1. Concepts and Connections © 2013 W. H. Freeman and Company
Benjamin A. Pierce
Genetics Essentials
Concepts and Connections
SECOND EDITION
CHAPTER 3
Basic Principles of Heredity
© 2013 W. H. Freeman and Company

2. 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

3. 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?
Red hair is caused by recessive mutations at the melanocortin 1 receptor gene. Reed Kaestner/Corbis

4. 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

5. Before we Continue…

6. 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

7. Experimental Outline
Mendel used this approach for all studied characteristics
Observe
Count
Analyze

8. 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

9. b. Both parental phenotypes reappeared in the F2 generation.
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.
Location: 3.2 Monohybrid Crosses Reveal the Principle of Segregation and the Concept of Dominance
What Monohybrid Crosses Reveal
Concept Check 3

10. 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)

11. 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

12. 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?

13. Yet another generation: F3

14. 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.

15. The Crosses and the Meiosis
Introduction to chromosome theory of heredity

16. Meiosis I- Crossing Over

17. Meiosis II- the Gametes and the Alleles

18. 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

19. 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

22. 3.3 Dihybrid Crosses Reveal the Principle of the Independent Assortment
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

23. 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

24. 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

25. 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

26. The Meiotic Behavior of Chromosomes Explains the Independent Assortment

27. Crosses and Probabilities- Branched Diagrams-Monohybrid

28. Crosses and Probabilities- Branched Diagrams-Dihybrid

29. Crosses and Probabilities- Branched Diagrams-Dihybrid Test Cross
½ Yy
½ yy
¼ Rr Yy
¼ Rr yy
¼ rr Yy
¼ rr yy
½ Rr
½ rr
½ Yy
½ yy

30. Human Traits- Pedigrees

31. Pedigrees- Autosomal Dominant
Waardenburg Syndrome- deafness might have fair skin, white forelock and visual problems

32. Pedigrees- Autosomal Recessive
consanguinity