An age-contingent reduction in physiological function and activity, with a concomitant increase in mortality rate and a diminution in reproductive rate. Aging is the process of becoming older, which is influenced by genetic and environmental factors. However, definitions of aging differ between biologists and behavioral scientists. Biologists regard aging as reflecting the sum of multiple and typical biological decrements occurring after sexual maturation; in contrast, behavioral scientists view it as reflecting regular and expected changes occurring in genetically representative organisms advancing through the life cycle under normal environmental conditions. It is difficult to define normal aging because many changes observed in older adults and perceived as concomitants of normal aging are effects of disease in later life. The behavioral science view allows for incremental as well as decremental changes with aging. Senescence is not always equated with aging; it is viewed as the increasing vulnerability or decreasing capacity of an organism to maintain homeostasis as it progresses through its life span, leading to death (Fig. 1). Gerontology refers to the study of aging. Geriatrics refers to the clinical science that is concerned with health and illness in the elderly. See also: Cell (biology); Cell senescence; Death; Genetic influences on aging; Genetics; Human genetics

Any of a number of alternative forms of a gene. An allele is a variant form of a gene (see illustration). Allele is a contraction of allelomorph, which is a term that William Bateson used to designate one of the alternative forms of a unit showing mendelian segregation. New alleles arise from existing ones by mutation. The diversity of alleles produced in this way is the basis for hereditary variation and evolution. See also: Gene; Genetics; Mendelism; Mutation

The application of genetic principles for improving heredity for economically important traits in domestic animals. The science of animal breeding is based on fundamental genetic concepts. Through controlled mating and selection, which can involve genetic engineering methods, animal breeders attempt to modify animal species with respect to one or more phenotypic traits (meaning traits relatable to an observable characteristic, Fig. 1). Examples of trait modification include improvement of milk production in dairy cattle, meatiness in pigs, growth rate in beef cattle, fleece weight in sheep, and egg production in chickens. Even after thousands of years of domestication, domestic animals respond readily to genetic selection. Selection permits the best parents to leave more offspring in the next generation than do genetically inferior parents. Many specialized breeds and strains have been developed to produce meat, fiber, and milk in different environmental and economic conditions. See also: Agricultural science (animal); Beef cattle production; Dairy cattle production; Domestication (anthropology); Genetic engineering; Genetics; Poultry production; Sheep; Swine production

Any treatment or procedure that includes the in-vitro handling of oocytes (immature ova or egg cells) and sperm or embryos for the purpose of establishing a pregnancy. Assisted reproductive technology (ART) is undertaken as a treatment of infertility, with a goal of achieving human reproduction. ART procedures include (but are not limited to) in-vitro fertilization (IVF) and embryo transfer (ET) [Fig. 1], gamete intrafallopian transfer, gamete and embryo cryopreservation (freezing, or vitrification, and storage of cells or tissues), oocyte and embryo donation, and surrogacy. Note that intracervical or intrauterine insemination with either partner or donor spermatozoa is not included under the umbrella of ART. See also: Animal reproduction; Biotechnology; Fertilization (animal); Infertility; Ovum; Pregnancy; Sperm cell

The study of gene structure and function in bacteria. Bacterial genetics is a scientific field concerned with the mechanisms of heredity in bacteria (Fig. 1). In general, genetics itself is concerned with determining the number, location, and character of the genes of an organism. The classical way to investigate genes is to mate two organisms with different genotypes and compare the observable properties (phenotypes) of the parents with those of the progeny. Bacteria, though, do not mate (in the usual way), so there is no way of getting all the chromosomes of two different bacteria into the same cell. However, there are a number of ways in which a part of the chromosome or genome from one bacterium can be inserted into another bacterium so that the outcome can be studied. See also: Bacteria; Bacterial physiology and metabolism; Bacteriology; Genetics

The study of the hereditary factors that shape behavior. Behavior genetics, that is, the scientific investigation of the hereditary factors that influence behavior, may be studied in animals and humans. Charles Darwin, who originated the theory that natural selection is the basis of biological evolution, was persuaded by Francis Galton that the principles of natural selection applied to behavior as well as to physical characteristics. Specifically, members of a species vary in the expression of certain behaviors because of variations in their genes (Fig. 1), and these behaviors have survival value in some environments. One example of such a behavior is curiosity: some organisms are more curious than others, and curiosity is advantageous for survival in some settings. Therefore, the science of behavior genetics is an extension of the aforementioned ideas and seeks (1) to determine to what extent the variation of a trait in a population (the extent of individual differences) is due to genetic processes, to what extent it is due to environmental variation, and to what extent it is due to joint functions of these factors (heredity–environment interactions and correlations); and (2) to identify the genetic architecture (genotypes) that underlies behavior. See also: Behavioral ecology; Genetics; Human genetics; Population genetics; Psychology

Generally, any technique that is used to make or modify the products of living organisms in order to improve plants or animals or to develop useful microorganisms. According to the general definition, biotechnology has actually been practiced for centuries, as exemplified by the use of yeast and bacteria in the production of various foods, including wine, bread, and cheese. However, in modern terms, biotechnology has come to mean the use of cell and tissue culture, cell fusion, molecular biology, and, in particular, recombinant deoxyribonucleic acid (DNA) technology (Fig. 1) to generate unique organisms with new traits or organisms that have the potential to produce specific products. The advances and products in the biotechnology arena have been developing at a rapid pace. Still, the manipulation of biological processes for commercial exploitation and benefit has led to controversy. Some examples of products in a number of important disciplines are described below. See also: Bioethics; Cell (biology); Deoxyribonucleic acid (DNA); Molecular biology; Recombination (genetics)

An experimental technique in which a normal gene is inserted into an organism to correct a genetic defect responsible for cancer development. Cancer is a serious medical condition that develops when the orderly relationship of cell division and cell differentiation becomes disordered, leading to the development of neoplasms (tumors) [Fig. 1]. Advances in knowledge of the molecular basis for cancer have led to the understanding that cancer is a multistep process involving a variety of gene alterations. Therefore, cancer gene therapy has been developed in order to introduce functional copies of a curative gene or genes into a cancer patient and thus achieve a therapeutic objective. In other words, cancer gene therapy involves the introduction of genes to either reverse or counteract the molecular defects or alterations associated with cancer cells. The advantage of cancer gene therapy is that it can be directed to tumor cells with greater specificity than conventional chemotherapy or radiation therapy. Hence, it is less likely to affect normal cells or to cause undesired systemic effects. See also: Cancer; Cancer cell metabolism; Cancer stem cells; Cell differentiation; Cell division; Chemotherapy and other antineoplastic drug treatments; Gene; Genetic engineering; Genetics; Oncology; Radiation therapy; Tumor

Any of the cells within a tumor that possess the capacity to self-renew and cause the heterogeneous lineages of cancerous cells that comprise the tumor. Cancer stem cells (CSCs) typically represent a small percentage of all cells in a tumor (rarely more than 1–3%). However, they are responsible for the regeneration of malignant cells, allowing the cancer to grow and, potentially, adapt. In general, the CSC concept postulates that malignant tumors, like many healthy tissues, can be hierarchically organized at the cellular level and that subpopulations of CSCs, representing the apex of such hierarchies, are exclusively responsible for tumor initiation and propagation. Hierarchical tumor organization denotes the concept that only tumor-initiating CSCs within cancers possess the capacity for generating further tumor-propagating CSC progeny, whereas more differentiated bulk populations of tumor cells with limited replicative potential, also derived from CSCs through asymmetric cell division, do not give rise to CSCs and are not capable of indefinite tumor propagation. According to the CSC model of tumor growth and progression, a CSC is thus defined as a cancer cell that possesses (1) a capacity for prolonged self-renewal that inexorably drives tumor growth and (2) a capacity for differentiation, that is, the production of heterogeneous lineages of daughter cells that represent the bulk of the tumor mass, but are dispensable for tumor propagation (Fig. 1). Consequently, CSCs are thought by proponents of the CSC concept to lie at the root of primary tumor formation and growth and to be also responsible for tumor dissemination, metastasis formation, therapeutic resistance, and posttreatment recurrence. The rapidly expanding interest in CSCs in the field of oncology is based on the exceptional promise of CSCs as paradigm-shifting novel targets for cancer therapy. See also: Cancer; Cancer cell metabolism; Cell (biology); Cell differentiation; Cell lineage; Oncology; Stem cells; Tumor

The succession of events that culminates in the asexual reproduction of a eukaryotic cell; also known as the cell division cycle. In a typical cell cycle (Fig. 1), the eukaryotic parent cell doubles its volume, mass, and complement of chromosomes, then sorts its doubled contents to opposite sides of the cell, and finally divides in half to yield two genetically identical offspring. Implicit in the term cycle is the idea that division brings the double-sized parent cell back to its original size and chromosome number, and ready to begin another cell cycle. This idea fits well with the behavior of many unicellular organisms; however, for multicellular organisms, the daughter cells may differ from their parent cell and from each other in terms of size, shape, and differentiation state. See also: Cell (biology); Cell biology; Cell differentiation; Cell division; Chromosome; Eukaryota; Genetics