13.1 Mendel solved the mystery of heredit Early Ideas about Heredity The road to mendel As far back as written records go, patterns of resemblance among the members of particular families have been noted and commented on(figure 13.2). Some familial features are unusual, such as the protruding lower lip of the European royal family Hapsburg, evident in pictures and descriptions of family members from the thirteenth century onward. Other characteristics, like the occur- rence of redheaded children within families of redheaded parents, are more common(figure 13.3). Inherited fea tures. the building blocks of evolution. will be our con cern in this chapter Classical Assumption 1: Constancy of Species Two concepts provided the basis for most of the thinking bout heredity before the twentieth century. The first that heredity occurs within species. For a very long time peo ple believed that it was possible to obtain bizarre compos- FIGURE 13.2 ite animals by breeding(crossing)widely different species. Heredity is responsible for family resemblance. Family The minotaur of Cretan mythology, a creature with the resemblances are often strong-a visual manifestation of the body of a bull and the torso and head of a man, is one e mechanism of heredity. This is the Johnson family, the wife and ample. The giraffe was thought to be another; its scien- daughters of one of the authors. While each daughter is different, tific name, Giraffa camelopardalis, suggests the belief that it all clearly resemble their mother. was the result of a cross between a camel and a leopard From the Middle Ages onward, however, people discov- ered that such extreme crosses were not possible and that variation and heredity occur mainly within the boundaries of a particular species. Species were thought to have been maintained without significant change from the time of Classical Assumption 2: Direct Transmission The second early concept related to heredity is that traits are transmitted directly. When variation is inherited by off- spring from their parents, wbat is transmitted? The ancient Greeks suggested that the parents' body parts were trans- mitted directly to their offspring. Hippocrates called this type of reproductive material gonos, meaningseed Hence, a characteristic such as a misshapen limb was the result of material that came from the misshapen limb of a parent. Information from each part of the body was sup- FIGURE 13.3 osedly passed along independently of the information Red hair is inherited. Many different traits are inherited i from the other parts, and the child was formed after the hereditary material from all parts of the parents' bodies had human families. This redhead is exhibiting one of these traits. come together. This idea was predominant until fairly recently For ex- ample, in 1868, Charles Darwin proposed that all cells and tissues excrete microscopic granules, or gemmules, " that 240 Part IV Reproduction and Heredity
240 Part IV Reproduction and Heredity Early Ideas about Heredity: The Road to Mendel As far back as written records go, patterns of resemblance among the members of particular families have been noted and commented on (figure 13.2). Some familial features are unusual, such as the protruding lower lip of the European royal family Hapsburg, evident in pictures and descriptions of family members from the thirteenth century onward. Other characteristics, like the occurrence of redheaded children within families of redheaded parents, are more common (figure 13.3). Inherited features, the building blocks of evolution, will be our concern in this chapter. Classical Assumption 1: Constancy of Species Two concepts provided the basis for most of the thinking about heredity before the twentieth century. The first is that heredity occurs within species. For a very long time people believed that it was possible to obtain bizarre composite animals by breeding (crossing) widely different species. The minotaur of Cretan mythology, a creature with the body of a bull and the torso and head of a man, is one example. The giraffe was thought to be another; its scientific name, Giraffa camelopardalis, suggests the belief that it was the result of a cross between a camel and a leopard. From the Middle Ages onward, however, people discovered that such extreme crosses were not possible and that variation and heredity occur mainly within the boundaries of a particular species. Species were thought to have been maintained without significant change from the time of their creation. Classical Assumption 2: Direct Transmission of Traits The second early concept related to heredity is that traits are transmitted directly. When variation is inherited by offspring from their parents, what is transmitted? The ancient Greeks suggested that the parents’ body parts were transmitted directly to their offspring. Hippocrates called this type of reproductive material gonos, meaning “seed.” Hence, a characteristic such as a misshapen limb was the result of material that came from the misshapen limb of a parent. Information from each part of the body was supposedly passed along independently of the information from the other parts, and the child was formed after the hereditary material from all parts of the parents’ bodies had come together. This idea was predominant until fairly recently. For example, in 1868, Charles Darwin proposed that all cells and tissues excrete microscopic granules, or “gemmules,” that 13.1 Mendel solved the mystery of heredity. FIGURE 13.2 Heredity is responsible for family resemblance. Family resemblances are often strong—a visual manifestation of the mechanism of heredity. This is the Johnson family, the wife and daughters of one of the authors. While each daughter is different, all clearly resemble their mother. FIGURE 13.3 Red hair is inherited. Many different traits are inherited in human families. This redhead is exhibiting one of these traits
are passed to offspring, guiding the growth forms of the characters Koelreuter was of the in the developing studying were distributed among the of embryo. Most similar theories of the direct transmission of hereditary material assumed character, a modern geneticist would say that the male and female contributions the alternative forms of each character were blend in the offspring. Thus, parents with red and brown hair would produce children ing,meaning that some offspring exhibited with reddish brown hair and tall and short one alternative form of a character (for ex- parents would produce children of interme ample, hairy leaves), while other offspring diate height alternative(smooth leaves). This segrega Koelreuter demonstrates tion of alternative forms of a character or Hybridization between Species traits,provided the clue that led Gregor lendel to his understanding of the nature Taken together however these two con- of heredity. cepts lead to a paradox. If no variation en- ters a species from outside, and if the varia- tion within each species blends in every Knight Studies Heredity in Peas generation, then all members of a species Over the next hundred years, other inves should soon have the same appearance tigators elaborated on Koelreuter's work Obviously, this does not happen. Individu- als within most species differ widely from genttenent among them were English each other, and they differ in characteris- FIGURE 13.4 eties of agricultural plants. In one such se- tics that are transmitted from generation to The garden pea, Pisum ries of experiments, carried out in the eneration sation. Easy to cultivate and 1790s, T. A. Knight crossed two true How could this paradox be resolved: Ac- able to produce many distinctive breeding varieties(varieties that remain hally, the resolution had been provided varieties, the garden pea was a uniform from one generation to the next) long before Darwin, in the work of the popular experimental subject in of the garden pea, Pisum sativum(tig German botanist Josef Koelreuter. In 1760, ure 13.4). One of these varieties had pur Koelreuter carried out successful hy- Mendels experiments ple flowers, and the other had white flow- bridizations of plant species, crossing dif- ers. All of the progeny of the cross had ferent strains of tobacco and obtaining fer- purple flowers. Among the offspring of tile offspring. The hybrids differed in appearance from these hybrids, however, were some plants with purple both parent strains. When individuals the hybrid flowers and others, less common, with white flowers. Just generation were crossed, their offspring were highly vari- as in Koelreuter's earlier studies, a trait from one of the able. Some of these offspring resembled plants of the hy- parents disappeared in one generation only to reappear brid generation(their parents), but a few resembled the in the next original strains(their grandparents) In these deceptively simple results were the makings of a scientific revolution. Nevertheless, another century passed The Classical Assumptions fail before the process of gene segregation was fully appreci ated. Why did it take so long? One reason was that early Koelreuter's work represents the beginning of modern workers did not quantify their results. A numerical record genetics,the first clues pointing to the modern theory of of results proved to be crucial to understanding the proc mer heredity. Koelreuter's experiments provided an impor- Knight and later experimenters who carried out other tant clue about how heredity works: the traits he was crosses with pea plants noted that some traits had a studying could be masked in one generation, only to "stronger tendency "to appear than others, but they did not reappear in the next. This pattern contradicts the theory record the numbers of the different classes of progeny. Sci- of direct transmission. How could a trait that is transmit- ence was young then, and it was not obvious that the num ted directly disappear and then reappear? Nor were the bers were important traits of Koelreuter's plants blended. A contemporary ac- count stated that the traits reappeared in the third gener Early geneticists demonstrated that some forms of an ation"fully restored to all their original powers and inherited character(1)can disappear in one generation pf it is worth repeating that the offspring in Koelreuter's only to reappear unchanged in future generations (2)segregate among the offspring of a cross; and crosses were not identical to one another. Some resembled ()are more likely to be represented than their the hybrid generation, while others did not. The alternative Iternatives Chapter 13 Patterns of Inheritance 241
are passed to offspring, guiding the growth of the corresponding part in the developing embryo. Most similar theories of the direct transmission of hereditary material assumed that the male and female contributions blend in the offspring. Thus, parents with red and brown hair would produce children with reddish brown hair, and tall and short parents would produce children of intermediate height. Koelreuter Demonstrates Hybridization between Species Taken together, however, these two concepts lead to a paradox. If no variation enters a species from outside, and if the variation within each species blends in every generation, then all members of a species should soon have the same appearance. Obviously, this does not happen. Individuals within most species differ widely from each other, and they differ in characteristics that are transmitted from generation to generation. How could this paradox be resolved? Actually, the resolution had been provided long before Darwin, in the work of the German botanist Josef Koelreuter. In 1760, Koelreuter carried out successful hybridizations of plant species, crossing different strains of tobacco and obtaining fertile offspring. The hybrids differed in appearance from both parent strains. When individuals within the hybrid generation were crossed, their offspring were highly variable. Some of these offspring resembled plants of the hybrid generation (their parents), but a few resembled the original strains (their grandparents). The Classical Assumptions Fail Koelreuter’s work represents the beginning of modern genetics, the first clues pointing to the modern theory of heredity. Koelreuter’s experiments provided an important clue about how heredity works: the traits he was studying could be masked in one generation, only to reappear in the next. This pattern contradicts the theory of direct transmission. How could a trait that is transmitted directly disappear and then reappear? Nor were the traits of Koelreuter’s plants blended. A contemporary account stated that the traits reappeared in the third generation “fully restored to all their original powers and properties.” It is worth repeating that the offspring in Koelreuter’s crosses were not identical to one another. Some resembled the hybrid generation, while others did not. The alternative forms of the characters Koelreuter was studying were distributed among the offspring. Referring to a heritable feature as a character, a modern geneticist would say the alternative forms of each character were segregating among the progeny of a mating, meaning that some offspring exhibited one alternative form of a character (for example, hairy leaves), while other offspring from the same mating exhibited a different alternative (smooth leaves). This segregation of alternative forms of a character, or traits, provided the clue that led Gregor Mendel to his understanding of the nature of heredity. Knight Studies Heredity in Peas Over the next hundred years, other investigators elaborated on Koelreuter’s work. Prominent among them were English gentleman farmers trying to improve varieties of agricultural plants. In one such series of experiments, carried out in the 1790s, T. A. Knight crossed two truebreeding varieties (varieties that remain uniform from one generation to the next) of the garden pea, Pisum sativum (figure 13.4). One of these varieties had purple flowers, and the other had white flowers. All of the progeny of the cross had purple flowers. Among the offspring of these hybrids, however, were some plants with purple flowers and others, less common, with white flowers. Just as in Koelreuter’s earlier studies, a trait from one of the parents disappeared in one generation only to reappear in the next. In these deceptively simple results were the makings of a scientific revolution. Nevertheless, another century passed before the process of gene segregation was fully appreciated. Why did it take so long? One reason was that early workers did not quantify their results. A numerical record of results proved to be crucial to understanding the process. Knight and later experimenters who carried out other crosses with pea plants noted that some traits had a “stronger tendency” to appear than others, but they did not record the numbers of the different classes of progeny. Science was young then, and it was not obvious that the numbers were important. Early geneticists demonstrated that some forms of an inherited character (1) can disappear in one generation only to reappear unchanged in future generations; (2) segregate among the offspring of a cross; and (3) are more likely to be represented than their alternatives. Chapter 13 Patterns of Inheritance 241 FIGURE 13.4 The garden pea, Pisum sativum. Easy to cultivate and able to produce many distinctive varieties, the garden pea was a popular experimental subject in investigations of heredity as long as a century before Gregor Mendel’s experiments
Mendel and the garden pea The first quantitative studies of inheritance were carried out by Gregor Mendel, an Austrian monk(figure 13.5) Born in 1822 to peasant parents, Mendel was educated in a monastery and went on to study science and mathematics at the University of Vienna, where he failed his examina tions for a teaching certificate. He returned to the monastery and spent the rest of his life there, eventually becoming abbot. In the garden of the monastery(figure 13.6), Mendel initiated a series of experiments on plant hy- bridization. The results of these experiments would ulti ly change our views of heredity Why mendel chose the Garden Pea For his experiments, Mendel chose the garden pea, the same plant Knight and many others had studied earlier The choice was a good one for several reasons. First, many earlier investigators had produced hybrid peas by crossing different varieties. Mendel knew that he could expect to observe segregation of traits among the offspring. Second a large number of true-breeding varieties of peas were available. Mendel initially examined 32. Then, for further tudy, he selected lines that differed with respect to seven easily distinguishable traits, such as round versus wrinkled seeds and purple versus white flowers, a character that night had studied. Third, pea plants are small and easy to grow,and they have a relatively short generation time Thus, one can conduct experiments involving numerous plants, grow several generations in a single year, and obtain results relatively quickly FIGURE 13.5 ing peas is that the sexual or- Gregor Johann Mendel. Cultivating his plants in the garden of a gans of the pea are enclosed within the flower(figure 13.7) monastery in Brunn, Austria(now Brno, Czech Republic), Mendel The flowers of peas, like those of many flowering plants, studied how differences among varieties of peas were inherited contain both male and female sex organs. Furthermore, the when the varieties were crossed. Similar experiments had been gametes produced by the male and female parts of the same done before, but Mendel was the first to quantify the results and flower, unlike those of many flowering plants, can fuse to appreciate their significand form viable offspring. Fertilization takes place automati lly within an individual flower if it not disturbed, resulting in offspring that are the progeny from a single indi in dividual flowers engage in se h vidual. Therefore one can either let fertilization, or remove the flower's male parts before fertilization and intro- duce pollen from a strain with a different trait, thus performing cross-pollination which results in cross-fertilization 田且日 FIGURE 13.6 The garden where Mendel carried out his plant-breeding experiments. Gregor Mendel did his key scientific experiments in this small garden in a monastery 242 Part Iv Reproduction and Heredity
Mendel and the Garden Pea The first quantitative studies of inheritance were carried out by Gregor Mendel, an Austrian monk (figure 13.5). Born in 1822 to peasant parents, Mendel was educated in a monastery and went on to study science and mathematics at the University of Vienna, where he failed his examinations for a teaching certificate. He returned to the monastery and spent the rest of his life there, eventually becoming abbot. In the garden of the monastery (figure 13.6), Mendel initiated a series of experiments on plant hybridization. The results of these experiments would ultimately change our views of heredity irrevocably. Why Mendel Chose the Garden Pea For his experiments, Mendel chose the garden pea, the same plant Knight and many others had studied earlier. The choice was a good one for several reasons. First, many earlier investigators had produced hybrid peas by crossing different varieties. Mendel knew that he could expect to observe segregation of traits among the offspring. Second, a large number of true-breeding varieties of peas were available. Mendel initially examined 32. Then, for further study, he selected lines that differed with respect to seven easily distinguishable traits, such as round versus wrinkled seeds and purple versus white flowers, a character that Knight had studied. Third, pea plants are small and easy to grow, and they have a relatively short generation time. Thus, one can conduct experiments involving numerous plants, grow several generations in a single year, and obtain results relatively quickly. A fourth advantage of studying peas is that the sexual organs of the pea are enclosed within the flower (figure 13.7). The flowers of peas, like those of many flowering plants, contain both male and female sex organs. Furthermore, the gametes produced by the male and female parts of the same flower, unlike those of many flowering plants, can fuse to form viable offspring. Fertilization takes place automatically within an individual flower if it is not disturbed, resulting in offspring that are the progeny from a single individual. Therefore, one can either let individual flowers engage in selffertilization, or remove the flower’s male parts before fertilization and introduce pollen from a strain with a different trait, thus performing cross-pollination which results in cross-fertilization. 242 Part IV Reproduction and Heredity FIGURE 13.5 Gregor Johann Mendel. Cultivating his plants in the garden of a monastery in Brunn, Austria (now Brno, Czech Republic), Mendel studied how differences among varieties of peas were inherited when the varieties were crossed. Similar experiments had been done before, but Mendel was the first to quantify the results and appreciate their significance. FIGURE 13.6 The garden where Mendel carried out his plant-breeding experiments. Gregor Mendel did his key scientific experiments in this small garden in a monastery
Mendel's Experimental Design Mendel was careful to focus on only a few specific differ Petals ences between the plants he was using and to ignore the countless other differences he must have seen He also had the insight to realize that the differences he selected to ana lyze must be comparable. For example, he appreciated that Anther d trying to study the inheritance of round seeds versus tall height would be useless Mendel usually conducted his experiments in three First, he allowed pea plants of a given variety to pro duce progeny by self-fertilization for several gene tions. Mendel thus was able to assure himself that the traits he was studying were indeed constant, transmitted unchanged from generation to genera- FIGURE 13.7 tion. Pea plants with white flowers, for example, Structure of the pea flower(longitudinal section). In a pea when crossed with each other, produced only off- plant flower, the petals enclose the male anther(containing spring with white flowers, regardless of the number pollen grains, which give rise to haploid sperm)and the female of generations carpel(containing ovules, which give rise to haploid eggs). This 2. Mendel then performed crosses between varieties es ensures that self-fertilization will take place unless the flower is exhibiting alternative forms of characters. For ex- disturbed ample, he removed the male parts from the flower of a plant that produced white flowers and fertilized it with pollen from a purple-flowered Pollen transferred from plant. He also carried out the white flower to stigma of purple flower from a white-flowered individual to fertilize a flower ol that produced purple flowers(fig 138) 3. Finally, Mendel perm brid offspring produced by these Anthers crosses to self-pollinate for several generations. By doing so, he al lowed the alternative forms of a character to segregate among the progeny. This was the same exper All purple flowers result mental design that Knight and others had used much earlier. But Mendel went an important step farther: he counted the numbers of ffspring exhibiting each trait in each succeeding generation. No one had ever done that before The quantitative results Mendel obtained proved to be of supreme mportance in revealing the process of heredity. FIGURE 13.8 How Mendel conducted his experiments. Mendel pushed aside the petals of a white flower and collected pollen from the anthers. He then placed that pollen onto the sign Mendel's experiments with the (part of the carpel) of a purple flower whose anthers had been removed, causing cross. E garden pea involved crosses between fertilization to take place. All the seeds in the pod that resulted from this pollination true-breeding varieties, followed by a After planting these seeds, Mendel observed the pea plants they produced. All of the were hybrids of the white-flowered male parent and the purple-flowered female parer generation or more of inbreeding progeny of this cross had purple flow Chapter 13 Patterns of Inheritance 243
Mendel’s Experimental Design Mendel was careful to focus on only a few specific differences between the plants he was using and to ignore the countless other differences he must have seen. He also had the insight to realize that the differences he selected to analyze must be comparable. For example, he appreciated that trying to study the inheritance of round seeds versus tall height would be useless. Mendel usually conducted his experiments in three stages: 1. First, he allowed pea plants of a given variety to produce progeny by self-fertilization for several generations. Mendel thus was able to assure himself that the traits he was studying were indeed constant, transmitted unchanged from generation to generation. Pea plants with white flowers, for example, when crossed with each other, produced only offspring with white flowers, regardless of the number of generations. 2. Mendel then performed crosses between varieties exhibiting alternative forms of characters. For example, he removed the male parts from the flower of a plant that produced white flowers and fertilized it with pollen from a purple-flowered plant. He also carried out the reciprocal cross, using pollen from a white-flowered individual to fertilize a flower on a pea plant that produced purple flowers (figure 13.8). 3. Finally, Mendel permitted the hybrid offspring produced by these crosses to self-pollinate for several generations. By doing so, he allowed the alternative forms of a character to segregate among the progeny. This was the same experimental design that Knight and others had used much earlier. But Mendel went an important step farther: he counted the numbers of offspring exhibiting each trait in each succeeding generation. No one had ever done that before. The quantitative results Mendel obtained proved to be of supreme importance in revealing the process of heredity. Mendel’s experiments with the garden pea involved crosses between true-breeding varieties, followed by a generation or more of inbreeding. Chapter 13 Patterns of Inheritance 243 Petals Anther Carpel FIGURE 13.7 Structure of the pea flower (longitudinal section). In a pea plant flower, the petals enclose the male anther (containing pollen grains, which give rise to haploid sperm) and the female carpel (containing ovules, which give rise to haploid eggs). This ensures that self-fertilization will take place unless the flower is disturbed. Pollen transferred from white flower to stigma of purple flower Anthers removed All purple flowers result FIGURE 13.8 How Mendel conducted his experiments. Mendel pushed aside the petals of a white flower and collected pollen from the anthers. He then placed that pollen onto the stigma (part of the carpel) of a purple flower whose anthers had been removed, causing crossfertilization to take place. All the seeds in the pod that resulted from this pollination were hybrids of the white-flowered male parent and the purple-flowered female parent. After planting these seeds, Mendel observed the pea plants they produced. All of the progeny of this cross had purple flowers.
What mendel found The Fi Generation The seven characters Mendel studied in his experiments When Mendel crossed two contrasting varieties of peas possessed several variants that differed from one another in such as white-flowered and purple-flowered plants,the score(figure 13.9). hybrid offspring he obtained did not have flowers of in We will examine in detail mendel's crosses with flower termediate color, as the theory of blending inheritance color. His experiments with other characters were similar, re and they produced similar results the offspring resembled one of their parents. It is custom- ary to refer to these offspring as the first filial (filius Character Dominant vs recessive trait F2 generation Ratio Dominant form Recessive form col 3.15:1 Purple White Seed color 6022 2001 3.01:1 Yellow Green Seed Round Wrinkled 152 Yellow Pod 295:1 Inflated Constricted Flowe 3.14:1 Axial Plar height 2.84:1 Tall Dwarf FIGURE 13.9 Mendel's experimental results. This table illustrates the seven characters Mendel studied in his crosses of the garden pea and presents the data he obtained from these crosses. Each pair of traits appeared in the F2 generation in very close to a 3: 1 ratio 244 Part IV Reproduction and Heredity
What Mendel Found The seven characters Mendel studied in his experiments possessed several variants that differed from one another in ways that were easy to recognize and score (figure 13.9). We will examine in detail Mendel’s crosses with flower color. His experiments with other characters were similar, and they produced similar results. The F1 Generation When Mendel crossed two contrasting varieties of peas, such as white-flowered and purple-flowered plants, the hybrid offspring he obtained did not have flowers of intermediate color, as the theory of blending inheritance would predict. Instead, in every case the flower color of the offspring resembled one of their parents. It is customary to refer to these offspring as the first filial ( filius is 244 Part IV Reproduction and Heredity Character Flower color Seed color Seed shape Pod color Pod shape Flower position Plant height Dominant vs. recessive trait F2 generation Dominant form Recessive form Ratio 3.15:1 3.01:1 2.96:1 2.82:1 2.95:1 3.14:1 2.84:1 705 224 6022 2001 5474 1850 428 152 882 299 651 207 787 277 Purple White Yellow Green Round Wrinkled Green Yellow Inflated Constricted Axial Terminal Tall Dwarf X X X X X X X FIGURE 13.9 Mendel’s experimental results. This table illustrates the seven characters Mendel studied in his crosses of the garden pea and presents the data he obtained from these crosses. Each pair of traits appeared in the F2 generation in very close to a 3:1 ratio