Mendelian Genetics
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KLUGMC03_038-060HR 10/18/06 1:01 PM Page 38 CHAPTER 3 Mendelian Genetics Gregor Johann Mendel, who in 1866 put forward the major postulates of transmission genetics as a result of experiments with the garden pea. C HAPTER C ONCEPTS ■ Inheritance is governed by information ■ During gamete formation, chromosomes other segregating homologs during gamete stored in discrete factors called genes. are distributed according to postulates first formation. ■ Genes are transmitted from generation described by Gregor Mendel, based on his ■ Genetic ratios, expressed as probabilities, to generation on vehicles called nineteenth-century research with the are subject to chance deviation and may be chromosomes. garden pea. evaluated statistically. ■ ■ Chromosomes, which exist in pairs, provide Mendelian postulates prescribe that ■ The analysis of pedigrees allows predictions the basis of biparental inheritance. homologous chromosomes segregate from involving the genetic nature of one another and assort independently with human traits. 38 KLUGMC03_038-060HR 10/18/06 1:01 PM Page 39 3.2 The Monohybrid Cross Reveals How One Trait Is Transmitted from Generation to Generation 39 lthough inheritance of biological traits has been recog- science for the next 16 years. Mendel received support from nized for thousands of years, the first significant the monastery for his studies and research throughout his life. insights into the mechanisms involved occurred about In 1856, Mendel performed his first set of hybridization A experiments with the garden pea. The research phase of his 135 years ago. In 1866, Gregor Johann Mendel published the results of a series of experiments that would lay the founda- career lasted until 1868, when he was elected abbot of the tion for the formal discipline of genetics. Mendel’s work went monastery. Although he retained his interest in genetics, his largely unnoticed until the turn of the century, but in the ensu- new responsibilities demanded most of his time. In 1884, ing years the concept of the gene as a distinct hereditary unit Mendel died of a kidney disorder. The local newspaper paid was established. The ways in which genes, as members of him the following tribute: chromosomes, are transmitted to offspring and control traits “His death deprives the poor of a benefactor, and were clarified. Research continued unabated throughout the mankind at large of a man of the noblest character, one twentieth century—indeed, studies in genetics, most recently who was a warm friend, a promoter of the natural at the molecular level, have remained continually at the fore- sciences, and an exemplary priest.” front of biological research since the early 1900s. When Mendel began his studies of inheritance using Pisum Mendel first reported the results of some simple genetic sativum, the garden pea, chromosomes and the role and mech- crosses between certain strains of the garden pea in 1865. anism of meiosis were totally unknown. Nevertheless, he Although his was not the first attempt to provide experimental determined that discrete units of inheritance exist and pre- evidence pertaining to inheritance, Mendel’s success where dicted their behavior during the formation of gametes. Subse- others failed can be attributed, at least in part, to his elegant quent investigators, with access to cytological data, were able model of experimental design and analysis. to relate their observations of chromosome behavior during Mendel showed remarkable insight into the methodology nec- meiosis to Mendel’s principles of inheritance. Once this corre- essary for good experimental biology. He chose an organism lation was made, Mendel’s postulates were accepted as the that is easy to grow and hybridize artificially. The pea plant is basis for the study of what is known as transmission genetics. self-fertilizing in nature but is easy to crossbreed experimental- ly. It reproduces well and grows to maturity in a single season. Mendel followed seven visible features (unit characters), each represented by two contrasting forms, or traits (Figure 3–1). How Do We Know? For the character stem height, for example, he experimented with the traits tall and dwarf. He selected six other visibly con- In this chapter, we focus on how Mendel was able to derive the trasting pairs of traits involving seed shape and color, pod shape essential postulates that explain inheritance. As you study this and color, and pod and flower arrangement. From local seed topic, you should try to answer several fundamental questions: merchants, Mendel obtained true-breeding strains—those in 1. How did Mendel know that unit factors existed as which each trait appeared unchanged generation after generation fundamental genetic components if he could not directly in self-fertilizing plants. observe them? There were several reasons for Mendel’s success. In addi- 2. How do we know that an organism expressing a dominant tion to his choice of a suitable organism, he restricted his trait is homozygous or heterozygous? examination to one or very few pairs of contrasting traits in each experiment. He also kept accurate quantitative records, a 3. In genetic data, how do we know that deviation from necessity in genetic experiments. From the analysis of the expected ratio is due to chance rather than another his data, Mendel derived certain postulates that became independent factor? principles of transmission genetics. 4. How do we know how a trait is inherited in humans? The results of Mendel’s experiments were unappreciated until the turn of the century, well after his death. However, once Mendel’s publications were rediscovered by geneticists investi- 3.1 Mendel Used a Model gating the function and behavior of chromosomes, the implica- Experimental Approach to Study tions of his postulates were immediately apparent. He had discovered the basis for the transmission of hereditary traits! Patterns of Inheritance Johann Mendel was born in 1822 to a peasant family in the central European village of Heinzendorf. An excellent student 3.2 The Monohybrid Cross Reveals in high school, he studied philosophy for several years after- How One Trait Is Transmitted ward, and in 1843 he was admitted to the Augustinian from Generation to Generation Monastery of St. Thomas in Brno, now part of the Czech Republic, taking the name Gregor. In 1849, he was relieved of Mendel’s simplest crosses involved only one pair of contrast- pastoral duties and accepted a teaching appointment that last- ing traits. Each such experiment is a monohybrid cross, ed several years. From 1851 to 1853, he attended the Univer- which is made by mating true-breeding individuals from two sity of Vienna, where he studied physics and botany. He parent strains, each exhibiting one of the two contrasting forms returned to Brno in 1854, where he taught physics and natural of the character under study. Initially, we examine the first KLUGMC03_038-060HR 10/18/06 1:01 PM Page 40 40 Chapter 3 Mendelian Genetics Character Contrasting traits F1 results F2 results F2 ratio 5474 round round/wrinkled all round 2.96:1 1850 wrinkled Seeds 6022 yellow yellow/green all yellow 3.01:1 2001 green 882 full full/constricted all full 2.95:1 299 constricted Pods 428 green green/yellow all green 2.82:1 152 yellow Flower violet/white 705 violet color all violet 224 white 3.15:1 Flower 651 axial axial/terminal all axial 3.14:1 position 207 terminal Stem 787 tall tall/dwarf all tall 2.84:1 length 277 dwarf FIGURE 3-1 Seven pairs of contrasting traits and the results of Mendel’s seven monohybrid crosses of the garden pea (Pisum sativum). In each case, pollen derived from plants exhibiting one trait was used to fertilize the ova of plants exhibiting the other trait. In the F1 generation, one of the two traits was exhibited by all plants. The contrasting trait reappeared in approximately 1 4 of the F plants. > 2 generation of offspring of such a cross, and then we consider shown in Figure 3–1. In every case, the outcome was similar the results of selfing, the offspring of self-fertilizing individu- to the tall/dwarf cross just described. All F1 offspring were als from this first generation. The original parents constitute identical to one of the parents, but in the F2 offspring, an the P1 , or parental, generation, their offspring are the F1 , or approximate ratio of 3:1 was obtained. That is, three-fourths first filial generation, and the individuals resulting from the looked like the F1 plants, while one-fourth exhibited the con- selfed F1 generation are the F2 , or second filial generation. trasting trait, which had disappeared in the F1 generation. We can continue to follow subsequent generations. We will point out one further aspect of Mendel’s monohy- The cross between true-breeding pea plants with tall stems brid crosses. In each cross, the F1 and F2 patterns of inheri- and dwarf stems is representative of Mendel’s monohybrid tance were similar regardless of which P1 plant served as the crosses. Tall and dwarf are contrasting traits of the character source of pollen (sperm) and which served as the source of the of stem height. Unless tall or dwarf plants are crossed togeth- ovum (egg). The crosses could be made either way—pollination er or with another strain, they will undergo self-fertilization of dwarf plants by tall plants or vice versa. These are called and breed true, producing their respective traits generation reciprocal crosses. Therefore, the results of Mendel’s mono- after generation. However, when Mendel crossed tall plants hybrid crosses were not sex-dependent. with dwarf plants, the resulting F1 generation consisted only To explain these results, Mendel proposed the existence of of tall plants. When members of the F1 generation were particular unit factors for each trait. He suggested that these selfed, Mendel observed that 787 of 1064 F2 plants were tall, factors serve as the basic units of heredity and are passed while the remaining 277 were dwarf.