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Observing Patterns in Inherited Traits
By the late nineteenth century…
 Charles Darwin’s theory of natural selection
suggested that a population could evolve if members
show variation in heritable traits
 Variations that improved an individual’s chances
of surviving would be more common in each
generation
 Over time, a population____________________
 Prevailing view:“____________ _______________”
 Hereditary material must be fluid
 Fluids from both parents blend at fertilization,
causing ___________________________
What’s Wrong with This Theory?
 Blending inheritance doesn’t explain the observable
variations in a population that allows for evolution
 Would produce uniform populations
 Many observations did not support blending

Example:
 The theory of natural selection does not fit with this
view of inheritance
Gregor Mendel
 Called:
 Used experiments in plant
breeding and his
knowledge of
mathematics to form his
own hypotheses about
inheritance
_________ _____
 Pisum sativum: the garden pea plant
 This plant can fertilize itself
 Its flowers produce both male and female gametes,
which fertilize and give rise to new plants
 Mendel could use true-breeding varieties
 Successive generations will be ____________ in a
certain trait
 Offspring grown from seeds of white-flowered parent
plants also have white flowers
 Peas can also be cross-fertilized by human manipulation
of pollen
Figure 10.3
Mendel’s
Experimental
Approach
 Mendel cross-fertilized
true-breeding garden
pea plants having
clearly contrasting
traits
 For example, white
flowers vs. purple
flowers
Producing Hybrids
 True-breeding lineage occurs when offspring
inherit __________ _________ in each
generation
 ___________ are the offspring of a cross
between two individuals that breed true for
different forms of a trait
 Each inherits ______________ ___________
for a trait being studied
Producing
_________
Figure 10.5
Terms Used in Modern Genetics
 _______ : heritable units of information about
traits
 Each is located at its own particular ________
on the chromosome
 _________ : different molecular forms of the same
gene
 ____________ : permanent change in a gene’s
information that alters a gene’s molecular
structure
Heritable Units
of Information
Figure 10.4
Alleles
 When both alleles are the same, the condition is the
_______________
 If the alleles differ, then it is the ______________
condition
 An allele is ___________ when its effect on a trait
masks the effect of any __________ allele paired
with it
 Dominant alleles are signified by capitalized
letters
 Lowercase letters signify recessive ones
Alleles
 A homozygous dominant individual has a
pair of dominant alleles: ______
 A heterozygous individual has a pair of
nonidentical alleles: ______
 Homozygous recessive: pair of recessive
alleles _______
Gene Expression
 Dominant allele may mask effect of recessive allele
on the homologous chromosome
 Gene expression is the process in which the gene’s
information is converted to its function.
 Expressed genes _________________________.
 _____________ : an individual’s alleles at any or all
gene loci (their genetic make up)
 ______________ : an individual’s observable traits;
how their genes are expressed
Mendel’s Experiments
 Mendel suspected that every plant inherits two “units”
of information for a trait, one from each parent
 Mendel’s first experiments were
_____________ __________
 Monohybrid crosses have two parents that are truebreeding for contrasting forms of a trait
 For example, pea plants with white flowers and pea
plants with purple flowers
 ____ = parent generation
 ____ = first-generation offspring
 ____ = second-generation offspring
Monohybrid Crosses
 In the first-generation offspring (F1):
 One form of the trait (white flowers) disappears
 All offspring have purple flowers
 When Mendel crossed those offspring (F1 x F1), the
white flowers reappeared!
 What is going on?
The parent pea plants must have been:
__________________________________________________
Monohybrid Cross
The parent pea plants must have been:
F1: The offspring, therefore, must ALL be:
F2: The offspring of the second cross (F1 x F1) will:
Monohybrid Cross
 We now know that all members of the F1 offspring
are heterozygous (Aa) because one parent could
produce only an A gamete and the other could
produce only an a gamete
 Mendel assumed that each sperm has an equal
probability of fertilizing an egg
Monohybrid Cross
 The numerical ratios of crosses suggested that
genes do not blend
 For example, the F2 offspring showed a 3:1
phenotypic ratio of purple to white
 Thus, each new plant has three chances in four
of having at least one dominant allele
Testcross
 ___________ : method of determining genotype
 One individual of unknown genotype is crossed
with another that is homozygous recessive
 The results will show if the individual is
homozygous or heterozygous for a dominant trait
 To support his concept of segregation, Mendel crossed
F1 plants with homozygous recessive individuals
 A ratio of ______ of recessive and dominant
phenotypes supported his hypothesis
Theory of Segregation
 Mendel’s Theory of Segregation: diploid organisms
have pairs of genes, on pairs of homologous
chromosomes
 States that diploid organisms inherit two genes per
trait on pairs of chromosomes. Each gene
segregates from the other during meiosis such that
each gamete will receive only one gene per trait.
Remember… there are
always exceptions to the
rules!
F2 Offspring:
Dominant and
Recessive Traits
Figure 10.6
Mendel’s Dihybrid Experiments
 _______________ are the offspring of parents that
breed true for different versions of two traits
 In addition to his monohybrid crosses, Mendel also
performed experiments involving two traits: a
___________ _______________
 True breeding parents: AABB x aabb
Gametes
 F1 hybrid offspring:
Dihybrid Crosses
 Mendel correctly predicted that all F1 plants would
show both of the dominant alleles
 Example:
 He wondered if the genes for flower color and plant
height would always travel together when two F1
plants were crossed
Dihybrid Cross
 The F2 results:
 tall and purple-flowered:
 dwarf and white-flowered:
 dwarf and purple-flowered:
 tall and white-flowered:
Punnett Square
 How did Mendel get these ratios?
*Reference Figure 10.9 in your textbook
Mendel’s Theory of
______________ ______________
 The Mendelian theory of independent
assortment states that as meiosis ends, genes
on pairs of homologous chromosomes have
been sorted out for distribution into one
gamete or another, independently of gene pairs
of other chromosomes
Requires qualification, because gene pairs do not always
assort independently
Why was Mendel lucky?
Beyond simple dominance patterns…
1)
3)
2)
4)
Codominance
 In _______________ , nonidentical alleles for a gene are
fully expressed in heterozygotes
 May occur in ___________ _________ __________, in which
three or more alleles of a gene persist among individuals
 Example: ______________________________
 Blood type is determined by markers produced by
three genes (a multiple allele system)
 Red blood cells have membrane glycolipids which
helps the body identify its own blood cells
 The ABO gene encodes an enzyme which determines
the form of the glycolipid
 IA and IB are each dominant to i, but are codominant
to each other
ABO Blood Types
 Blood type is determined by the alleles carried for the
ABO gene: _________________
 A and B are codominant when paired; this means that
some people can express both genes and have _____
blood
 The O allele is
__________
when paired
with A or B
Figure 10.10
Incomplete Dominance
 In ___________ __________, one dominant allele
cannot completely mask the expression of another
 One allele of a pair is not fully dominant over its
partner
 Example: ______________________________
 A true-breeding red-flowered snapdragon crossed
with a white-flowered snapdragon will produce
_________ ____________
 This pink color is because there is not enough red
pigment (produced by the dominant) allele to
completely mask the effects of the white allele
Epistasis
 Traits are expressed through ____________ :
interactions among products of two or more gene pairs
 Two alleles can mask the expression of another gene’s
alleles because of these interactions
 Examples: variations in chicken combs and Labrador
retriever coat colors
Single Genes with a Wide Reach
 Sometimes the expression of alleles at one location can
have effects on two or more traits, which is called
______________
 Marfan syndrome: genetic disorder which arises by
mutations in the fibrillin gene
 Fibrillin protein allows elasticity in many tissues,
including the heart, skin, blood vessels, skeleton, and
tendons
 Characterized by these effects: lanky skeleton, leaky heart
valves, weakened blood vessels, deformed air sacs in
lungs, pain, and lens displacement in the eyes