Chapter

The Mendelian View of the World

It is easy to consider human beings unique among living organisms. We alone have developed complicated languages that allow mean* ingful and complex interplay of ideas and emotions. Great civilizations have developed and changed our world's environment in ways inconceivable for any other form of life. There has always been a tendency, therefore, to think that something special differentiates humans from every other species. This belief has found expression in the many forms of religion through which we seek the origin and explore the reasons for our existence and, in so doing, try to create workable rules lor conducting our lives. Little more than a century ago, it seemed natural to think that, just as every human life begins and ends at a fixed time, the human species and all other forms of life must also have been created at a fixed moment.

This belief was first seriously questioned 140 years ago, when diaries Darwin and Alfred R. Wallace proposed their theories of evolution. based on the selection of the most fit. They slated that the various forms of life are not constant but continually give rise to slightly different animals and plants, some of which adapt to survive and multiply more effectively. At the time of this theory, they did not know the origin of this continuous variation, but they did correctly realize that these new characteristics must persist in the progeny if such variations are to form the basis of evolution,

At first, there was a great furor against Darwin, most of it coming from people who did not like to believe that humans and the rather obscene-looking apes could have a common ancestor, even if this ancestor had lived some 10 million years ago. There was also initial opposition from many biologists who failed to find Darwin's evidence convincing. Among these was the famous naturalist lean L. Agassiz, then at Harvard, who spent many years writing against Darwin and Darwin's champion, Thomas H. Huxley, the most successful of the popularizes of evolution. But by the end of the nineteenth century, the scientific argument was almost complete; both the current geographic distribution of plants and animals and their selective occurrence in the fossil records of the geologic past were explicable only by postulating that continuously evolving groups of organisms had descended from a common ancestor. Today, evolution is an accepted fact for everyone except a fundamentalist minority, whose objections are based not on reasoning but on doctrinaire adherence to religious principles.

An immediate consequence of Darwinian theory is the realization that life first existed on our Karth more than 4 billion years ago in a simple form, possibly resembling the bacteria—the simplest variety of life known today. The existence of such small bacteria tells us that the essence of the living slate is found in very small organisms. Evolutionary theory further suggests that ihe basic principles of life apply to all living forms.

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Chromosomal Theory of Heredity {p 8) Gene l inkage and Crossing Over (p 9)

The Origin of Genetic Variability through Mutations (p. 15)

Early Speculations about What Genes Are and How They Ad (p. 16) •

Preliminary Attempts to Find a Gene-Protein Relationship (p. 16)

MENDEL'S DISCOVERIES

Gregor Mendel's experiments traced the results of breeding experiments (genetic crosses) between strains of peas differing in well-defined characteristics, like seed shape (round or wrinkled), seed color [yellow or green), pod shape {inflated or wrinkled), and stem length (long or short). His concentration on well-defined differences was of great importance; many breeders had previously tried to fallow the inheritance of mare gross qualities, like body weight, and were unable to discover any simple rules about their transmission from parents to offspring (see Box 1-1, Mendelian I^ws).

The Principle of Independent Segregation

After ascertaining that each type of parental strain bred true—that is, produced progeny with particular qualities identical to those of the parents—Mendel performed a number of crosses between parents (P) differing in single characteristics (such as seed shape or seed color).

Box 1-1 Mendeiian Laws

The most striking attribute of a living cell is its ability to transmit hereditary properties from one ceil generation to another. The existence of heredity must have been noticed by early humans, who witnessed the passing of characteristics, like eye Of hair color, from parents to offspring. Its physical basis, however, was not understood until the first years of the twentieth century, when, during a remarkable pertod of creative activity, the chromosomal theory of heredity was established.

Hereditary transmission through the sperm and egg became known by i860, and in 1868 Ernst Hacckel, noting that sperm consists largely of nudear material, postulated that the nudeus ts responsible For heredity. Almost 20 years passed before the chromosomes were singled out as the active factors, because the details of mitosis, meiosis, and fertilization had to be worked out first When this was accomplished, it could be seen that, unlike other cellular constituents, the chromosomes are equally divided between daughter cells. Moreover, the complicated chromosomal changes that reduce the sperm and egg chromosome number to the haploid number during meiosis became understandable as necessary for keeping the chromosome number constant. These facts, however, merely suggested that chromosomes carry heredity.

Proof came at the turn of the century with the discovery of the basic rules of heredity. The concepts were first proposed by Gregor Mendel in 1865 in a paper entitled "Experiments on Plant Hybrids" given to the Natural Soence Scoety at Brno. In his presentation, Mendel described in great detail the patterns of transmission of traits in pea plants (which we discuss in detail bebw), his conclusions of the principles of heredity, and their relevance to the controversial theories of evolution. The climate of scientific opinion, however, was not favorable, and these ideas were completely ignored, despite some early efforts un Mendel's part to interest the prominent biologists of his time. In 1900, years after Mendel's death, three plant breeders working independently on different systems confirmed the significance of Mendel's forgotten work. Hugo Dc Vries, Karl Cotfens, and Erich Tscher-mak, all doing experiments related to Mendel's, reached similar conclusions before they knew of Mendel's work

All the progeny (F, = first filial generation} had the appearance of one parent only. For example, in a cross between peas having yellow seeds and peas having green seeds, all the progeny had yellow seeds. The trait that appears in the F, progeny is called dominant, whereas the trait that does not appear in Ft is called recessive.

The meaning of these results became clear when Mendel set up genetic crosses between F, offspring. These crosses gave the important result thai the recessive trait reappeared in approximately 25% of the F; progeny, whereas the dominant trait appeared in 75% of these offspring. For each of the seven trails he followed, the ratio in V, of dominant to recessive traits was always approximately 3:1, When these experiments were carried to a third (F;,) progeny generation, all the F2 peas with recessive traits bred true (produced progeny with the recessive traits). Those with dominant traits fell into two groups: one-third bred true {produced only progeny with the dominant trait); the remaining two-thirds again produced mixed progeny in a 3:1 ratio of dominant to recessive.

Mendel correctly interpreted his results as follows (Figure l-l): the various traits are controlled by pairs of factors (which we now call genes), one factor derived from the male parent, the other from the female. For example, pure-breeding strains of round peas contain two versions (or alleles) of the roundness gene (RR), whereas pure-breeding wrinkled strains have two copies of the wrinkledness (rr) allele. The round-strain gametes each have one gene for roundness (if); the wiinklnd-strain gametes each have one gene for wrinkledness (r). In a cross between RR and rr, fertilization produces an F| plant with both alleles (/ir). The seeds look round because R is dominant over r. We refer to the appearance or physical structure of an individual as its phenotype, and to its genetic composition as its genotype, individuals with identical phenotypes may possess different genotypes; thus, to determine the genotype of an organism, it is frequently necessary to perform genetic crosses For several generations. The term homozygous refers to a gene pair in which both the maternal and paternal genes are identical (lor example, RR or rr). In contrast, those gene pairs in which paternai and maternal genes are different (for example, Br) are called heterozygous.

One or several letters or symbols may be used to represent a particular gene. The dominant allele of the gene may lie indicated by a capital letter (H), by a superscript I (r), or by a + standing alone. fn our discussions here, we use the first convention in which the dominant allele is represented by a capital fetter and the recessive allele by the lowercase letter.

It is important to notice that a given gamete contains only one of the two copies (one allele) of the genes present in the organism it comes from (for example, either R or r, but never both) and that the two types of gametes are produced in equal numbers. Thus, there is a 50-50 chance that a given gamete from an Fj pea will contain a particular gene [R or r). This choice is purely random. We do not expect to find exact 3:1 ratios when we examine a limited number of F^ progeny. The ratio will sometimes be slightly higher and other times slightly lower. But as we look at increasingly larger samples, we expect that the ratio of peas with the dominant trait to peas with the recessive trait will approximate the 3:1 ratio more and more closely.

The reappearance of the recessive characteristic in the Fz generation indicates that recessive alleles are neither modified nor lost in the F, (Hr) generation, but that the dominant and recessive genes are parental generation

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