Epistasis: Labrador Retriever
Animation: Coat Color in Labrador Retrievers
Pleiotropy
• A pleiotropic gene influences multiple traits
• Mutations in pleiotropic genes are associated with complex
genetic disorders such as sickle-cell anemia, cystic fibrosis,
and Marfan syndrome
• pleiotropic
• Refers to a gene whose product influences multiple traits
Pleiotropy: Marfan Syndrome
• In Marfan syndrome,
mutations affect
elasticity of tissues of
the heart, skin, blood
vessels, tendons, and
other body parts
• Haris Charalambous
died when his aorta
burst – he was 21
ANIMATION: Pleiotropic effects of Marfan
syndrome
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Animation: Comb Shape in Chickens
13.6 Complex Variation in Traits
• Phenotype often results from complex interactions among
gene products and the environment
• Many traits show a continuous range of variation
Continuous Variation
• Some traits appear in two or three forms; others occur in a
range of small differences (continuous variation)
• The more genes and environmental factors that influence a
trait, the more continuous is its variation
• continuous variation
• In a population, a range of small differences in a shared
trait
Continuous Variation (Cont.)
• If a graph line drawn around the top of the bars showing the
distribution of values for a trait is bell-shaped (a bell curve)
the trait varies continuously
• bell curve
• Bell-shaped curve
• Typically results from graphing frequency versus
distribution for a trait that varies continuously
Continuous Variation (Cont.)
• Human height and eye
color are traits that vary
continuously
Continuous Variation (Cont.)
Environmental Effects on Phenotype
• Environmental factors often affect gene expression, which in
turn affects phenotype:
• Seasonal change in animal fur colors
• Spines grow in presence of predators
• Different plant heights when grown at different altitudes
Environmental Effects on Phenotype
• In summer, the snowshoe hare’s fur is brown; in winter, white
– offering seasonal camouflage from predators
Animation: Continuous Variation in Height
Environmental Effects on Phenotype
• Daphnia at right developed a longer tail spine and a pointy
head in response to chemicals emitted by predatory insects
Environmental Effects on Phenotype
• Yarrow plant (Achillea millefolium) grew to different heights at
three different elevations
Environmental Effects on Phenotype
A Plant grown at
high elevation
(3,060 meters
above sea level)
B Plant grown at
mid-elevation
(1,400 meters
above sea level)
C Plant grown
at low elevation
(30 meters
above sea level)
Fig. 13.16, p. 199
Key Concepts
• Variations on Mendel’s Theme
• Not all traits appear in Mendelian inheritance patterns
• An allele may be partly dominant over a nonidentical
partner, or codominant with it
• Multiple genes may influence a trait; some genes influence
many traits
• The environment also influences gene expression
Menacing Mucus (revisited)
• The ΔF508 allele that causes cystic fibrosis in homozygotes
may persist because it offers heterozygous individuals a
survival advantage against certain deadly infectious diseases
• People who carry it may have a decreased susceptibility to
typhoid fever and other bacterial diseases that begin in the
intestinal tract