heredity
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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.
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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.
The blood theory of heredity, if this notion can be dignified with such a name, is really a part of the folklore antedating scientific biology. It is implicit in such popular phrases as “half blood,” “new blood,” and “blue blood.” It does not mean that heredity is actually transmitted through the red liquid in blood vessels; the essential point is the belief that a parent transmits to each child all its characteristics and that the hereditary endowment of a child is an alloy, a blend of the endowments of its parents, grandparents, and more-remote ancestors. This idea appeals to those who pride themselves on having a noble or remarkable “blood” line. It strikes a snag, however, when one observes that a child has some characteristics that are not present in either parent but are present in some other relatives or were present in more-remote ancestors. Even more often, one sees that brothers and sisters, though showing a family resemblance in some traits, are clearly different in others. How could the same parents transmit different “bloods” to each of their children?
Mendel disproved the blood theory. He showed (1) that heredity is transmitted through factors (now called genes) that do not blend but segregate, (2) that parents transmit only one-half of the genes they have to each child, and they transmit different sets of genes to different children, and (3) that, although brothers and sisters receive their heredities from the same parents, they do not receive the same heredities (an exception is identical twins). Mendel thus showed that, even if the eminence of some ancestor were entirely the reflection of his genes, it is quite likely that some of his descendants, especially the more remote ones, would not inherit these “good” genes at all. In sexually reproducing organisms, humans included, every individual has a unique hereditary endowment.
Lamarckism—a school of thought named for the 19th-century pioneer French biologist and evolutionist Jean-Baptiste de Monet, chevalier de Lamarck—assumed that characters acquired during an individual’s life are inherited by his progeny, or, to put it in modern terms, that the modifications wrought by the environment in the phenotype are reflected in similar changes in the genotype. If this were so, the results of physical exercise would make exercise much easier or even dispensable in a person’s offspring. Not only Lamarck but also other 19th-century biologists, including Darwin, accepted the inheritance of acquired traits. It was questioned by German biologist August Weismann, whose famous experiments in the late 1890s on the amputation of tails in generations of mice showed that such modification resulted neither in disappearance nor even in shortening of the tails of the descendants. Weismann concluded that the hereditary endowment of the organism, which he called the germ plasm, is wholly separate and is protected against the influences emanating from the rest of the body, called the somatoplasm, or soma. The germ plasm–somatoplasm are related to the genotype–phenotype concepts, but they are not identical and should not be confused with them.
The noninheritance of acquired traits does not mean that the genes cannot be changed by environmental influences; X-rays and other mutagens certainly do change them, and the genotype of a population can be altered by selection. It simply means that what is acquired by parents in their physique and intellect is not inherited by their children. Related to these misconceptions are the beliefs in “prepotency”—i.e., that some individuals impress their heredities on their progenies more effectively than others—and in “prenatal influences” or “maternal impressions”—i.e., that the events experienced by a pregnant female are reflected in the constitution of the child to be born. How ancient these beliefs are is suggested in the Book of Genesis, in which Jacob produces spotted or striped progeny in sheep and goats by showing the flocks striped rods while the animals are breeding. Another such belief is “telegony,” which goes back to Aristotle; it alleged that the heredity of an individual is influenced not only by his father but also by males with whom the female may have mated and who have caused previous pregnancies. Even Darwin, as late as 1868, seriously discussed an alleged case of telegony: that of a mare mated to a zebra and subsequently to an Arabian stallion, by whom the mare produced a foal with faint stripes on his legs. The simple explanation for this result is that such stripes occur naturally in some breeds of horses.
All these beliefs, from inheritance of acquired traits to telegony, must now be classed as superstitions. They do not stand up under experimental investigation and are incompatible with what is known about the mechanisms of heredity and about the remarkable and predictable properties of genetic materials. Nevertheless, some people still cling to these beliefs. Some animal breeders take telegony seriously and do not regard as purebred the individuals whose parents are admittedly “pure” but whose mothers had mated with males of other breeds. Soviet biologist and agronomist Trofim Denisovich Lysenko was able for close to a quarter of a century, roughly between 1938 and 1963, to make his special brand of Lamarckism the official creed in the Soviet Union and to suppress most of the teaching and research in orthodox genetics. He and his partisans published hundreds of articles and books allegedly proving their contentions, which effectively deny the achievements of biology for at least the preceding century. The Lysenkoists were officially discredited in 1964.
Mendelian genetics
Discovery and rediscovery of Mendel’s laws
Gregor Mendel published his work in the proceedings of the local society of naturalists in Brünn, Austria (now Brno, Czech Republic), in 1866, but none of his contemporaries appreciated its significance. It was not until 1900, 16 years after Mendel’s death, that his work was rediscovered independently by botanists Hugo de Vries in Holland, Carl Erich Correns in Germany, and Erich Tschermak von Seysenegg in Austria. Like several investigators before him, Mendel experimented on hybrids of different varieties of a plant; he focused on the common pea plant (Pisum sativum). His methods differed in two essential respects from those of his predecessors. First, instead of trying to describe the appearance of whole plants with all their characteristics, Mendel followed the inheritance of single, easily visible and distinguishable traits, such as round versus wrinkled seed, yellow versus green seed, purple versus white flowers, and so on. Second, he made exact counts of the numbers of plants bearing each trait; it was from such quantitative data that he deduced the rules governing inheritance.
Since pea plants reproduce usually by self-pollination of their flowers, the varieties Mendel obtained from seedsmen were “pure”—i.e., descended for several to many generations from plants with similar traits. Mendel crossed them by deliberately transferring the pollen of one variety to the pistils of another; the resulting first-generation hybrids, denoted by the symbol F1, usually showed the traits of only one parent. For example, the crossing of yellow-seeded plants with green-seeded ones gave yellow seeds, and the crossing of purple-flowered plants with white-flowered ones gave purple-flowered plants. Traits such as the yellow-seed colour and the purple-flower colour Mendel called dominant; the green-seed colour and the white-flower colour he called recessive. It looked as if the yellow and purple “bloods” overcame or consumed the green and white “bloods.”
That this was not so became evident when Mendel allowed the F1 hybrid plants to self-pollinate and produce the second hybrid generation, F2. Here, both the dominant and the recessive traits reappeared, as pure and uncontaminated as they were in the original parents (generation P). Moreover, these traits now appeared in constant proportions: about 3/4 of the plants in the second generation showed the dominant trait and 1/4 showed the recessive, a 3 to 1 ratio. It can be seen in the table that Mendel’s actual counts were as close to the ideal ratio as one could expect, allowing for the sampling deviations present in all statistical data.
| number dominant | number recessive | ratio | ||
|---|---|---|---|---|
| round seed | 5,474 | wrinkled seed | 1,850 | 2.96:1 |
| yellow seed | 6,022 | green seed | 2,001 | 3.01:1 |
| purple flowers | 705 | white flowers | 224 | 3.15:1 |
| tall plants | 787 | short plants | 277 | 2.84:1 |
Mendel concluded that the sex cells, the gametes, of the purple-flowered plants carried some factor that caused the progeny to develop purple flowers, and the gametes of the white-flowered variety had a variant factor that induced the development of white flowers. In 1909 the Danish biologist Wilhelm Ludvig Johannsen proposed to call these factors genes.
An example of one of Mendel’s experiments will illustrate how the genes are transmitted and in what particular ratios. Let R stand for the gene for purple flowers and r for the gene for white flowers (dominant genes are conventionally symbolized by capital letters and recessive genes by lowercase letters). Since each pea plant contains a gene endowment half of whose set is derived from the mother and half from the father, each plant has two genes for flower colour. If the two genes are alike—for instance, both having come from white-flowered parents (rr)—the plant is termed a homozygote. The union of gametes with different genes gives a hybrid plant, termed a heterozygote (Rr). Since the gene R, for purple, is dominant over r, for white, the F1 generation hybrids will show purple flowers. They are phenotypically purple, but their genotype contains both R and r genes, and these alternative (allelic or allelomorphic) genes do not blend or contaminate each other. Mendel inferred that, when a heterozygote forms its sex cells, the allelic genes segregate and pass to different gametes. This is expressed in the first law of Mendel, the law of segregation of unit genes. Equal numbers of gametes, ovules, or pollen grains are formed that contain the genes R and r. Now, if the gametes unite at random, then the F2 generation should contain about 1/4 white-flowered and 3/4 purple-flowered plants. The white-flowered plants, which must be recessive homozygotes, bear the genotype rr. About 1/3 of the plants exhibiting the dominant trait of purple flowers must be homozygotes, RR, and 2/3 heterozygotes, Rr. The prediction is tested by obtaining a third generation, F3, from the purple-flowered plants; though phenotypically all purple-flowered, 2/3 of this group of plants reveal the presence of the recessive gene allele, r, in their genotype by producing about 1/4 white-flowered plants in the F3 generation.
Mendel also crossbred varieties of peas that differed in two or more easily distinguishable traits. When a variety with yellow round seed was crossed to a green wrinkled-seed variety, the F1 generation hybrids produced yellow round seed. Evidently, yellow (A) and round (B) are dominant traits, and green (a) and wrinkled (b) are recessive. By allowing the F1 plants (genotype AaBb) to self-pollinate, Mendel obtained an F2 generation of 315 yellow round, 101 yellow wrinkled, 108 green round, and 32 green wrinkled seeds, a ratio of approximately 9 : 3 : 3 : 1. The important point here is that the segregation of the colour (A–a) is independent of the segregation of the trait of seed surface (B–b). This is expected if the F1 generation produces equal numbers of four kinds of gametes, carrying the four possible combinations of the parental genes: AB, Ab, aB, and ab. Random union of these gametes gives, then, the four phenotypes in a ratio 9 dominant–dominant : 3 recessive–dominant : 3 dominant–recessive : 1 recessive–recessive. Among these four phenotypic classes there must be nine different genotypes, a supposition that can be tested experimentally by raising a third hybrid generation. The predicted genotypes are actually found. Another test is by means of a backcross (or testcross); the F1 hybrid (phenotype yellow round seed; genotype AaBb) is crossed to a double recessive plant (phenotype green wrinkled seed; genotype aabb). If the hybrid gives four kinds of gametes in equal numbers and if all the gametes of the double recessive are alike (ab), the predicted progeny of the backcross are yellow round, yellow wrinkled, green round, and green wrinkled seed in a ratio 1 : 1 : 1 : 1. This prediction is realized in experiments. When the varieties crossed differ in three genes, the F1 hybrid forms 23, or eight, kinds of gametes (2n = kinds of gametes, n being the number of genes). The second generation of hybrids, the F2, has 27 (33) genotypically distinct kinds of individuals but only eight different phenotypes. From these results and others, Mendel derived his second law: the law of recombination, or independent assortment of genes.
Universality of Mendel’s laws
Although Mendel experimented with varieties of peas, his laws have been shown to apply to the inheritance of many kinds of characters in almost all organisms. In 1902 Mendelian inheritance was demonstrated in poultry (by English geneticists William Bateson and Reginald Punnett) and in mice. The following year, albinism became the first human trait shown to be a Mendelian recessive, with pigmented skin the corresponding dominant.
In 1902 and 1909, English physician Sir Archibald Garrod initiated the analysis of inborn errors of metabolism in humans in terms of biochemical genetics. Alkaptonuria, inherited as a recessive, is characterized by excretion in the urine of large amounts of the substance called alkapton, or homogentisic acid, which renders the urine black on exposure to air. In normal (i.e., nonalkaptonuric) persons the homogentisic acid is changed to acetoacetic acid, the reaction being facilitated by an enzyme, homogentisic acid oxidase. Garrod advanced the hypothesis that this enzyme is absent or inactive in homozygous carriers of the defective recessive alkaptonuria gene; hence, the homogentisic acid accumulates and is excreted in the urine. Mendelian inheritance of numerous traits in humans has been studied since then.
In analyzing Mendelian inheritance, it should be borne in mind that an organism is not an aggregate of independent traits, each determined by one gene. A “trait” is really an abstraction, a term of convenience in description. One gene may affect many traits (a condition termed pleiotropic). The white gene in Drosophila flies is pleiotropic; it affects the colour of the eyes and of the testicular envelope in the males, the fecundity and the shape of the spermatheca in the females, and the longevity of both sexes. In humans many diseases caused by a single defective gene will have a variety of symptoms, all pleiotropic manifestations of the gene.
Allelic interactions
Dominance relationships
The operation of Mendelian inheritance is frequently more complex than in the case of the traits recorded by Mendel. In the first place, clear-cut dominance and recessiveness are by no means always found. When red- and white-flowered varieties of four-o’clock plants or snapdragons are crossed, for example, the F1 hybrids have flowers of intermediate pink or rose colour, a situation that seems more explicable by the blending notion of inheritance than by Mendelian concepts. That the inheritance of flower colour is indeed due to Mendelian mechanisms becomes apparent when the F1 hybrids are allowed to cross, yielding an F2 generation of red-, pink-, and white-flowered plants in a ratio of 1 red : 2 pink : 1 white. Obviously the hereditary information for the production of red and white flowers had not been blended away in the first hybrid generation, as flowers of these colours were produced in the second generation of hybrids.
The apparent blending in the F1 generation is explained by the fact that the gene alleles that govern flower colour in four-o’clocks show an incomplete dominance relationship. Suppose then that a gene allele R1 is responsible for red flowers and R2 for white; the homozygotes R1R1 and R2R2 are red and white respectively, and the heterozygotes R1R2 have pink flowers. A similar pattern of lack of dominance is found in Shorthorn cattle. In diverse organisms, dominance ranges from complete (a heterozygote indistinguishable from one of the homozygotes) to incomplete (heterozygotes exactly intermediate) to excessive or overdominance (a heterozygote more extreme than either homozygote).
Another form of dominance is one in which the heterozygote displays the phenotypic characteristics of both alleles. This is called codominance; an example is seen in the MN blood group system of human beings. MN blood type is governed by two alleles, M and N. Individuals who are homozygous for the M allele have a surface molecule (called the M antigen) on their red blood cells. Similarly, those homozygous for the N allele have the N antigen on the red blood cells. Heterozygotes—those with both alleles—carry both antigens.
Multiple alleles
The traits discussed so far all have been governed by the interaction of two possible alleles. Many genes, however, are represented by multiple allelic forms within a population. (One individual, of course, can possess only two of these multiple alleles.) Human blood groups—in this case, the well-known ABO system—again provide an example. The gene that governs ABO blood types has three alleles: IA, IB, and IO. IA and IB are codominant, but IO is recessive. Because of the multiple alleles and their various dominance relationships, there are four phenotypic ABO blood types: type A (genotypes IAIA and IAIO), type B (genotypes IBIB and IBIO), type AB (genotype IAIB), and type O (genotype IOIO).
Gene interactions
Many individual traits are affected by more than one gene. For example, the coat colour in many mammals is determined by numerous genes interacting to produce the result. The great variety of colour patterns in cats, dogs, and other domesticated animals is the result of different combinations of complexly interacting genes. The gradual unraveling of their modes of inheritance was one of the active fields of research in the early years of genetics.
Two or more genes may produce similar and cumulative effects on the same trait. In humans the skin-colour difference between so-called blacks and so-called whites is due to several (probably four or more) interacting pairs of genes, each of which increases or decreases the skin pigmentation by a relatively small amount.
Epistatic genes
Some genes mask the expression of other genes just as a fully dominant allele masks the expression of its recessive counterpart. A gene that masks the phenotypic effect of another gene is called an epistatic gene; the gene it subordinates is the hypostatic gene. The gene for albinism (lack of pigment) in humans is an epistatic gene. It is not part of the interacting skin-colour genes described above; rather, its dominant allele is necessary for the development of any skin pigment, and its recessive homozygous state results in the albino condition regardless of how many other pigment genes may be present. Albinism thus occurs in some individuals among dark- or intermediate-pigmented peoples as well as among light-pigmented peoples.
The presence of epistatic genes explains much of the variability seen in the expression of such dominantly inherited human diseases as Marfan syndrome and neurofibromatosis. Because of the effects of an epistatic gene, some individuals who inherit a dominant, disease-causing gene show only partial symptoms of the disease; some in fact may show no expression of the disease-causing gene, a condition referred to as nonpenetrance. The individual in whom such a nonpenetrant mutant gene exists will be phenotypically normal but still capable of passing the deleterious gene on to offspring, who may exhibit the full-blown disease.
Examples of epistasis abound in nonhuman organisms. In mice, as in humans, the gene for albinism has two variants: the allele for nonalbino and the allele for albino. The latter allele is unable to synthesize the pigment melanin. Mice, however, have another pair of alleles involved in melanin placement. These are the agouti allele, which produces dark melanization of the hair except for a yellow band at the tip, and the black allele, which produces melanization of the whole hair. If melanin cannot be formed (the situation in the mouse homozygous for the albino gene), neither agouti nor black can be expressed. Hence, homozygosity for the albinism gene is epistatic to the agouti/black alleles and prevents their expression.
Complementation
The phenomenon of complementation is another form of interaction between nonallelic genes. For example, there are mutant genes that in the homozygous state produce profound deafness in humans. One would expect that the children of two persons with such hereditary deafness would be deaf. This is frequently not the case, because the parents’ deafness is often caused by different genes. Since the mutant genes are not alleles, the child becomes heterozygous for the two genes and hears normally. In other words, the two mutant genes complement each other in the child. Complementation thus becomes a test for allelism. In the case of congenital deafness cited above, if all the children had been deaf, one could assume that the deafness in each of the parents was owing to mutant genes that were alleles. This would be more likely to occur if the parents were genetically related (consanguineou
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