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Modification of Mendel's principles developed as knowledge of the chromosomes increased; many discoveries have helped to account for apparent deviations from Mendelian ratios. For example, Mendel's studies emphasized genes that behave independently from one another during transmission to offspring. But we now know that genes are transmitted as constituents of chromosomes, each of which carries many different genes, which sheds light on the tendency of certain characteristics to appear in combination with one another (linkage). It also has been found that some characteristics are sex-linked, i.e., are transmitted by genes carried by the sex chromosomes (see sex); and that a non-sex-linked gene inherited from the father may differ in its expression from the same gene inherited from the mother, a phenomenon called "imprinting." Other research has shown that there may be multiple alleles (more than two alternative genes) for a given characteristic: the human blood groups are determined by a combination of several possible alleles. It is apparent that homologous portions of paired chromosomes may be interchanged during meiosis (crossing over) and that the interaction of many genes is responsible for determining many of the traits of individuals. Since the discovery (1953) of the structure of DNA, work on nucleic acids has begun to explain how genes determine life processes by directing the synthesis of proteins. It has also explained mutations as alterations in gene or chromosome structure. It has been found, for example, that mutations in the form of repeated sequences of otherwise normal chemical bases, can grow in length with succeeding generations, in some cases causing diseases (e.g., myotonic muscular dystrophy) that increase in severity each time they are inherited.
Most of the knowledge of chromosome structure and the behavior of genes has come from studies of the vinegar, or fruit, fly (Drosophila melanogaster), which reproduces so rapidly that many generations can be studied over a short time. The work of T. H. Morgan and his associates on Drosophila was the basis of much of the early progress of genetics in the United States. Certain other small laboratory animals, plants, and microorganisms such as the E. coli bacteria are now used, also largely because of their ability to reproduce rapidly. For obvious reasons human beings are poor subjects for experimental genetic studies; however, much that aids understanding heredity in humans has been learned from the "lower" forms of life. Also, by tracing the appearance of certain abnormal characteristics (e.g., hemophilia, color blindness, and certain mental disorders and anatomical defects) and blood groups through a number of generations the hereditary pattern of these conditions has been established. The increasing ability of scientists to decode genetic information (see Human Genome Project) has led to a considerable expansion of knowledge about the nature and role of genes in humans and other organisms. Application of this knowledge has played an important role in the fields of gene therapy, genetic engineering, and evolutionary studies, and has resulted in a better understanding of the genetic components of disease, physical characteristics, mental illness, and even personality.
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