History of Biology: Inheritance
Heredity (colloquially synonymous with "inheritance") refers to the process by which certain features (heritable characteristics) are transmitted from parent to offspring. This process has long been a source of intense interest to scientists. Just why do children look like their parents, but not exactly? This question can be separated into two parts: (1) In sexually reproducing organisms, how do the features of the parents get combined and transmitted to the offspring? (2) What actually gets transmitted? To ask these questions requires the materialist belief that some physical substance is transmitted that corresponds to particular traits, an assumption that was not widely held before the late nineteenth century.
From Aristotle to Weismann
Before the nineteenth century, questions about offspring looking like their parents were asked within a conceptual framework that embraced very different assumptions than scientists do today. The contributions of the parents to the offspring were not necessarily assumed to be equal, or even to be purely material. The ancient Greek philosopher Aristotle, for example, thought that the male semen contributed the "active element" to the offspring, bringing it to life, while the female contributed only nutritional material for the offspring.
Theorists who did think both parents contributed some material elements generally assumed that blending inheritance held true: the parental contributions were believed to blend together so that the offspring's characteristics were usually intermediate between those of the parents. If one parent had a short nose and another a long one, the child could be expected to have a nose somewhere in between. Moreover, in this conceptual framework, heredity was not separated sharply from environment; it was "common sense" that environmental effects on parental characteristics could reappear in their offspring. (This would later be called "the inheritance of acquired characters," or "Lamarckism," after the early-nineteenth-century biologist Jean-Baptiste Lamarck.) Thus, if parents were well educated, it was assumed that their children would be smart.
In the late nineteenth century, this framework was gradually abandoned. Two shifts in outlook were especially important. First, spurred on by new observations, scientists came to view hereditary transmission as a purely material process (possibly exempt from the effects of the environment). Starting in the 1860s, biologists developed new microscopic techniques to study the physical processes of the cell (a branch of biology called cytology ). In 1875, the German anatomist Oscar Hertwig was the first to observe a sperm penetrating an egg (of a sea urchin), thereby lending credence to the idea that a material substance was actually physically transferred via the sperm.
In the 1870s, new structures in the nucleus were discovered, called chromosomes (which means "colored bodies") because they absorbed dyes more intensely than the surrounding nuclear material. Although their function was mysterious, the fact that they came in pairs (perhaps one from each parent) suggested a possible role in heredity. As cytologists raced to sort out the complex and confusing cell-division events of mitosis and meiosis from the late 1870s to the early 1900s, they constructed innovative theories of heredity to accommodate these new observations. In contrast with earlier work, most of these theories postulated that some physical substance carried by the sperm and egg combined during fertilization to produce the offspring.
August Weissmann. At the same time, theorists began to challenge a second fundamental assumption of the old framework: blending inheritance. Instead, they suggested that inheritance was particulate: each parent contributed to the offspring its own share of discrete units corresponding to some hereditary trait (such as height or eye color), which were somehow then combined and sorted in the offspring. In the 1880s and 1890s, the German zoologist August Weismann influentially combined the two new concepts (material transfer and particulate inheritance), postulating a substance called the "germ plasm" that was carried in the chromosomes of the reproductive cells from generation to generation, and that was made up of invisible particles corresponding to particular body structures. Though Weismann's theory was highly speculative, by the early 1900s studies of chromosomal action during fertilization and early development seemed to confirm important parts of it, especially the role of the chromosomes as bearers of particulate hereditary material.
Weismann was not the only theorist to propose that the hereditary material was made up of discrete particles: Charles Darwin had conceived of heredity as particulate in the late 1860s (though his theory of heredity was not well regarded), and the Dutch plant breeder Hugo de Vries theorized a hereditary particle he called the "pangene." Thus, in 1900 scientists were already thinking about hereditary particles when de Vries and the German botanist Carl Correns rediscovered an obscure paper published in 1865 by the Austrian monk Gregor Mendel.
Gregor Mendel. Describing his breeding experiments on the common garden pea, Mendel developed his basic concept of paired, discrete hereditary "factors" (he did not call them "genes" or "alleles"). Each parent contributed one factor for each trait, and each trait came in one of two forms, dominant or recessive. Although only the dominant form would be visible in any combination of dominant and recessive, the recessive factor was still there, hidden, and could be passed to the next generation. If two recessives combined together, then the recessive form would be "expressed." Mendel's results also supported the idea that traits such as height and seed texture were not generally linked but recombined randomly during reproduction, showing independent assortment. A tall pea plant could thus have either smooth or wrinkled seeds; so could a short pea plant. In 1909, the Danish Mendelian Wilhelm Johannsen named these presumed hereditary particles "genes."
Mendel's ideas commanded immediate, widespread interest. His peabreeding experiments, which ran over many generations of plants to yield impressively stable statistical ratios of hereditary traits, provided biological theorists with compelling new evidence for the hypothesis of paired hereditary characters that sorted independently. Mendel's results appeared to offer practical guidance as well. Animal and plant breeders believed that they would help them develop rational systems for combining desirable traits in livestock and agriculturally important plants. Eugenicists, who sought to improve the human race through breeding "the best" traits together (such as strength and intelligence), thought Mendelism would provide rules for rational human breeding.
Thomas Hunt Morgan. By the early 1900s, then, the existence of discrete genes that governed heredity seemed plausible to most biologists. However, the location and the physical nature of these theoretical entities was still uncertain. In particular, the relation between genes (which seemed to come in pairs) and chromosomes (which also came in pairs) was still a matter of some debate. Then in the 1910s, Thomas Hunt Morgan at Columbia University united the cytological focus on chromosome activity with the Mendelian breeding approach.
Combining breeding experiments on fruit flies ( Drosophilia ) with microscopic study of their chromosomes, Morgan and his students established beyond any doubt that hereditary material was carried on the chromosomes and that the theoretical entity known as the gene corresponded to particular identifiable traits. They also refined the theory of the gene substantially, developing explanations for "linked" traits that did not sort randomly (genes near each other on the same chromosome), positing the existence of more than two forms of a gene (multiple alleles ), and developing the idea that some genes could act as modifiers on others, changing their effect.
Morgan's student Alfred H. Sturtevant combined breeding experiments, statistical analysis, and the study of chromosomes under the microscope to draw up chromosome "maps" that showed how far apart the genes for various traits must be on the chromosome. Although some scientists outside Morgan's powerful circle—especially in Europe—contested the view that the chromosomal gene was the sole bearer of hereditary material (arguing, for example, that the cytoplasm surrounding the nucleus might also play a role in heredity), the views established by Morgan and his school in the 1910s and 1920s largely prevailed, and have come to be known as classical genetics.
Biochemistry. Biologists in the Morgan tradition, however, were unequipped to answer the question, What is the gene made of? Answering this question required attention to biochemistry. In the 1930s and 1940s, the leading candidate was protein , though a minority view held that it might be deoxyribonucleic acid, or DNA. In 1944, Oswald Avery, Colin MacLeod, and Maclyn McCarty published results of their experiments with the pneumonia-causing bacterium Streptococcus pneumoniae that indicated that DNA was the right pick, and in 1952 this view gained strong verification by the famous "Waring blender" experiments of Alfred Hershey and Martha Chase, which showed that the protein of the bacteriophage virus was a mere protective coating, while the stuff that created genetic transformation was DNA.
In 1953, James Watson and Francis Crick went further, postulating a double-helical structure for DNA, arguing that the four nucleotide bases guanine, cytosine, thymine, and adenine were its building blocks. The parallel structure of the helices suggested the possibility that it "unzipped" in replication, such that each side of the zipper, each helix, could then act as a template for the synthesis of a complementary strand of DNA, thus creating a perfect replica, ideally suited for passing on to offspring. Finally, in the early 1960s, scientists interpreted the sequence of nucleotides along the chromosome as a code for the sequence of amino acids in protein. This insight illuminated the means by which the gene dictates the physical characteristics of the organism possessing it. Although many details needed to be resolved, it seemed to many that the most basic keys to heredity had been discovered.
By the late twentieth century, then, biologists had come to view the gene from two directions. Working from the "outside in," organismal and population biologists continued to operate with the classical concept that a gene (or some combination of genes) corresponds to a trait (as in "a gene for X"). Working from the "inside out," biochemists and molecular biologists defined the gene as the amount of DNA that codes for one protein or one polypeptide . Since a protein is not the same as a trait, much work continues to aim at unravelling the complex nature of gene expression . As research continues to develop, and the field of genomics continues to expand, the idea of the gene continues to evolve.
Lynn K. Nyhart
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Why do children look like their parents, but not exactly? Heredity refers to the process by which certain features (heritable characters) are transmitted from parent to offspring.