Basic Principles of Heredity Benjamin A. Pierce GENETICS A Conceptual Approach SIXTH EDITION CHAPTER 3 Basic Principles of Heredity © 2017 W. H. Freeman and Company

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

Blond hair occurs in 5%–10% of dark-skinned Solomon Islanders Blond hair occurs in 5%–10% of dark-skinned Solomon Islanders. Research demonstrates that blond hair in this group is a recessive trait and has a different genetic basis from blond hair in Europeans due to a point mutation in the TYRP1 gene.

3.2 Gregor Johann Mendel discovered the principles of heredity by experimenting with peas.

3.1 Gregor Mendel Discovered the Basic Principles of Heredity Gregor Mendel and his success in genetics Proper experimental model Used an experimental approach and analyzed results mathematically Studied easily differentiated characteristics

3.3 Mendel used the pea plant Pisum sativum in his studies of heredity. He examined seven characteristics that appeared in the seeds and in plants grown from the seeds. [Photograph by Wally Eberhart/Visuals Unlimited.]

Concept Check 1 Which of the following factors did not contribute to Mendel’s success in his study of heredity? a. His use of the pea plant b. His study of plant chromosomes c. His adoption of an experimental approach d. His use of mathematics

Concept Check 1 Which of the following factors did not contribute to Mendel’s success in his study of heredity? a. His use of the pea plant b. His study of plant chromosomes c. His adoption of an experimental approach d. His use of mathematics

Summary of important genetic terms TABLE 3.1 Summary of important genetic terms Term Definition Gene An inherited factor (encoded in the DNA) that helps determine a characteristic Allele One of two or more alternative forms of a gene Locus Specific place on a chromosome occupied by an allele Genotype Set of alleles possessed by an individual organism Heterozygote An individual organism possessing two different alleles at a locus Homozygote An individual organism possessing two of the same alleles at a locus Phenotype or trait The appearance or manifestation of a characteristic Characteristic or character An attribute or feature possessed by an organism Table 3.1 Summary of important genetic terms

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

Figure 3.4 Mendel conducted monohybrid crosses.

Figure 3.4 Mendel conducted monohybrid crosses.

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 Monohybrid crosses are explained by the principle of segregation. The symbols in genetic crosses correspond to alleles on chromosomes. Sutton: Chromosomal theory of heredity

3.6 Mendel’s monohybrid crosses revealed the principle of segregation and the concept of dominance.

Comparison of the principles of segregation and independent assortment TABLE 3.2 Comparison of the principles of segregation and independent assortment Principle Observation State of Meiosis* Segregation (Mendel’s first law) 1. Each individual organism possesses two alleles encoding a trait. Before meiosis 2. Alleles separate when gametes are formed. Anaphase I 3. Alleles separate in equal proportions. Independent assortment (Mendel’s second law) Alleles at different loci separate independently. *Assumes that no crossing over occurs. If crossing over takes place, then segregation and independent assortment may also occur in anaphase II of meiosis. Table 3.2 Comparison of the principles of segregation and independent assortment

3.2 Monohybrid Crosses Reveal the Principle of Segregation and the Concept of Dominance Tested the theory of inheritance of dominant traits using backcrosses Predicted the outcomes of genetic crosses The Punnett square Figure 3.8

Figure 3.7 The Punnett square can be used to determine the results of a genetic cross.

Figure 3.7 The Punnett square can be used to determine the results of a genetic cross.

Concept Check 2 If an F1 plant depicted in Figure 3.6 is backcrossed to the parent with round seeds, what proportion of the progeny will have wrinkled seeds? (Use a Punnett square.) a. ¾ b. ½ c. ¼ d. 0

Concept Check 2 If an F1 plant depicted in Figure 3.6 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

Probability Probability: the likelihood of the occurrence of a particular event Used in genetics to predict the outcome of a genetic cross Multiplication rule Addition rule

3.9 The multiplication and addition rules can be used to determine the probability of a combination of events.

Concept Check 3 If the probability of being blood-type A is 1/8 and the probability of blood-type O is 1/2, what is the probability of being either blood-type A or O? a. 5/8 b. 1/2 c. 1/8 d. 1/16

Concept Check 3 If the probability of being blood-type A is 1/8 and the probability of blood-type O is 1/2, what is the probability of being either blood-type A or O? a. 5/8 b. 1/2 c. 1/8 d. 1/16

Useful for complex situations Binomial takes the form (p + q)n Binomial Expansion Useful for complex situations Binomial takes the form (p + q)n p equals the probability of one event q equals the probability of the alternative event n equals the number of times the event occurs Table 3.4

TABLE 3.4 Coefficients and terms for the binomial expansion ( p + q)n for n = 1 through 5 n Binomial Expansion 1 a + b 2 a2 + 2ab + b2 3 a3 + 3a2b + 3ab2 + b3 4 a4 + 4a3b + 6a2b2 + 4ab3 + b4 5 a5 + 5a4b + 10a3b2 + 10a2b3 + 5ab4 + b5 Table 3.4 Coefficients and terms for the binomial expansion ( p + q)n for n = 1 through 5

Ratios in Simple Crosses 3.2 Monohybrid Crosses Reveal the Principle of Segregation and the Concept of Dominance The Testcross - Figure 3.8 Ratios in Simple Crosses Tables 3.5 and 3.6

TABLE 3.5 Phenotypic ratios for simple genetic crosses (crosses for a single locus) with dominance Phelotypic Ratio Genotypes of Parents Genotypes of Progeny 3 : 1 Aa × Aa 3/4 A_  :  1/4 aa 1 : 1 Aa × aa 1/2 Aa  :  1/2 aa Uniform progeny AA × AA aa × aa AA × aa AA × Aa All AA All aa All Aa All A_ Table 3.5 Phenotypic ratios for simple genetic crosses (crosses for a single locus) with dominance

TABLE 3.6 Genotypic ratios for simple genetic crosses (crosses for a single locus Genotypic Ratio Genotypes of Parents Genotypes of Progeny 1 : 2 : 1 Aa × Aa 1/4 AA  :  1/2 Aa :  1/4 aa 1 : 1 Aa × aa AA × AA 1/2 Aa  :  1/2 aa 1/2 Aa  :  1/2 AA Uniform progeny aa × aa AA × aa All AA All aa All Aa Table 3.6 Genotypic ratios for simple genetic crosses (crosses for a single locus)

3.3 Dihybrid Crosses Reveal the Principle of Independent Assortment Examine two traits at a time The principle of independent assortment Figure 3.11

Figure 3.10 Mendel’s dihybrid crosses revealed the principle of independent assortment.

Figure 3.10 Mendel’s dihybrid crosses revealed the principle of independent assortment.

Figure 3.10 Mendel’s dihybrid crosses revealed the principle of independent assortment.

Figure 3.11 Mendel’s dihybrid crosses revealed the principle of independent assortment.

3.3 Dihybrid Crosses Reveal the Principle of Independent Assortment Relate the principle of independent assortment to meiosis Gametes located on different chromosomes will sort independently

3.3 Dihybrid Crosses Reveal the Principle of Independent Assortment Applying probability and the branch diagram to dihybrid crosses Figure 3.13

3.13 A branch diagram can be used to determine the phenotypes and expected proportions of offspring from a dihybrid cross (Rr Yy × Rr Yy).

3.14 A branch diagram can be used to determine the phenotypes and expected proportions of offspring from a dihybrid testcross (Rr Yy × rr yy).

3.4 Observed Ratios of Progeny May Deviate from Expected Ratios by Chance - Chi-square goodness of fit - Indicates the probability that the difference between the observed and expected values is due to chance

3.15 A chi-square goodness-of-fit test is used to determine the probability that the difference between observed and expected values is due to chance.

Concept Check 4 How are the principles of segregation and independent assortment related and how are they different?

Concept Check 4 How are the principles of segregation and independent assortment related and how are they different? Answer: Genes encoding different characteristics separate and assort 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.