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Chapter 13
Observing Patterns
in Inherited Traits
Albia Dugger • Miami Dade College
13.1 Menacing Mucus
• Cystic fibrosis (CF) is the most common fatal genetic disorder
in the United States
• The CFTR gene encodes CFTR protein which maintains a
thin film of water on the surface of the epithelial sheets
• A deletion of three base pairs (ΔF508, deletion) prevents
proper membrane trafficking of CFTR so it can’t do its job
• Mucus obstructs the airways and bacteria infect the intestine
and lungs – most CF patients live no longer than thirty years
ATP
ΔF508
Figure 13-1a p203
13.2 Mendel, Pea Plants,
and Inheritance Patterns
• Recurring inheritance patterns are observable evidence of
how heredity works
• Before the discovery of genes, it was thought that inherited
traits resulted from a blend of parental characters
Mendel’s Experimental Approach
• Gregor Mendel was a monk with training in plant breeding
and mathematics
• He studied the garden pea (Pisum sativum), which breeds
true for a number of traits
• Mendel discovered that traits of offspring of cross-fertilized
pea plants often appear in predictable patterns
• Mendel’s work led him to conclude that hereditary information
passes from one generation to the next in discrete units
Gregor Mendel
Garden Pea Plant:
Self Fertilization and Cross-Fertilization
B
A
carpel
C
anther
D
E
ANIMATED FIGURE: Crossing garden pea
plants
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Terms Used in Modern Genetics
• Genes are heritable units of information about traits
• Each gene has a specific locus on a chromosome
• Diploid cells (chromosome number 2n) have pairs of genes
on homologous chromosomes
• Alleles are different molecular forms of a gene
Loci of Some Human Genes
ribosomal RNA
skin pigmentation
fibrillin 1 (Marfan
syndrome)
(Tay–Sachs
disease)
15
Figure 13-3a p205
(Canavan disease)
p53 tumor antigen
NF1 (neurofibromatosis)
serotonin transporter
BRCA1 (breast,
ovarian cancer)
Growth hormone
17
Figure 13-3b p205
LDL receptor
(coronary artery disease)
insulin receptor
brown hair color
green/blue eye color
(Warfarin resistance)
HCG, β chain
19
LH, β chain
Figure 13-3c p205
prion protein
(Creutzfeldt–
Jakob disease)
oxytocin
GHRH
(acromegaly)
20
Figure 13-3d p205
dystrophin
(muscular dystrophy)
(anhidrotic ectodermal
dysplasia)
IL2RG (SCID-X1)
XIST X chromosome
inactivation control
(hemophilia B)
(hemophilia A)
(red-deficient color blind)
X
(green-deficient color blind)
Figure 13-3e p205
Terms Used in Modern Genetics
• The particular set of alleles that an individual carries is the
individual’s genotype
• An individual with two identical alleles of a gene is
homozygous for that gene
• An individual with nonidentical alleles of a gene is
heterozygous for that gene
Terms Used in Modern Genetics
• A hybrid is the heterozygote offspring of a cross between two
individuals that breed true for different forms of a trait
• An individual’s genotype determines its phenotype, which
refers to an individual’s observable traits
• Any mutated gene is a new allele, whether or not it affects
phenotype
Terms Used in Modern Genetics
• An allele is dominant if its effect masks the effect of a
recessive allele paired with it
• Capital letters (P) signify dominant alleles; lowercase
letters (p) signify recessive alleles
• Homozygous dominant (PP)
• Homozygous recessive (pp)
• Heterozygous (Pp)
Genotypes Give Rise to Phenotypes
genotype:
phenotype:
PP
(homozygous for
dominant allele P)
pp
(homozygous for
recessive allele p)
Pp
(heterozygous at
the P gene locus)
Take-Home Message:
How do alleles contribute to traits?
• Gregor Mendel discovered the role of alleles in inheritance by
breeding pea plants and tracking traits of their offspring
• Genotype refers to the particular set of alleles carried by an
individual’s somatic cell; phenotype refers to the individual’s
set of observable traits; genotype is the basis of phenotype
• A homozygous individual has two identical alleles at a
particular locus; a heterozygous individual has nonidentical
alleles at the locus
• Dominant alleles mask the effects of recessive ones in
heterozygous individuals
13.3 Mendel’s Law of Segregation
• Pairs of genes on homologous chromosomes separate during
meiosis, so they end up in different gametes
• Mendel showed that garden pea plants inherit two “units” of
information for a trait, one from each parent
Gene Segregation
• Homologous chromosomes (and all the alleles they carry)
segregate into separate gametes during meiosis
• Plants homozygous for the dominant allele (PP) can only
make gametes that carry the allele P
• Plants homozygous for the recessive allele (pp) can only
make gametes that carry the allele p
• Heterozygous plants produce both type of gametes
Calculating Probabilities
• Probability
• A measure of the chance that a particular outcome will
occur
• Punnett square
• A grid used to calculate the probability of genotypes and
phenotypes in offspring
DNA replication
meiosis I
2
1
meiosis II
3
gametes (P)
gametes (p)
zygote (Pp)
male gametes
4
female gametes
Stepped Art
Figure 13-5 p206
male gametes
female gametes
Figure 13-5b p206
Testcrosses
• A testcross is a method of determining if an individual is
heterozygous or homozygous dominant
• An individual with unknown genotype is crossed with one that
is homozygous recessive (PP x pp) or (Pp x pp)
Monohybrid Crosses
• A monohybrid cross is a testcross that checks for a
dominance relationship between two alleles at a single locus
• May be a cross between true breeding (homozygous)
individuals (PP x pp), or between identical heterozygotes (Pp
x Pp)
Generations in a Monohybrid Cross
• P stands for parents, F for filial (offspring)
• F1: First generation offspring of parents
• F2: Second generation offspring of parents
Mendel’s Monohybrid Crosses
• Mendel used monohybrid crosses to find dominance
relationships among pea plant traits
• When he crossed plants that bred true for white flowers with
plants that bred true for purple flowers, all F1 plants had
purple flowers
• When he crossed two F1 plants, ¾ of the F2 plants had purple
flowers, ¼ had white flowers
Table 13-1 p207
Mendel’s 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
Offspring of Mendel’s Monohybrid Cross
Mendel’s Law of Segregation
• Mendel observed a phenotype ratio of 3:1 in the F2 offspring
of his monohybrid crosses
• Consistent with the probability of the pp genotype in the
offspring of a heterozygous cross (Pp x Pp)
• This is the basis of Mendel’s law of segregation
• Diploid cells have pairs of genes on pairs of homologous
chromosomes
• The two genes of each pair separate during meiosis, and
end up in different gametes
Take-Home Message:
What is Mendel’s law of segregation?
• Diploid cells carry pairs of genes, on pairs of homologous
chromosomes
• The two genes of each pair are separated from each other
during meiosis, so they end up in different gametes
• Mendel discovered patterns of inheritance in pea plants by
tracking the results of many monohybrid crosses
ANIMATED FIGURE: Monohybrid cross
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13.4 Mendel’s Law
of Independent Assortment
• Mendel’s law of independent assortment
• During meiosis, members of a pair of genes on
homologous chromosomes get distributed into gametes
independently of other gene pairs
Dihybrid Crosses
• Dihybrid crosses test for dominance relationships between
alleles at two loci
• Individuals that breed true for two different traits are crossed
(PPTT x pptt)
• F2 phenotype ratio is 9:3:3:1 (four phenotypes)
• Individually, each dominant trait has an F2 ratio of 3:1 –
inheritance of one trait does not affect inheritance of the other
The Contribution of Crossovers
• Independent assortment also occurs when the genes are on
the same chromosome, but far enough apart that crossing
over occurs between them very frequently
• Genes that have loci very close to one another on a
chromosome tend to stay together during meiosis and not
assort independently
Linkage Groups
• All genes on one chromosome are called a linkage group
• The farther apart two genes are on a chromosome, the more
often crossing over occurs between them
• Linked genes are very close together; crossing over rarely
occurs between them
• The probability that a crossover will separate alleles of two
genes is proportional to the distance between those genes
What is
Mendel’s law of independent assortment?
Take-Home Message:
• Each member of a pair of genes on homologous
chromosomes tends to be distributed into gametes
independently of how other genes are distributed during
meiosis
ANIMATED FIGURE: Independent
assortment
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ANIMATED FIGURE: Dihybrid cross
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13.5 Beyond Simple Dominance
• Mendel focused on traits based on clearly dominant and
recessive alleles; however, the expression patterns of genes
for some traits are not as straightforward
Codominance
• Codominance
• Two nonidentical alleles of a gene are both fully expressed
in heterozygotes, so neither is dominant or recessive
• May occur in multiple allele systems
• Multiple allele systems
• Genes with three or more alleles in a population
• Example: ABO blood types
Codominance in ABO Blood Types
Genotypes:
AA
or
AO
Phenotypes
(blood type):
A
AB
BB
or
BO
OO
AB
B
O
Incomplete Dominance
• Incomplete dominance
• One allele is not fully dominant over its partner
• The heterozygote’s phenotype is somewhere between the
two homozygotes, resulting in a 1:2:1 phenotype ratio in F2
offspring
• Example: Snapdragon color
• RR is red
• Rr is pink
• rr is white
homozygous (RR)
homozygous (rr)
heterozygous (Rr)
Figure 13-10 p210
Figure 13-10b p210
INTERACTION: Incomplete dominance
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Epistasis
• Epistasis
• Two or more gene products influence a trait
• Typically, one gene product suppresses the effect of
another
• Example: Coat color in dogs
• Alleles B and b designate colors (black or brown)
• Two recessive alleles ee suppress color
Coat Colors in Labrador Retrievers
Figure 13-11b p211
Pleiotropy
• A pleiotropic gene
influences multiple traits
• Example: Some tall, thin
athletes have Marfan
syndrome, a potentially
fatal genetic disorder
Take-Home Message
Are all alleles dominant or recessive?
• An allele may be fully dominant, incompletely dominant, or
codominant with its partner on a homologous chromosome
• In epistasis, two or more gene products influence a trait
• The product of a pleiotropic gene influences two or more traits
13.6 Nature and Nurture
• Variations in traits aren’t always the result of differences in
alleles – many traits are influenced by environmental factors
Environment and Gene Expression
• The environment affects the expression of many genes, which
in turn affects phenotype – including behavioral traits
• We can summarize this relationship as:
genotype + environment → phenotype
Environment and Epigenetics
• Environmentally driven changes in gene expression patterns
can be permanent and heritable
• Such changes are implemented by gene controls such as
chromatin modifications and RNA interference that act on
DNA itself
• Example: Many environmental factors affect DNA methylation
patterns, enhancing or suppressing gene expression
Effects of Temperature
on Gene Expression
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)
Figure 13-14a p212
A Light micrograph of a living water flea.
Figure 13-15a p213
B Electron micrographs comparing Daphnia body form that develops in the presence of few
predators (left) with the form that develops in the presence of many predators (right). Note the
difference in the length of the tail spine and the pointiness of the head. Chemicals emitted by the
water flea’s insect predators provoke the change.
Figure 13-15b p213
Mood Disorders in Humans
• Environment is a factor in schizophrenia, bipolar disorder,
depression, and other mood disorders
• Example: Stress-induced depression causes methylationbased silencing of a particular nerve growth factor – some
antidepressants work by reversing this methylation
• Future treatments for many disorders may involve deliberate
modification of epigenetic marks in one’s DNA
Take-Home Message: Is genotype the only
factor that gives rise to phenotype?
• The environment influences gene expression, and therefore
can alter phenotype
• Cell-signaling pathways link environmental cues with
epigenetic marks such as methylation and other chromatin
modifications
ANIMATION: Coat color in the Himalayan
rabbit
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13.7 Complex
Variations in Traits
• Individuals of most
species vary in some of
their shared traits
• Many traits (such as
eye color) show a
continuous range of
variation
Continuous Variation
• Continuous variation
• Traits with a range of small differences
• The more factors that influence a trait, the more
continuous the distribution of phenotype
• Bell curve
• When continuous phenotypes are divided into measurable
categories and plotted as a bar chart, they form a bellshaped curve
Continuous Variation in Height (Females)
Continuous Variation in Height (Males)
The Bell Curve
Regarding the Unexpected Phenotype
• Phenotype results from complex interactions among gene
products and the environment
• Enzymes and other gene products control steps of most
metabolic pathways
• Mutations, interactions among genes, and environmental
conditions may result in unpredictable traits
• Example: Camptodactyly can affect any fingers on either or
both hands
Camptodactyly
Take-Home Message:
Do all traits occur in distinct forms?
• The more genes and other factors that influence a trait, the
more continuous is its range of variation
• Unpredictable phenotypes can be caused by interactions
among genes with a range of expression among individuals