Читать книгу Welcome to the Genome - Michael Yudell - Страница 15

For close to two millennia few scientists approached Aristotle’s understanding of heredity, though other theories were put forth during the centuries. Some, like the idea of the homunculus—the belief that every being was miniaturized and preformed in a reproductive cell—or the belief in panspermia—the idea that secretions from the entire body contribute to offspring—held sway for varying lengths of time. (10)

But before the late eighteenth century, ideas about what we today understand as heredity were quite different than our modern concept. Although similarities were recognized between parents and offspring and among families, such similarities, in a pre‐hereditarian worldview, were not generated by a hereditary mechanism, but by the act of conception itself, the pregnancy that followed, the development of the embryo, the birth, and, finally, lactation. There could be no laws of heredity in a system that viewed each creation of plant and animal life as isolated events. (11)

However, beginning in the eighteenth century, disparate fields of thought concerning hereditary phenomena would begin to converge on the road to developing hereditary theories. (12) Medical science, for example, began to systematically characterize disease. The taxonomic language of natural history moved toward uniformity. Professional animal and plant breeders more actively sought to breed specific features. Scientists investigated preformationist theories. And anthropology, in seeking to understand physical differences between peoples and populations, investigated the origins of human diversity. (13) From these various scientific investigations would slowly emerge both a popular and scientific discourse that would, over time, shape emerging concepts of heredity.

The work of Austrian monk Gregor Mendel, who bred peas in his abbey garden, built upon these growing discussions of heredity, and is credited with making the jump to studying heredity experimentally. But Mendel was not just a monk tending peas. The child of peasant farmers, he was a classically trained scientist raised in the greatest traditions of the Enlightenment. Intellectually nurtured by his family and schooled in the best academies and universities of Central Europe, the German‐speaking Mendel spent his life dividing his affection between God and science. (14) In 1843, at the age of 21, Mendel entered the St. Thomas Monastery in Brünn in what is now the Czech Republic. (15)

In the Church Mendel found a community of scientists—botanists, zoologists, and geologists among them—working diligently in their fields and making important contributions to the scientific literature. Perhaps the most important event in Mendel’s early career occurred 10 years into his stay at St. Thomas. In 1851, at the behest of his abbot, Mendel was sent to Vienna University to study at the institute of Professor Christian Doppler, one of the pioneers of modern physics. For 2 years at Doppler’s institute Mendel honed his scientific skills, taking courses in physics, chemistry, and mathematics, as well as entomology, botany, and plant physiology. The influence of physics was important to Mendel’s later work on heredity. Physics taught Mendel that laws governed the natural world and that these laws could be uncovered through experimentation. (16) But it was ultimately Mendel’s exposure to ongoing debates in heredity that transformed him into the scientist we remember today.

Mendel and his predecessors understood that traits could be passed between generations. A child with his mother’s eyes and his father’s nose was easy evidence of that. Breeding experiments with domesticated animals also suggested that traits were passed to offspring.

The prevailing theory during the nineteenth century, one to which even Charles Darwin mistakenly ascribed, was “blended inheritance.” (17) This theory held that the characteristics of parents blended in their offspring. Experimentation in this area failed because, as Mendel was able to eventually determine, heredity was not a lump sum but rather a series of individual traits.

In 1856 Mendel began to study the mechanisms of inheritance, working with varieties of garden peas from the genus Pisum. (18) In the course of his experiments his garden flowered, as did his understanding of heredity. Mendel discovered several generalities from his experiments that remain the foundation of twentieth‐century genetics. Any student of biology knows Mendel’s work. Known as Mendel’s laws, these basic tenets describe heredity in two simple mechanisms: the law of independent assortment and the law of segregation.

Figure 1.3 Although it took decades for Gregor Mendel’s work on pea plants to revolutionize hereditary theory, his impact is today still felt in the biological sciences.

Credit: American Museum of Natural History

Mendel began an experiment with purebred peas. One breed had yellow seeds, the other green seeds. When purebred yellow‐seeded peas were bred with each other, their offspring through the generations would have yellow seeds. Under the same circumstances, the green‐seeded peas would always have green‐seeded progeny. However, when he bred the purebred pea with yellow seeds to a purebred pea with green seeds, the offspring, or the first generation of this breeding cross, always had yellow seeds. The green seed trait seemed to be gone. Mendel called traits like the yellow‐seed trait dominating (now called dominant) because in first‐generation crosses they would always appear. (19) Traits like the green‐seed trait were called recessive—although they disappeared completely in the first generation, they reappeared in the second. Thus, when Mendel took the yellow seeds from the first generation and either self‐pollinated them or pollinated them with pollen from other yellow peas from the same first‐generation breed, he discovered that some of these offspring, the second generation, again had the green seed trait. The plants, Mendel concluded, retained the ability to produce green seeds—of the second‐generation seeds, 6022 were yellow and 2001 were green. Likewise, when he used six other traits, he found the same pattern in the second generation—traits that had disappeared in the first generation reappeared in the second. (20) The chart below shows the relationship between dominant and recessive traits in second‐generation pea plants in the seven traits Mendel experimented with.

Dominant trait		Recessive trait
Round seeds	5474	Wrinkled seeds	1850
Yellow seeds	6022	Green seeds	2001
Gray seed coats	705	White seed coats	224
Green pods	428	Yellow pods	152
Inflated pods	882	Constricted pods	299
Long stems	787	Short stems	277
Axial flowers	651	Terminal flowers	207

(21)

From these experimental data, Mendel made several conclusions that are at the heart of his revolutionary contribution to hereditary theory. From the 3:1—dominant to recessive—ratio in the second generation, Mendel concluded that the traits he studied came in two different forms and that these forms existed in pairs in the plant. Mendel called these forms factors. Today we call them genes. During the process of making reproductive cells, Mendel deduced, these genes segregate from each other—that is, the two copies of a gene that you get from each parent segregate, and in the subsequent reproductive cells, only one half of the pair is passed on to offspring. At fertilization, a gene from each parent reconstitutes the pair. How else could Mendel explain how two yellow‐seeded pea plants could produce offspring with green seeds? In this case, the green‐seed trait was as much a part of the pea plant as the yellow‐seed trait despite sometimes being hidden. Mendel also concluded that the factors that were dominant (in the left‐hand column above) somehow overcame the factors that were recessive (in the right‐hand column) when they were combined in offspring from crosses. When all first‐generation plants were crossed, they had both kinds of factors. Mendel’s calculations allowed him to predict a 3:1 ratio if the two factors were segregated. This is Mendel’s first law, the law of segregation, which states that the factors specifying different alleles are separate or segregated, that only one may be carried by a gamete (an egg or sperm), and that gametes combine randomly. Therefore, a child has the same chance of inheriting allele A as it does allele B. (22)

Without the assistance of a calculator or computer, Mendel counted thousands and thousands of plants. Even more remarkable, he constructed lineages that had all possible combinations of two of the seven traits together. For example, he crossed a line of pea plants with round yellow seeds with a line whose seeds were wrinkled and green. This cross gave rise to first‐generation plants with seeds that were all yellow and smooth. But when he crossed these first‐generation plants to each other (a self‐cross), an amazingly regular ratio in the offspring arose—the seeds of nine were yellow and round, three yellow and wrinkled, three green and round, and one green and wrinkled. Mendel reasoned that mating these first‐generation plants was like taking the two possible types of each trait (e.g., seed texture and seed color) and throwing them into a hat. Nature then randomly chose from the hat how to combine the genes. Although the choice is random, the outcome is a remarkably regular ratio of 9:3:3:1. (23).

Figure 1.4 Mendel’s first law of segregation says that alleles will segregate randomly between generations. Mendel’s second law, the law of independent assortment, represented in the figure above, says that pairs of alleles will segregate independently between generations. (P = parents, F₁ = first generation, F₂ = second generation).

Credit: Wiley

These observations are now known as Mendel’s second law, the law of independent assortment—if two traits (genes) are being controlled with different controllers (alleles), offspring will be produced by random combinations of the controllers (alleles). (24) In other words, a trait is independently and randomly distributed among offspring.

Mendel was either very lucky or very perceptive: it turns out that seven is the number of chromosomes of Pisum. For all seven of the traits he examined to show true independent assortment with respect to one another, none of them can be linked—that is, none of them can be on the same chromosome (or in the case of one of the traits he examined, they have to be very far apart on the same chromosome). (25) Mendel must have watched his peas very closely. Perhaps he recognized the pattern of segregation as he was weeding his garden and thus performed his experiment with an expectation based on his knowledge as a pea biologist. Or, perhaps, he selectively looked at his data and forgot to record crosses that deviated from the ratios 3:1 and 9:3:3:1. Either way, his conclusions have not been overturned.

Mendel died on January 6, 1884, nearly 20 years after his momentous study with Pisum had been published. (26) Even though its significance remained unheralded, Mendel’s work as a scientist and as a servant of God was recognized by his peers. If Mendel had been luckier in choosing the journal in which to publish his findings, he might have been famous in his own time, but he published in an obscure scientific journal and died in genetic obscurity (his monastic calling guaranteed that). (27) Although his contemporaries did cite his work with Pisum, they probably did not comprehend its deeper meanings for what would become a cornerstone of hereditary theory. A tribute to Mendel by a fellow scientist in Brünn lauded him as one of the great scientists of his day who worked “almost exclusively on detailed natural scientific studies, in which he displayed a totally independent, unique way of thinking.” (28) Unfortunately, it would take the world another 16 years after his death to uncover the greatness of Mendel’s investigations.

The lack of attention to Mendel’s work may also be explained by the near obsession with evolution in the mid‐nineteenth century after the publication of Darwin’s The Origin of Species. Darwin’s work was published just 6 years before Mendel’s and captured public attention well into the twentieth century, leaving Mendel’s theory to languish quietly. (29)

The “rediscovery” of Mendel in 1900 was driven in part by what biologist Ernst Mayr calls “an accelerating interest in the problem of inheritance.” (30) Incredibly, in the spring of that year three botanists—Hugo de Vries, Carl Correns, and Erich Tschermak—all claimed to have discovered laws of inheritance. They soon learned, unfortunately, that Mendel’s work was nearly identical and had preceded them by 35 years. (31) In the coming decades, Mendel’s laws of segregation and independent assortment would be tested on a wide variety of species.