Chapter 2 1 Chapter 2 Mendel's Breakthrough: Patterns, Particles and Principles of Heredity Synopsis: Chapter 2 covers the basic principles of inheritance, first described by Mendel, that form the foundation of the Laws of Segregation and Independent Assortment. You will see in chapter 4 how these laws relate to chromosome segregation during meiosis. Chapter 2 contains most of the essential terminology used to describe inheritance. You should become very familiar with and fluent in the use of these terms because they will be used in increasingly sophisticated ways in subsequent chapters. A good way to assure that you have a solid grasp of the meanings of the new terms is to pretend you are describing each word or phenomenon to a friend or relative who is not a science major. Often giving an example of each term is useful. The first problem at the end of the chapter is also a useful gauge of how well you know these terms. A few of the terms defined in this chapter are very critical yet often misunderstood. Learn to be precise about the way in which you use these terms: genes and alleles of genes - a gene determines a trait; and there are different alleles or forms of a gene. The color gene in pea has two alleles: the yellow allele and the green allele; genotype and phenotype - genotype is the genetic makeup of an organism (written as alleles) and phenotype is what the organism looks like; homozygous and heterozygous - when both alleles of a gene are the same, we say the organism is homozygous for that gene; if the two alleles are different, the organism is heterozygous; dominant and recessive - the dominant allele is the one that controls the phenotype in the heterozygous genotype. Significant Elements: After reading the chapter and thinking about the concepts you should be able to: ♦ Remember there are two alleles of each gene when describing genotypes of individuals. If you are describing gametes remember there is only one allele of each gene per gamete. ♦ Recognize if a trait is dominant or recessive by considering the phenotype of the F1 generation. ♦ Recognize ratios: (i) in the Study Guide monohybrid ratio refers to any ratio involving one gene, e.g. the phenotypic monohybrid ratio from a cross between two individuals heterozygous for one gene (3 dominant :1 recessive);2 Chapter 2 (ii) in the Study Guide dihybrid ratio refers to any ratio involving two genes, for e.g. a phenotypic dihybrid ratio from a cross between two individuals heterozygous for two genes (9:3:3:1). ♦ Recognize the need for and be able to set up a test cross. ♦ Determine probabilities using the basic rules of probability: (i) Product rule: If two outcomes must occur together, the probability of one outcome AND the other occurring is the product of the two individual probabilities. (The final outcome is the result of two independent events.) So, the probability of getting a 4 on one die AND a 4 on the second die is the product of the two individual probabilities. (ii) Sum rule: If there is more than one way in which an outcome can be produced, the probability of either one OR the other occurring is the sum of the individual probabilities. In this case, the outcomes are mutually exclusive. ♦ Draw and interpret pedigrees as in Figures 2.21 and 2.22 using the helpful information and hints found in Figure 2.20 and Table 2.2. ♦ Set up Punnett squares by determining the gametes produced by the parents and the probabilities of the potential offspring as in Figures 2.11 and 2.15. ♦ Use and interpret branched line diagrams as in Figure 2.17 and Problem 2-24a. Problem Solving - How to Begin: ♦ As you work on problems in the first few chapters, you will begin to recognize some similarities in the type of problem. The majority of problems in the first few chapters involve genetic crosses. It is best to BE CONSISTENT in your approach and formatting to solve such problems. To begin, rewrite the information given in a format that will be useful in solving the problem: in other words, learn to DIAGRAM THE CROSS. phenotype of one parent x phenotype of other parent → phenotype(s) of progeny The goal is to assign genotypes to the parents then use these predicted genotypes to generate genotypes, phenotypes and ratios of progeny. If the predicted progeny match the observed data you were given in the problem then the genetic explanation you have created is correct. ♦ Use the THREE ESSENTIAL QUESTIONS to determine basic information about the genotypes and/or phenotypes of the parents and/or offspring. These questions are distilled from many years of helping students figure out how to solve problems. They are designed to force you to focus on the underlying genetic basis of the information in a problem. Each of the questions has an identifying characteristic that helps you answer the question - see the Hints for an idea of what sort of information allows you to formulate specific answers. As you go through further chapters the Hints will be further refined.Chapter 2 3 THREE ESSENTIAL QUESTIONS (3EQ): 1. How many genes are involved in the cross? 2. For each gene involved in the cross: what are the phenotypes associated with the gene? Which phenotype is the dominant one and why? Which phenotype is the recessive one and why? [3. For each gene involved in the cross: is it X-linked or autosomal?] At this point, only questions 3EQ #1 and 3EQ #2 may be applied. The material that is the basis of question 3EQ #3 will be covered in Chapter 4. Hints: For 3EQ #1 look for the number of different phenotypes or phenotypic classes in the progeny. In Chapter 2 each gene has only 2 phenotypes. For 3EQ #2 if the parents of a cross are true-breeding, look at the phenotype of the F1 individuals. Their genotype must be heterozygous, and their phenotype is thus controlled by the dominant allele of the gene. Also, look at the F2 progeny – the 3/4 portion of the 3:1 phenotypic monohybrid ratio is the dominant one. Solutions to Problems: Vocabulary 2-1. a. 4; b. 3; c. 6; d. 7; e. 11; f. 13; g. 10; h. 2; i. 14; j. 9; k. 12; l. 8; m. 5; n. 1. Section 2.1 - Background 2-2. People held two basic misconceptions about inheritance. Firstly, it was believed that one parent contributes the most to an offspring's inherited features. Second was the idea of blended inheritance - the parental traits become mixed and forever changed in the offspring. 2-3. There are several advantages to