GENETICS

A Conceptual ApproachFOURTH EDITION

GENETICS

A Conceptual ApproachFOURTH EDITION

Benjamin A. Pierce

© 2012 W. H. Freeman and Company

CHAPTER 3Basic Principles of Heredity

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, 56

• 3.4 Observed Ratios of Progeny May Deviate from Expected Ratios by Chance, 61

3.1 Gregor Mendel Discovered the Basic Principles of Heredity

• Gregor Mendel and his success in genetics

• Genetic terminology

Table 3.1 & Figure 3.2

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. 3.4

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.

3.2 Monohybrid Crosses Reveal the Principle of Segregation and the Concept

of Dominance

• Relating Genetic Crosses to Meiosis– Chromosome theory of heredity

Fig. 3.6

• Predicting the outcomes of genetics crosses– The Punnett square

Fig. 3.7bProbability as a tool in genetics

The multiplication ruleThe addition ruleThe application of probability to genetics crossesThe binomial expansion and probability

If an F1 plant depicted in Figure 3.4 is backcrossed to the parent with round seeds, what proportion of the progeny will have winkled seeds? ( Use a Punnett square.)

a. ¾b. ½c. ¼d. 0

Concept Check 1

If an F1 plant depicted in Figure 3.4 is backcrossed to the parent with round seeds, what proportion of the progeny will have winkled seeds? ( Use a Punnett square.)

a. ¾b. ½c. ¼d. 0

Concept Check 1

If the probability of being blood-type A is 1/8 and the probability of blood-type O is ½, what is the probability of being either blood-type A or O?

a. 5/8

b. ½

c. 1/8

d. 1/16

Concept Check 2

If the probability of being blood-type A is 1/8 and the probability of blood-type O is ½, what is the probability of being either blood-type A or O?

a. 5/8

b. ½

c. 1/8

d. 1/16

Concept Check 2

3.2 Monohybrid Crosses Reveal the Principle of Segregation and the Concept

of Dominance

• The Testcross

- Fig. 3.7

• Ratios in Simple Crosses– Table 3.3 & 3.4

3.3 Dihybrid Crosses Reveal the Principle of the Independent Assortment

Dihybrid Crosses– The principle of independent assortment

• Fig. 3.10

– Relating the principle of independent assortment to meiosis

– Applying probability and the branch diagram to dihybrid crosses

• Fig. 3.11

– The Dihybrid testcross• Fig. 3.12

• How are the principles of segregation and independent assortment related and how are they different?

Concept Check 3

• How are the principles of segregation and independent assortment related and how are they different?Answer:

– Genes encoding different characteristics separate and assorted independently of one another when they do not locate close together on the same chromosome.

– During this process, two alleles of the same gene encoding one characteristic still have to be segregated from each other during the formation of gametes.

Concept Check 3