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Key People:Gregor MendelThomas Hunt MorganErnst HaeckelAugust WeismannKarl Pearson…(Show more)Related Topics:geneticsgenechimerachromosomeDNA…(Show more)
heredity, the sum of all biological processes by which particular characteristics are transmitted from parents to their offspring. The concept of heredity encompasses two seemingly paradoxical observations about organisms: the constancy of a species from generation to generation and the variation among individuals within a species. Constancy and variation are actually two sides of the same coin, as becomes clear in the study of genetics. Both aspects of heredity can be explained by genes, the functional units of heritable material that are found within all living cells. Every member of a species has a set of genes specific to that species. It is this set of genes that provides the constancy of the species. Among individuals within a species, however, variations can occur in the form each gene takes, providing the genetic basis for the fact that no two individuals (except identical twins) have exactly the same traits.
Each offspring is a combination of its two parents, receiving some dominant traits from its mother and others from its father.
The set of genes that an offspring inherits from both parents, a combination of the genetic material of each, is called the organism’s genotype. The genotype is contrasted to the phenotype, which is the organism’s outward appearance and the developmental outcome of its genes. The phenotype includes an organism’s bodily structures, physiological processes, and behaviours. Although the genotype determines the broad limits of the features an organism can develop, the features that actually develop, i.e., the phenotype, depend on complex interactions between genes and their environment. The genotype remains constant throughout an organism’s lifetime; however, because the organism’s internal and external environments change continuously, so does its phenotype. In conducting genetic studies, it is crucial to discover the degree to which the observable trait is attributable to the pattern of genes in the cells and to what extent it arises from environmental influence.
Because genes are integral to the explanation of hereditary observations, genetics also can be defined as the study of genes. Discoveries into the nature of genes have shown that genes are important determinants of all aspects of an organism’s makeup. For this reason, most areas of biological research now have a genetic component, and the study of genetics has a position of central importance in biology. Genetic research also has demonstrated that virtually all organisms on this planet have similar genetic systems, with genes that are built on the same chemical principle and that function according to similar mechanisms. Although species differ in the sets of genes they contain, many similar genes are found across a wide range of species. For example, a large proportion of genes in baker’s yeast are also present in humans. This similarity in genetic makeup between organisms that have such disparate phenotypes can be explained by the evolutionary relatedness of virtually all life-forms on Earth. This genetic unity has radically reshaped the understanding of the relationship between humans and all other organisms. Genetics also has had a profound impact on human affairs. Throughout history humans have created or improved many different medicines, foods, and textiles by subjecting plants, animals, and microbes to the ancient techniques of selective breeding and to the modern methods of recombinant DNA technology. In recent years medical researchers have begun to discover the role that genes play in disease. The significance of genetics only promises to become greater as the structure and function of more and more human genes are characterized.
This article begins by describing the classic Mendelian patterns of inheritance and also the physical basis of those patterns—i.e., the organization of genes into chromosomes. The functioning of genes at the molecular level is described, particularly the transcription of the basic genetic material, DNA, into RNA and the translation of RNA into amino acids, the primary components of proteins. Finally, the role of heredity in the evolution of species is discussed.
Basic features of heredity
Prescientific conceptions of heredity
Heredity was for a long time one of the most puzzling and mysterious phenomena of nature. This was so because the sex cells, which form the bridge across which heredity must pass between the generations, are usually invisible to the naked eye. Only after the invention of the microscope early in the 17th century and the subsequent discovery of the sex cells could the essentials of heredity be grasped. Before that time, ancient Greek philosopher and scientist Aristotle (4th century bc) speculated that the relative contributions of the female and the male parents were very unequal; the female was thought to supply what he called the “matter” and the male the “motion.” The Institutes of Manu, composed in India between 100 and 300 ad, consider the role of the female like that of the field and of the male like that of the seed; new bodies are formed “by the united operation of the seed and the field.” In reality both parents transmit the heredity pattern equally, and, on average, children resemble their mothers as much as they do their fathers. Nevertheless, the female and male sex cells may be very different in size and structure; the mass of an egg cell is sometimes millions of times greater than that of a spermatozoon.
The ancient Babylonians knew that pollen from a male date palm tree must be applied to the pistils of a female tree to produce fruit. German botanist Rudolph Jacob Camerarius showed in 1694 that the same is true in corn (maize). Swedish botanist and explorer Carolus Linnaeus in 1760 and German botanist Josef Gottlieb Kölreuter, in a series of works published from 1761 to 1798, described crosses of varieties and species of plants. They found that these hybrids were, on the whole, intermediate between the parents, although in some characteristics they might be closer to one parent and in others closer to the other parent. Kölreuter compared the offspring of reciprocal crosses—i.e., of crosses of variety A functioning as a female to variety B as a male and the reverse, variety B as a female to A as a male. The hybrid progenies of these reciprocal crosses were usually alike, indicating that, contrary to the belief of Aristotle, the hereditary endowment of the progeny was derived equally from the female and the male parents. Many more experiments on plant hybrids were made in the 1800s. These investigations also revealed that hybrids were usually intermediate between the parents. They incidentally recorded most of the facts that later led Gregor Mendel (see below) to formulate his celebrated rules and to found the theory of the gene. Apparently, none of Mendel’s predecessors saw the significance of the data that were being accumulated. The general intermediacy of hybrids seemed to agree best with the belief that heredity was transmitted from parents to offspring by “blood,” and this belief was accepted by most 19th-century biologists, including English naturalist Charles Darwin.