Download Genetics Essentials 2e

Benjamin A. Pierce
•Genetics Essentials
• Concepts and Connections
• SECOND EDITION
CHAPTER 3
Basic Principles of Heredity
© 2013 W. H. Freeman and Company
CHAPTER 3 OUTLINE
• 3.1 Gregor Mendel Discovered the Basic Principles
of Heredity, 44
• 3.2 Monohybrid Crosses Reveal the Principle of
Segregation and the Concept of Dominance, 47
• 3.3 Dihybrid Crosses Reveal the Principle of
Independent Assortment, 55
• 3.4 Observed Ratios of Progeny May Deviate from
Expected Ratios by Chance, 60
The Case of the Red Hair
• Did you read the Chapter 3?
• What do you think, what is the case?
• Did you find any new research on the subject?
• Are you ready to present?
Mendel and His Study of Heredity
• His success was result of his choice of model organism
Pisum sativum
• Rapid growth
• Many offspring
• Large number of varieties with known genetic makeup
• 7 characteristics he investigated
• And his mathematical knowledge
Before we Continue…
3.2 MONOHYBRID CROSSES REVEAL THE
PRINCIPLE OF SEGREGATION AND THE
CONCEPT OF DOMINANCE
• Monohybrid cross: cross between two parents
that differ in a single characteristic.
• Conclusion 1: one character is encoded by two genetic
factors.
• Conclusion 2: two genetic factors (alleles) separate
when gametes are formed.
• Conclusion 3: The concept of dominant and recessive
traits.
• Conclusion 4: Two alleles separate with equal
probability into the gametes.
Fig. 3.3
Experimental Outline
• Mendel used this approach for all studied characteristics
Observe
Count
Analyze
The Monohybrid Cross
Conclusion 1: one character is encoded by two genetic factors.
• Cross between parents
different in a single
characteristic (in this example
seed shape; round or wrinkled)
• Parental (P) generation
• Pure breed
(homozygous)
• Filial 1 (F1) generation
• Uniform only one
characteristic
• Filial 2 (F2) generation
• Both characteristics
appear in 3:1 ratio
In Mendel’s pea plants, how did he know that each F1
generation contained two alleles encoding different
characteristics?
a. The F1 generation had a blended phenotype of
the two parental phenotypes.
b. Both parental phenotypes reappeared in the F2
generation.
c. Each F1 plant had a different phenotype.
d. The F1 plants did not have two alleles.
e. None of the above.
The Labeling of Factors (Alleles)
Conclusion 3: The concept of dominant and recessive traits.
• Remember meiosis and gamete formation?
• Each gamete and representative allele is labeled with a
letter that corresponds to the characteristic studied
• R for round (dominant)
• r for wrinkled (recessive)
The Factors (Alleles) in the Experiment
Conclusion 2: two genetic factors (alleles) separate when gametes
are formed.
• What happens with the alleles in the cross of the P
generation?
• F1 generation is heterozygous Rr
• F1 generation form gametes
• Each plant two different gametes
The Alleles in the Next Generation
Conclusion 4: Two alleles separate with equal probability into the
gametes.
• Self fertilization of F1 plants
• The two gametes combine and fertilize randomly
• F2 generation
• Two phenotypes (3:1 ratio)
• Three genotypes
• RR
• Rr
• rr
• How will that be determined?
Yet another generation: F3
3.2 MONOHYBRID CROSSES REVEAL THE
PRINCIPLE OF SEGREGATION AND THE
CONCEPT OF DOMINANCE
• Principle of segregation: (Mendel’s first law)
Each individual diploid organism possesses two alleles
for any particular characteristic. These two alleles
segregate when gametes are formed, and one allele
goes into each gamete.
• The concept of dominance: when two different alleles
are present in a genotype, only the trait encoded by one
of them―the “dominant” allele―is observed in the
phenotype.
The Crosses and the Meiosis
• Introduction to chromosome theory of heredity
Meiosis I- Crossing Over
Meiosis II- the Gametes and the Alleles
Predicting the Outcomes of Genetic
Crosses- The Punnett Square
• The example of backcross on tall and
short plants (T and t alleles)
• F1 heterozygous (Tt) plant with
homozygous recessive (tt)
parental variety
• Generate possible gametes from each
parent
• Align the gametes from one parent
vertically on the left
• Alight the gametes of the other parent
horizontally on top
• MIX and count
Predicting the Outcomes of Genetic
Crosses- Multiplication Rule
• Two or more independent events
taking place together
• Key word is and
• Probability to roll 4 is 1/6
• What is the probability to obtain
4 in the first and the second try?
• 1/6 in the first and 1/6 in the
second
• 1/6 X 1/6 = 1/36
• Independent means that the first does
not influence the second
3.3 DIHYBRID CROSSES REVEAL THE PRINCIPLE
OF THE INDEPENDENT ASSORTMENT
Dihybrid Crosses
• The principle of independent assortment
• Relating the principle of independent assortment to
meiosis
• Applying probability and the branch diagram to
dihybrid crosses
• The Dihybrid testcross
Dihybrid Cross
• Cross between parents different in
two characteristic (in this example
seed shape and color; round/wrinkled
and yellow/green)
• Parental (P) generation
• Pure breed (homozygous for
two characteristics)
• Filial 1 (F1) generation
• Uniform round/yellow
• Filial 2 (F2) generation
• Various combinations of the two
characteristics appear in
9:3:3:1 ratio
The Factors (Alleles) in the Experiment
Conclusion: genetic factors (alleles) of the first trait separate
independently from the alleles of the second trait
• What happens with the alleles in the cross of the P generation?
• F1 generation is heterozygous for both traits RrYy
• F1 generation form gametes
• Each sex forms four different
gametes
The Alleles in the Next Generation
Conclusion: genetic factors (alleles) of the first trait separate
independently from the alleles of the second trait
• Self fertilization of F1 plants
• The four gametes fertilize randomly
• F2 generation
• Four phenotypes
• 16 genotypes in 9:3:3:1 ratio
The Meiotic Behavior of Chromosomes
Explains the Independent Assortment
Crosses and Probabilities- Branched
Diagrams-Monohybrid
Crosses and Probabilities- Branched
Diagrams-Dihybrid
Crosses and Probabilities- Branched
Diagrams-Dihybrid Test Cross
½ Yy
¼ Rr Yy
½ yy
¼ Rr yy
½ Rr
½ Yy
¼ rr Yy
½ yy
¼ rr yy
½ rr
Human Traits- Pedigrees
Pedigrees- Autosomal Dominant
• Waardenburg Syndrome- deafness might have fair skin, white
forelock and visual problems
Pedigrees- Autosomal Recessive
consanguinity