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LECTURE CONNECTIONS 3 | Basic Principles of Heredity © 2009 W. H. Freeman and Company 3.2 Monohybrid Crosses Reveal the Principle of Segregation and the Concept of Dominance • Predicting the outcomes of genetics crosses • The Punnett square Concept Check 1 If an F1 plant depicted in Figure 3.4 (Rr) is backcrossed to the parent with round seeds (RR), what proportion of the progeny will have winkled seeds? a. b. c. d. ¾ ½ ¼ 0 The multiplication and addition rules • Multiplication rule: the probability of two or more independent events occurring TOGETHER is calculated by multiplying their independent probabilities. Example: what is the probability to roll a dice and obtain three times the number 5? • Addition rule: the probability of any one of two or more mutually exclusive events is calculated by adding the probabilities of these events. Concept Check 2 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. b. c. d. 5/8 1/2 1/8 1/16 3.2 Monohybrid Crosses Reveal the Principle of Segregation and the Concept of Dominance • The Testcross: one individual of unknown genotype is crossed with another individual with a homozygous recessive genotype for the trait in question. The testcross reveals the genotype of the first individual. • Incomplete Dominance: dominance is lacking • Ratios in Simple Crosses When a trait displays incomplete dominance, the genotypic ratios and phenotypic ratios of the offspring are the SAME, because each genotype has its own phenotype. Ratios in simple crossing 3.3 Dihybrid Crosses Reveal the Principle of Independent Assortment • Dyhybrid Crosses • The Principle of Independent Assortment: alleles at different loci separate independently of one another. • Relating the Principle of Independent Assortment to Meiosis: genes located on different pairs of homologous chromosomes will assort independently. • Applying Probability and the Branch Diagram to Dihybrid Crosses (slides 16 and 17) • Dihybrid Testcross Round and Yellow Round and green wrinkled and Yellow wrinkled and green Consider the cross Aa Bb Cc x Aa bb Cc. What is the probability of obtaining offspring with the genotype aa bb cc? Concept Check 3 How are the principles of segregation and independent assortment related and how are they different? • 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.