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Magill's Encyclopedia of Science: Animal Life Genetics Types of Animal Science: Evolution, fields of study, reproduction Fields of Study: Biochemisty, cell biology, developmental biology, embryology, evolutionary science, reproduction science Genetics is the study of inheritance of characteristics from one generation to the next. Humans have been studying genetics since prehistoric times with the first selective breeding of wolves for companion animals. In the 1800’s, an Austrian monk, Gregor Mendel, described the basic laws that govern the inheritance of genetic traits. In the twentieth century, the field of molecular genetics was created as biologists determined the actual chemical makeup of genes. Principal Terms allele: alternative forms of a single gene chromosome: a long strand of DNA with supporting proteins, that contains many genes deoxyribonucleic acid (DNA): the chemical polymer that is the genetic material of multicellular organisms gene: factors in cells that are responsible for an observable characteristic of an organism genome: all of the genetic material of an organism genotype: the actual genetic makeup of an organism mutation: any heritable change in the genetic material phenotype: the observable characteristics of an organism (for example, black fur color in a cat) Before any recorded history, ancient man chose alert pups from a litter of wolves for breeding. This practice of selectively breeding the wolves that were good companions eventually gave rise to the domesticated dog. The oldest undisputed dog bones known, excavated from a twenty-thousand-year-old Alaskan settlement, demonstrate that prehistoric humans knew that traits could be passed from one generation to the next, and that selectively breeding animals (or plants) could produce an organism that possessed desired characteristics. This practice of deliberate breeding is known as artificial selection. Humans have practiced artificial selection on numerous animals, including pigs, cattle, goats, and sheep. Homer and other Greek poets wrote about selective breeding, and part of the wealth of the ancient city of Troy was attributed to its expertise in horse breeding. Although humans had some control over the traits of domesticated animals through selective breeding, the results of matings were not always predictable, and nothing was known about the mechanism through which traits were passed from one generation to another until the mid-1800’s. Mendelian Genetics Gregor Mendel, an Austrian monk, is the undisputed father of the science of genetics. Working with garden peas, Mendel analyzed thousands of breeding experiments to describe laws that governed the inheritance of traits. Though Mendel studied a plant, his laws for the inheritance of traits apply to all sexually reproducing organisms, including humans. Mendel chose seven distinct traits to study in his garden peas: flower color, plant height, seed shape, seed color, pod shape, pod color, and flower position. He concluded that each of these traits was determined by a single, discrete factor called a gene. For instance, there was a gene for flower color and a gene for seed shape. Each gene had several variations, or alleles. The gene for flower color had a white allele that produced white flowers and a purple allele that produced purple flowers. Mendel’s experiments revealed that organisms have two copies of any gene for a trait. Those two copies can be identical, two purple alleles of the flower color gene, for instance; or those two alleles can be different. A pea plant could have one purple allele of the flower color gene and one white allele of the flower color gene. When an organism has two identical alleles of a gene, it is homozygous for that gene. When an organism has two different alleles of a gene, it is heterozygous for that gene. An organism inherits one allele, or copy of a gene, from one parent and one allele from the other parent. An organism, or cell, that has two copies of all of its genetic information is called diploid. In most sexually reproducing animals, the offspring are formed when a sperm cell from the male parent fertilizes an egg from the female parent. The sperm and the egg only contain half of all the genetic information. They are said to be haploid. However, the new organism they create is diploid because it gets one copy of the genetic information from the sperm and a second copy from the egg. Mendel’s first law, or the law of segregation, states that the two copies of each gene separate during the formation of gametes (eggs and sperm), and that fertilization of the egg by the sperm is a random event. Any sperm containing any allele of a gene can fertilize any egg of the same species, regardless of the allele carried by that egg. Mendel noted that certain alleles seemed to dominate over others. For instance, when a plant had a purple allele for flower color and a white allele for flower color, the plant always had purple flowers. Mendel called the allele that was seen in the heterozygote, in this case the purple allele, the dominant allele. The allele that was hidden or masked, he called the recessive allele. In order to show a recessive allele, an organism has to have two identical copies of a gene, both containing the same recessive allele. This is known as the homozygous recessive condition. Garden peas that have white flowers are homozygous recessive for the white allele of the flower color gene. Homozygous recessive describes the organism’s genotype, or its genetic makeup. It has two copies of the recessive allele of the gene. The observable characteristic of the organism, having white flowers, is called its phenotype. Mendel also demonstrated that the segregation of alleles of any one gene is not dependent on the segregation of alleles of any other gene. For instance, a gamete could receive a dominant allele for an eye color gene and a recessive allele for height, or that gamete could receive the recessive alleles for both genes or the dominant alleles of both genes. This is Mendel’s second law, the law of independent assortment, and it applies to any genes that are located on separate chromosomes. Mendel’s work was far ahead of its time. Although Mendel published his research in the 1800’s, it was not until after his death that his work gained recognition in the scientific community. In 1900, three other scientists, each working separately on inheritance, came across Mendel’s work in the course of their research. They gave him credit for his insights, and Mendel’s research provided the foundation for the new discipline of genetics. Genes and Chromosomes Although Mendel described the gene as the factor that was responsible for a particular trait, nothing was known about the physical makeup of a gene. One of the first questions scientists needed to answer was where genes are found in cells. Early studies in frogs and sea urchins indicated that the nucleus of the sperm and the nucleus of the egg combined with each other during fertilization. This observation suggested that the genetic material that determined how the fertilized egg would develop might reside in the nucleus. As microscopes improved, scientists were able to distinguish structures within the nuclei of cells. These long, threadlike structures stained blue and were called chromosomes (Greek chroma, “color”). Several scientists observed that when animal and plant cells divided, the chromosomes duplicated, then separated, and each daughter cell inherited a complete set of chromosomes. The one exception to this was the cell division that produced the gametes (eggs and sperm). When an egg or a sperm cell was produced, it only contained half the number of chromosomes as the cell that produced it. If genetic information was carried on chromosomes, scientists reasoned that a sperm and an egg could each contribute half of the genetic information to the new organism at fertilization. Some of the first evidence that chromosomes were linked to observable traits came from the studies of American graduate student Walter S. Sutton. 1 Sutton studied grasshoppers, and his observations indicated that male grasshoppers 1 always had an X and a Y chromosome, whereas female grasshoppers contained two X chromosomes. Several other scientists observed similar things in other organisms, such as fruit flies, 1 and concluded that the physical characteristic of sex was determined by the kind of chromosomes an organism possessed. Since chromosomes determined the trait of sex, it was possible that chromosomes contained the genes that Mendel had shown to determine physical characteristics. The first scientist to demonstrate that genes were located on chromosomes was Thomas Hunt Morgan, 1 who showed that an eye-color gene in the fruit fly, Drosophila melanogaster, 1 was located on the X chromosome. Next, scientists wanted to know what kind of chemical molecule actually carried the genetic information. Chromosomes contain two kinds of molecules, protein and a weak acid called deoxyribonucleic acid (DNA). Experiments in the early 1930’s first demonstrated that DNA is the genetic material. Oswald Avery, 2 Colin MacLeod, 2 and Maclyn McCarty 2 showed that adding DNA to these bacterial cells could change their physical traits. In their experiments, they mixed a harmless strain of bacteria with DNA from bacteria that caused disease in mice. When they did this, the previously harmless bacteria changed (or transformed) into disease-causing bacteria. Two other scientists, Alfred Hershey 2 and Martha Chase, 2 later obtained similar results by studying a virus that infects E. coli. Molecular Genetics By the 1940’s, scientists knew that genetic information was carried by genes made of DNA molecules inside cell nuclei. However, scientists did not know how the genetic information was copied accurately from one generation to the next—from one cell division to the next. Nor did scientists know how the DNA could account for the appearance of inherited changes or mutations. In order to answer these questions, scientists needed to know the precise chemical structure of DNA. Many scientists contributed to the understanding of the structure of DNA. Erwin Chargaff 2 obtained data that indicated that specific molecular components of the DNA molecule were always present in equal parts. These components were nitrogen-containing molecules (or nitrogenous bases). Chargaff determined that the nitrogen-containing bases adenosine and thymine were always present in a one to one ratio, and the bases guanine and cytosine were always present in a one to one ratio, no matter what species’ DNA was analyzed. Simultaneously, two scientists at Kings College in London, Rosalind Franklin 2 and Maurice Wilkins, 2 were attempting to make X-ray pictures of DNA molecules. Rosalind Franklin obtained an X-ray film that indicated that DNA was a helical molecule. Just previous to Franklin’s work, an American chemist, Linus Pauling, 2 had made a breakthrough in solving the structure of the protein alpha helix using a model-building approach. Two scientists working at Cambridge University in England, James Watson 2 and Francis Crick, 3 decided to use Pauling’s method of model building to attempt to solve the structure of the DNA molecule. Combining the data from a variety of sources including the data of Chargaff, Wilkins, and the crucial X-ray crystallography data of Rosalind Frankin, Watson and Crick solved the structure of the DNA molecule. Watson and Crick created a model of DNA: a double helix, 3 like a twisted ladder. The DNA molecule was a long polymer of repeating nucleotides. Each nucleotide contained three chemical parts: a sugar, a phosphate group, and a nitrogen-containing base. The sides of the double helix ladder were formed by alternating sugars and phosphates, and the rungs were formed on the inside of the helix by specific pairings of the nitrogen-containing bases. Adenine paired with thymine to form one kind of rung. Guanine paired with cytosine to form a second kind of rung. The order of the bases provided the information within DNA. Certain combinations of bases could form “words” that stood for parts of proteins or other molecules encoded by the DNA. The double helix could unzip like a zipper, each strand serving as a template to guide the construction of a new strand. This provided an accurate means for copying the DNA molecules from a parent cell to a daughter cell.
The details of how DNA is passed from one generation to the next, of how mutations arise, and of how the information of DNA is actually translated into the activities of cells forms the basis of genetic research at the beginning of the twenty-first century. One of the most important scientific discoveries that led to modern genetic technology was the discovery of a particular kind of protein, a restriction enzyme, from bacteria that cuts DNA molecules at specific sequences of bases. These restriction enzymes 4 4 gave scientists the tool they needed to break DNA down into smaller pieces, eventually allowing the isolation of individual genes from the huge amount of DNA inside the nucleus of the cell. Herbert Boyer 4 and Stanley Cohen 4 combined their knowledge of restriction enzymes and bacterial transformation (getting bacteria to take up DNA from the environment) to clone genes. Gene cloning 4 involves isolating a gene of interest by using a restriction enzyme to cut it away from other DNA, and placing it in a piece of DNA called a vector that can be taken up by bacterial cells. One of the first applications of this technology was the production of human insulin. 4 Scientists isolated the gene that encodes the information for making insulin from human DNA, cloned it into a bacterial vector, and placed the vector with the insulin gene in E. coli. The E. coli cells were able to produce large quantities of insulin. This new insulin was considerably cheaper and safer than insulin purified from human tissue. Variations on this technique of taking a piece of DNA from one species and inserting it into the cells of another species are involved in genetic engineering of multicellular organisms. In multicellular organisms such as plants or monkeys, the DNA vector is usually a modified virus. These techniques are the basis of human gene therapy. 4 In the last decade of the twentieth century, entire organisms have been cloned. In Scotland, Ian Wilmut 4 and colleagues reported the first mammalian cloning of a sheep named Dolly. 4 In Wisconsin and Japan, scientists have cloned cattle. When an organism is cloned, all of its DNA, usually contained within an intact nucleus from a cell of the adult animal, is transferred to an egg cell from which all the genetic information has been removed. The egg is then allowed to develop into a new organism. Although the new organism is young, it has the same DNA as the parent from which the nucleus was obtained. Scientists have also developed techniques for sequencing DNA, determining the exact order and number of nitrogenous bases within the DNA of an organism’s genome. In 2000, the Human Genome Project 4 announced that the entire genome of the human had been sequenced. Many other genomes have been sequenced, including the roundworm, C. elegans, 5 several plants, and even baker’s yeast. The sequence of an organism gives scientists another tool in answering questions about how DNA regulates and determines the activities of cells. The ethical consequences of genetic engineering are not clear. DNA forensic evidence is now used to convict or exonerate criminal suspects on a routine basis. The genetic engineering of food crops that are pest resistant or contain additional nutrients is fairly routine. With the cloning of entire organisms now possible, the cloning of a human is not science fiction. Parents can have an embryo tested for devastating genetic diseases before it is born. While many of these advances are clearly positive, many of them are double-edged swords, begging for informed public debate. Michele Arduengo See AlsoAsexual reproduction; Breeding programs; Cleavage, gastrulation, and neurulation; Cloning of extinct or endangered species; Copulation; Courtship; Determination and differentiation; Development: Evolutionary perspective; Estrus; Fertilization; Gametogenesis; Hermaphrodites; Hydrostatic skeletons; Mating; Parthenogenesis; Pregnancy and prenatal development; Reproduction; Reproductive strategies; Reproductive system of female mammals; Reproductive system of male mammals; Sexual development. Bibliography Hartwell, L., L. Hood, M. Goldberg, A. Reynolds, L. Silver, and R. Veres. Genetics: From Genes to Genomes. Boston: McGraw-Hill, 2000. Up-to-date, definitive genetic textbook covering all major fields of investigation in genetics: Mendelian, bacterial and viral, human, population, and genetic engineering. Marshall, Elizabeth L. The Human Genome Project: Cracking the Code Within Us. New York: Franklin Watts. 1996. Discusses the goals and structure of the Human Genome Project and its implications. Contains a chapter on the contributions of animal model systems to this project. Mousseau, T. A., B. Sinervo, J. A. Endler, eds. Adaptive Genetic Variation in the Wild. New York: Oxford University Press, 1999. Provides excellent background and current information for readers interested in population genetics and evolution in natural systems. Sayre, Anne. Rosalind Franklin and DNA. New York: W. W. Norton, 1975. Details the invaluable contribution of Dr. Rosalind Franklin to the discovery of the structure of the DNA molecule. Sponenberg, D. P. Equine Color Genetics. Ames: Iowa State University Press, 1996. A fascinating, in-depth look at the genetic mechanisms that cover coat color in horses and donkeys. Watson, James D. The Double Helix: A Personal Account of the Discovery of the Structure of DNA. Edited by Gunther S. Stent. New York: W. W. Norton, 1980. Watson’s personal story of the characters and events of the race to solve the structure of the double helix. This edition provides viewpoints and commentary from other players in the story as well as reprints of the original research articles. |
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