Benjamin A. Pierce
GENETICS
A Conceptual Approach
FOURTH EDITION
CHAPTER 3
Basic Principles of Heredity
© 2012 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, 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. 3.7b
Probability as a tool in genetics
The multiplication rule
The addition rule
The application of probability to genetics crosses
The binomial expansion and probability
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. ¾
b. ½
c. ¼
d. 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. ¾
b. ½
c. ¼
d. 0
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
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
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
Concept Check 3
• 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.