An educator is a person who systematically works to improve another's understanding of a topic. The role of educator encompasses both those who teach in classrooms and the more informal educators who, for example, work in zoos, museums, and recreational areas. The work of educators varies depending on the institution that employs them and the age or grade level of the people he or she teach…
Mammalian embryonic stem (ES) cells have the special property of being able to differentiate into virtually every cell type. Because ES cells can be genetically manipulated in vitro and can be transplanted into embryos and adults, they are a powerful tool in biological experiments and hold promise for future medical therapies. The ability to differentiate into all cell types, a property known as p…
Epidemiologists are scientists that study the factors influencing the health status of populations. These populations may be defined by geography (such as the residents of a particular city), occupation (such as members of the armed forces), or any other common trait (such as age, race, or sex). Epidemiologists look for trends in measures of the health of the population, such as the average life s…
Epistasis, first defined by the English geneticist William Bateson in 1907, is the masking of the expression of a gene at one position in a chromosome, or locus, at one or more genes at other positions. Epistasis should not be confused with dominance, which refers to the interaction of genes at the same locus. The human genome contains from 30,000 to 70,000 gene loci. Some of them are involved in …
Escherichia coli Escherichia coli (E. coli) is a very common bacterium that normally inhabits the digestive tract of animals, including humans. It is widespread in the natural world and can also be found in soil and water. It is a member of the bacterial family Enterobacteriaciae, which also includes the bacteria Shigella, Salmonella, and Yersinia, among others. Some of these organisms, in…
The Eubacteria, also called just "bacteria," are one of the three main domains of life, along with the Archaea and the Eukarya. Eubacteria are prokaryotic, meaning their cells do not have defined, membrane-limited nuclei. As a group they display an impressive range of biochemical diversity, and their numerous members are found in every habitat on Earth. Eubacteria are responsible for…
While the idea of improving humans through selective breeding is at least as old as the ancient Greeks, it gained widespread prominence after 1869. In 1883, Sir Francis Galton coined the word "eugenics," from the Greek word eugenes, meaning "well-born" or "hereditarily endowed with noble qualities," to describe this new science of directed human evolution.…
All life on Earth is cellular and uses DNA to store genetic information. However, evidence suggests that, on ancient Earth, much complex chemical activity preceded cellular development, and it was probably not DNA-based at the start. What was the nature of this activity, and how did it lead to life? "Molecular evolution" is a term used to describe the stages that preceded the origin …
Individual genes and whole genomes change over time. Indeed, evolution of genes ultimately accounts for the evolution of organisms that is seen in the fossil record: Humans evolved from earlier apes, and those creatures from their ancestors, by gene changes in the earlier creatures. Just as the fossil record can be examined to understand the patterns of organismic evolution, so too can genes be co…
When someone asks, "What color are your eyes?" he should really be asking "What color are your irises?" because it is the iris that contains the pigment that determines the color of your eyes. Despite the fascination eye color holds for us, the genes responsible for it in humans are not well-known and are more complex than most people think. …
Fertilization is the fusion of a female's egg cell (oocyte) and a male's sperm cell (spermatozoa) to form the first cell of a new and unique being. While on the surface this sounds like a simple process, there are many factors that make this possible. …
The term "founder effect" refers to the observation that when a small group of individuals breaks off from a larger population and establishes a new population, chance plays a large role in determining which alleles are represented in the new population. The particular alleles may not be representative of the larger population. As the new population grows, the allele frequencies will…
Fragile X syndrome is one of the most common causes of inherited mental retardation. Individuals with fragile X syndrome can exhibit moderate to severe mental retardation. Additional characteristics may include autistic-like behavior, hyperactivity, mitral valve prolapse (a heart valve defect), a large head circumference, a long face with a prominent forehead and jaw, protruding ears, flat feet, h…
Drosophila melanogaster, a common fruit fly, was one of the first model organisms used in genetic research, and continues to be one of the most important. Thomas Hunt Morgan (1866-1945) developed Drosophila as a model system in 1909. Morgan, along with his students, Calvin Bridges, Alfred Sturtevant, and Hermann Muller, made some of the most important discoveries in genetics through their work wi…
Gel electrophoresis is a widely used technique for separating electrically charged molecules. It is a central technique in molecular biology and genetics laboratories, because it lets researchers separate and purify the nucleic acids DNA and RNA and proteins, so they can be studied individually. Gel electrophoresis is often followed by staining or blotting procedures used to identify the separated…
Genes are functional units of DNA that contain the instructions for making proteins or RNA. Genes also act as units of heredity, transferring the same instructions from parent to offspring. The nature, structure, and regulation of genes has been a central topic of scientific research for more than 100 years. …
Questions of "nature versus nurture" have been asked of most human traits: Is it our genes, inherited from our parents, that make us the way we are, or is it the environment in which we live? A phenotype is a trait that can be observed and described in a population. Although some phenotypes may be totally controlled by genetic or environmental factors, most are influenced by a compl…
Gene discovery is the process of identifying genes that contribute to the development of a trait or phenotype. Researchers often try to discover the genes that are involved in specific diseases. They also try to find the genes that contribute to many other traits. Polymorphic markers along the chromosomes (here shown as different colored bars) are examined to determine which is coinherited wit…
The chromosomes of an organism contain genes that encode all of the RNA and protein molecules required to construct that organism. Gene expression is the process through which information in a gene is used to produce the final gene product: an RNA molecule or a protein. Each cell in a multicellular organism such as a human contains the same genes as every other cell. Nonetheless, there are hundred…
Gene families are groups of DNA segments that have evolved by common descent through duplication and divergence. They are multiple DNA segments that have evolved from one common ancestral DNA segment that has been copied and changed over millions of years. The members of a gene family may include expressed genes as well as nonexpressed sequences. Such nonexpressed sequences include promoters, oper…
Gene flow is the transfer of genetic material between separate populations. Many organisms are divided into separate populations that have restricted contact with each other, possibly leading to reproductive isolation. Many things can fragment a species into a collection of isolated populations. For example, a treacherous mountain pass may cut off one herd of mountain goats from another. In human …
Gene targeting is a method for modifying the structure of a specific gene without removing it from its natural environment in the chromosome in a living cell. This process involves the construction of a piece of DNA, known as a gene targeting vector, which is then introduced into the cell where it replaces or modifies the normal chromosomal gene through the process of homologous recombination. …
Gene therapy is a new and largely experimental branch of medicine that uses genetic material (DNA) to treat patients. Researchers hope one day to use this therapy to treat several different kinds of diseases. While rapid progress has been made in this field in recent years, very few patients have been successfully treated by gene therapy, and a great deal of additional research remains to be done …
Gene therapy introduces or alters genetic material to compensate for a genetic mistake that causes disease. It is hoped that gene therapy can treat or cure diseases for which no other effective treatments are available. However, many unique technical and ethical considerations have been raised by this new form of treatment, and several levels of regulatory committees have been established to revie…
The sequence of nucleotides in DNA determines the sequence of amino acids found in all proteins. Since there are only four nucleotide "letters" in the DNA alphabet (A, C, G, T, which stand for adenine, cytosine, guanine, and thymine), but there are 20 different amino acids in the protein alphabet, it is clear that more than one nucleotide must be used to specify an amino acid. Even t…
Over the last half-century, our understanding of genetic disorders has increased spectacularly. When facts about inherited disorders first came to light, health professionals began to inform families about probable inheritance patterns and recurrence risks (the likelihood that offspring or other relatives might also inherit the disease). A couple receives genetic counseling during their sixth …
Genetic counselors are health professionals trained in genetics, genetic disorders, genetic testing, molecular biology, psychology and psychosocial issues, and the ethical and legal issues of genetic medicine. Most genetic counselors have a master's degree from a genetic counseling training program. The very first class of genetic counselors was graduated from Sarah Lawrence College in 1971…
The potentially stigmatizing nature of genetic information and its history of abuse necessitate special provisions for its protection. Although the U.S. Supreme Court has established a basic right to privacy (in the case of Roe v. Wade), the nonspecific nature of this privacy protection fails to adequately guard against the unauthorized disclosure of personal genetic information. There have been r…
Genetic drift is the random change in the genetic composition of a population due to chance events causing unequal participation of individuals in producing succeeding generations. Along with natural selection, genetic drift is a principal force in evolution. …
Genetic testing involves examining a person's DNA in order to find changes or mutations that might put an individual, or that individual's children, at risk for a genetic disorder. These changes might be at the chromosomal level, involving extra, missing, or rearranged chromosome material. Or the changes might be extremely small, affecting just one or more of the chemical bases that …
Genetic testing is the name given to a variety of laboratory techniques, all of which ultimately provide information about a person's underlying genetic makeup, also called their genotype. Genetic information is of interest because it can help in the diagnosis of a current health condition or provide insight into future health. …
A genetically modified (GM) food is a plant that has a genetic change in each of its cells that a researcher has introduced. The modification may add a gene from a different species and thereby create a transgenic plant, or it may overexpress or silence a preexisting plant gene. Overexpression is accomplished by altering the promoter region of a gene, which controls how rapidly and in which cells …
Genetics intersects almost every other field of biology. For this reason, professionals with a genetics education have a broad range of career opportunities. The recent success of the Human Genome Project has created a great demand for genetics professionals with a variety of expertise in all areas of genetics, not only those applicable to human disease. Geneticists are involved in identifying gen…
Genetics is the scientific study of the structure, function, and transmission of genes in living things. The field of genetics includes many disciplines and uses many different techniques. Historically, genetic scientists (geneticists) investigated patterns of inheritance in whole organisms by observing the distribution and segregation of physical characteristics across several generations of bree…
A genome is the complete collection of hereditary information for an individual organism. In cellular life forms, the hereditary information exists as DNA. There are two fundamentally distinct types of cells in the living world, prokaryotic and eukaryotic, and the organization of genomes differs in these two types of cells. Prokaryotes comprise the bacteria and archaea. The latter were originally …
A quote commonly heard these days is that in the history of medicine, the greatest advancements in the treatment of patients have occurred within the past 50 years. But what if a doctor could prevent a disease from occurring, treating the cause rather than the symptoms? We all agree this would be wonderful, but how could a doctor predict a patient's medical future? This dream is now within …
Genomics is a recent scientific discipline that strives to define and characterize the complete genetic makeup of an organism. Its primary approaches are to determine the entire sequence and structure of an organism's DNA (its genome) and then to determine how that DNA is arranged into genes. This second goal is accomplished by determining the structure and relative abundance of all messeng…
Fundamental to the myriad of genomic research efforts in operation around the world is the mapping and sequencing of whole genomes. The entire genomes of more than seventy organisms had been completed by early 2002, including the working drafts of the human genome, first published in 2001. The successful completion of the sequencing of these genomes was made possible in part by companies developin…
An individual's genotype is the composition, in the individual's genome, of a specific region of DNA that varies within a population. (The genome of the individual is the total collection of the DNA in a cell's chromosomes. It includes all of the individual's genes, as well as the DNA sequences that lie between them.) Alleles differ in their DNA sequence, and may le…
Growth, which usually refers to skeletal growth since it determines final adult height, is an extremely complex process. As such, it is susceptible to a wide range of genetic and physiologic disturbances. Indeed, growth is adversely affected by many if not most chronic diseases of childhood, through many different mechanisms. Skeletal growth depends on hormonal signals for regulation. It also requ…
The Hardy-Weinberg equilibrium is the statement that allele frequencies in a population remain constant over time, in the absence of forces to change them. Its name derives from Godfrey Hardy, an English mathematician, and Wilhelm Weinberg, a German physician, who independently formulated it in the early twentieth century. The statement and the set of assumptions and mathematical tools that accomp…
Hemoglobinopathies are diseases caused by the production of abnormal hemoglobin or by a deficiency of hemoglobin synthesis. Hemoglobin is the protein in red blood cells (erythrocytes) that binds to oxygen, to distribute it throughout the body. The major hemoglobinopathies are sickle cell disease and several forms of thalassemia. …
Hemophilia A and hemophilia B are genetic disorders in the blood-clotting system, characterized by bleeding into joints and soft tissues, and by excessive bleeding into any site experiencing trauma or undergoing surgery. Hemophilia A and B are clinically indistinguishable. Both have the same type of bleeding manifestations, and both affect males almost exclusively. The coagulation cascade invo…
Heterozygote advantage is the superior fitness often seen in hybrids, the cross between two dissimilar parents. A heterozygote is an organism with two different alleles, one donated from each parent. Fitness means the ability to survive and have offspring. Heterozygote advantage also refers more narrowly to superior fitness of an organism that is heterozygous for a particular gene, usually one gov…
High-throughput screening (HTS) is an automated method for rapidly analyzing the activity of thousands of chemical compounds. It has become a key tool in modern drug discovery. Paired with combinatorial chemistry and bioinformatics, HTS allows potential drugs to be quickly and efficiently screened to find candidates that should be explored in more detail. …
HIV, the human immunodeficiency virus, is the virus that causes AIDS, a debilitating and deadly disease of the human immune system. HIV is one of the world's most serious health problems: at the end of 2001, more than 40 million people worldwide were infected with HIV and living with the virus or AIDS. The World Health Organization estimates that about 20 million people have died from AIDS …
Homology is used to describe two things that share a common evolutionary origin. In genetics and molecular biology, homology means that the sequences of two different genes or two different proteins are so similar that they must have been derived from the same ancestral gene or protein. The word "homology" has several meanings in biology, each related to the word's origin, mea…
All types of cells are capable of receiving signals from their environment and mounting an appropriate response to the signal, such as chemotaxis toward a nutrient source or toward other cells emitting a pheromone. The A steroid hormone binds to a two-part receptor within the cell. This links with a coactivator, making a complex that binds to DNA. This triggers transcription of the target gene…
High-performance liquid chromatography (HPLC) is an advanced form of liquid chromatography used in separating the complex mixture of molecules encountered in chemical and biological systems, in order to understand better the role of individual molecules. In liquid chromatography, a mixture of molecules dissolved in a solution (mobile phase) is separated into its constituent parts by passing throug…
In order to help treat human diseases, it is important to understand what causes them to occur. Understanding what causes a disease is the first step in understanding the entire abnormal course of disease. Sometimes it is fairly easy to determine what causes a disease. For example, pneumonia is caused by the Pneumococcus bacterium. However, in other cases it is not nearly as easy to tell what is c…
The genome represents the entire complement of DNA in a cell. The Human Genome Project is the determination of the entire nucleotide sequence of all 3 billion + bases of DNA within the nucleus of a human cell. It is one of the greatest scientific undertakings in the history of mankind. The first draft of the human genome sequence was completed in the year 2001 and published simultaneously in the B…
The immune system is the set of cells and glands that protects the body from invasion and infection by viruses, bacteria, and other pathogens. The immune system must be able to recognize any foreign target, or antigen, of which there are potentially millions. Pathogenic organisms change over time, and new antigens evolve that must also be targeted. At the same time, the immune system must distingu…
Imprinting refers to the chemical modification of the DNA in some genes that affects how or whether those genes are expressed. One particular kind of DNA imprinting found in mammals is known as parental genomic imprinting, in which the sex of the parent from whom a gene is inherited determines how the gene is modified. While imprinting has been found in only about fifty human genes to date, some e…
In situ hybridization is a technique used to detect specific DNA and RNA sequences in a biological sample. Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are macromolecules made up of different sequences of four nucleotide bases (adenine, guanine, uracil, cytosine, and thymidine). In situ hybridization takes advantage of the fact that each nucleotide base binds with a complementary nucleot…
Inbreeding is defined as mating between related individuals. It is also called consanguinity, meaning "mixing of the blood." Although some plants successfully self-fertilize (the most extreme case of inbreeding), biological mechanisms are in place in many organisms, from fungi to humans, to encourage cross-fertilization. In human populations, customs and laws in many countries have b…
"Variety is the spice of life," or so the saying goes. In fact, it is probably more precise to say that variety is the key to life. It is genetic variation that contributes to the diversity in phenotype that provides for richness in human variation, and it is genetic variation that gives evolution the tool that it needs for selection and for trying out different combinations of allel…
An information systems manager (ISM) is a professional whose skills are needed to handle the large amounts of information generated by and analyzed in the modern genetics laboratory. A successful information systems manager needs to be experienced with the technical aspects of computer hardware and networking systems. The daily work may involve managing a team of information technology workers, so…
Less than a decade after the rediscovery of Mendel's laws describing the inheritance of genes in the nucleus, hereditary traits were discovered that obey a different set of laws. The genes involved in this non-Mendelian pattern of inheritance reside outside the nucleus, in the cytoplasm of the cell. Specifically, they were found to reside in mitochondria, chloroplasts, or intracellular symb…
Inheritance patterns are the predictable patterns seen in the transmission of genes from one generation to the next, and their expression in the organism that possesses them. (A gene is said to be expressed when it is read by cellular mechanisms that result in the production of a protein.) While people have long noted that offspring resemble parents, the formal description of inheritance patterns …
The roles of genes and environment in the determination of intelligence have been controversial for more than 100 years. Studies of the question have often been marred by untested assumptions, poor design, and even racism, faults that more modern studies have striven to avoid. Nonetheless, examining the biology of intelligence is an enterprise that continues to be fraught with difficulty, and ther…
Biologists often use two terms to describe alternative approaches for conducting experiments. "In vitro" (Latin for "in glass") refers to experiments typically carried out in test tubes with purified biochemicals. "In vivo" ("in life") experiments are performed directly on living organisms. In recent years, the indispensable use of computers…
