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Chapter
3
Mendelian
Genetics
Lecture Presentation by
Dr. Cindy Malone,
California State University Northridge
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© 2015 Pearson Education, Inc.
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Chapter 3 Contents
Mendel Used a Model Experimental Approach to
Study Patterns of Inheritance
The Monohybrid Cross Reveals How One Trait Is
Transmitted from Generation to Generation
3.3 Mendel's Dihybrid Cross Generated a Unique F2
Ratio
3.4 The Trihybrid Cross Demonstrates That Mendel's
Principles Apply to Inheritance of Multiple Traits
3.5 Mendel's Work Was Rediscovered in the Early
Twentieth Century
Continued
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3.1
3.2
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Chapter 3 Contents
Independent Assortment Leads to Extensive Genetic
Variations
Laws of Probability Help to Explain Genetic Events
Chi-Square Analysis Evaluates the Influence of
Chance on Genetic Data
3.9 Pedigrees Reveal Patterns of Inheritance of Human
Traits
3.10 Mutant Phenotypes Have Been Examined at the
Molecular Level
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3.6
3.7
3.8
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Slide 4
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3.1 Mendel Used a Model Experimental
Approach to Study Patterns of Inheritance
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Section 3.1: Mendel’s Experimental Approach
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 Mendel’s model organism: peas
– easy to grow
– true-breeding strains
– controlled matings: self-fertilization or crossfertilization
– grow to maturity in one season
– observable characteristics with two distinct forms
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Section 3.1: Mendel’s Experimental Methods
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 Mendel used seven visible features
– each with two contrasting traits
– true-breeding strains
 Mendel kept quantitative records
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Figure 3.1
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Section 3.1: Mendel’s Legacy
 Mendel was not appreciated during his lifetime
 His work was rediscovered
– Geneticists recognized Mendel had discovered
the basis for the transmission of hereditary
traits
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Slide 9
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3.2 The Monohybrid Cross Reveals How
One Trait Is Transmitted from
Generation to Generation
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Section 3.2: Monohybrid Crosses
Monohybrid crosses
 True-breeding
 Involve a single pair of contrasting traits
 P1 generation: Original parents
 F1 generation: Offspring
 F2 generation: Offspring of F1 generation crossed
(self-fertilizing: “Selfing”)
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Section 3.2: Constraining Traits
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 F1 generation monohybrid cross
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– All plants have just one of two contrasting traits
 F2 generation
– 3/4 of plants exhibit same trait as F1 generation
– 1/4 exhibit contrasting trait that disappeared in the
F1 generation
– 3:1 ratio (Figure 3.1)
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Figure 3.1
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Section 3.2: Particulate Unit Factors
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 Particulate unit factors (genes)
– Basic units of heredity
 Are passed unchanged from generation to generation
 Determine various traits expressed by each individual
plant
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Section 3.2: Mendel’s Three Postulates
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– Unit factors exist in pairs
 Genetic characters controlled by unit factors
– Dominance/Recessiveness
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 In a pair of unit factors, one unit is dominant, the other
recessive
– Segregation
 Paired unit factors segregate (separate) independently
during gamete formation
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Section 3.2: Genetic Terminology
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 Phenotype
– Physical expression of a trait
 Gene
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– Unit of inheritance
 Allele
– Alternative form of a single gene
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 Genotype
– Genetic makeup of individual
– Alleles written in pairs (DD, Dd, or dd)
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Section 3.2: Genetic Terminology (continued)
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 Organism inherits two alleles
– One from each parent
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 Homozygous/Homozygote
– Both alleles are the same (DD, dd)
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 Heterozygous/Heterozygote
– Alleles are different (Dd)
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Section 3.2: Punnett Square
 Punnett square
– Reginald C. Punnett devised this approach
– Genotypes and phenotypes resulting from
combining gametes can be visualized
– Displays all possible random fertilization events
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Monohybrid cross
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Figure 3.2
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F2 generation
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Figure 3.3
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Section 3.2: Testcross: One Character
 Testcross
– Determines if individual displaying dominant
phenotype is homozygous or heterozygous for
that trait
– Cross between dominant phenotype and
homozygous recessive (see Figure 3.4)
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Figure 3.4
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3.3 Mendel’s Dihybrid Cross Generated a
Unique F2 Ratio
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Section 3.3: Dihybrid Cross
 Dihybrid cross
– Two pairs of contrasting traits
– Generates unique F2 generation (Figure 3.5)
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Figure 3.5
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Section 3.3: Mendel’s Fourth Postulate
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 Independent assortment
– Unit factors (traits) assort independently during
gamete formation
– All possible gamete combinations form with equal
frequency (Figure 3.6)
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Section 3.3: Product Law
 Product law
– Used to predict frequency of two independent
events occurring simultaneously
 Example:
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– F2 plant having yellow and round seeds
 3/4  3/4, or 9/16
(Figure 3.6)
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Figure 3.6
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Section 3.3: Mendel’s 9:3:3:1 Dihybrid Ratio
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 F1  F1 fertilization event
 Each zygote can receive one of four
combinations
 Example:
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– 9/16 yellow, round seeds
– 3/16 yellow, wrinkled seeds
– 3/16 green, round seeds
– 1/16 green, wrinkled seeds
– Gives 9:3:3:1 dihybrid ratio
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