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the ABI Model 3700 Automated Sequencer, developed by ABI, addressed these limitations by using multiple capillaries, thin, hollow glass tubes filled with a gel polymer.

The ABI PRISM Model 3700 Automated Sequencer, developed with the Hitachi Corporation and having a price tag of $300,000, uses ninety-six capillaries, each not much wider than a strand of human hair. The capillaries are automatically cleaned and filled with fresh gel polymer between each electrophoresis run. The instrument is also equipped with a robot arm that automatically loads the sequencing reactions into the capillaries, greatly decreasing the amount of human labor required for its operation. The Model 3700 Automated Sequencer can read over 400,000 bases in a day, a greater than twenty-fold increase over the maximum capacity of the Model 377. Beginning in September 1999 and using 300 of these instruments, the Celera Corporation had sequenced the entire human genome five times over within four months. see also Cycle Sequencing; Gel Electrophoresis; Nucleotide; Sanger, Fred; Sequencing DNA.

Frank H. Stephenson and Maria Cristina Abilock

Bibliography

Smith, Lloyd M., et al. "Fluorescence Detection in Automated DNA Sequence Analysis." Nature 321 (1986): 674-679.

Bacteria See Eubacteria, Archaea

Balanced Polymorphism

Balanced polymorphism is a situation in which two different versions of a gene are maintained in a population of organisms because individuals carrying both versions are better able to survive than those who have two copies of either version alone. The evolutionary process that maintains the two versions over time is called balancing selection.

Genes are carried on chromosomes. Different versions of a gene are called alleles. The standard allele found in a population is referred to as the wild-type allele. Most plants and animals have at least two copies of each chromosome, one inherited from each parent. The copies of the genes found on these homologous chromosomes may be identical or different; that is, the organism may carry two copies of one allele, or one each of two different alleles. In the first case, the organism is called homozygous for that gene, and, in the second, it is called heterozygous.

Alleles differ from each other in their sequence of nucleotides, which may change the structure and function of the protein the gene codes for. Because of this, different alleles may have different effects on an organism's appearance or ability to survive. These effects can be helpful, harmful, or neutral.

An example of balanced polymorphism can be illustrated with the set of enzymes in the liver that act like an assembly line (or, more accurately, a disassembly line) to detoxify poisons and other chemicals. Different alleles for these enzymes can affect how well an organism can protect itself from a, a homologous similar in structure nucleotides building blocks of DNA or RNA, composed of sugars, phosphates, and bases enzymes proteins that control a reaction in a cell

OOOOCOO, exposure to harmful chemicals. An especially active form of a detoxifying enzyme, which is encoded by a specific allele, can cause accumulation of potentially harmful intermediates. If the other allele encodes an enzyme with low activity, the potential for this enzyme to cause harm is lessened, and the benefits of its activity will be felt by the organism. If an individual has two copies of the very active allele or two copies of the low-activity allele, it may not survive well. In the first case, too much enzyme activity will result in high levels of the harmful intermediate, and in the second case, too little enzyme activity will be present for detoxification. Therefore, the best situation for the organism is to have one copy of each allele. Because of this, both copies are maintained in the population.

The effects of alleles and whether they are maintained in a population can be influenced by the environment. A classic case of balanced polymorphism in humans that is influenced by the environment is the sickle-cell p the Greek letter beta allele of the p-globin gene. This gene forms part of hemoglobin, which carries oxygen in red blood cells.

Individuals who have two copies of the /8-globin sickle-cell allele develop sickle-cell disease and generally do not survive into adulthood without intensive medical care. Individuals with one copy of the ^-globin sickle-cell allele and one ^-globin wild-type allele have red blood cells that are functional and resistant to the organism that causes malaria. Because individuals with

?this combination of alleles tend to survive malaria better than those who carry only the wild-type allele, the combination is advantageous to those who live in areas where malaria is present. This is called "heterozygote advantage." As a result, the beta-globin sickle-cell allele will be maintained along with the wild-type allele in populations exposed to malaria—an example of balancing selection. see also Alzheimer's Disease; Hemoglobinopathies; Heterozygote Advantage; Population Genetics.

R. John Nelson

Bibliography

Weaver, Robert F., and Philip W. Hedrick. Genetics, 2nd ed. Dubuque, IA: William C. Brown, 1992.

Barr Body See Mosaicism

Behavior

What is behavior? A dictionary definition reveals that behavior consists of our activities and actions, especially actions toward one another. As such definitions suggest, many behavioral terms have meaning only in social comparisons: We identify others as contentious, courteous, or conscientious only by their actions in social contexts. A long-standing question in science and in everyday affairs inquires about the causes of individual differences in behavior: Why are some people gregarious extroverts and others timid, shy introverts?

hybrid combination of Behavior genetics is a hybrid area of science, at the intersection of two different types human genetics and psychology. Its focus is on how genes and environments contribute to differences in behavior. It is a young discipline. A book that ooc

Identical twins share all their genes, and, if raised together, much of their environment. Together, genes and environment determine many aspects of behavior.

gave the field its name was published in 1960, and a decade later the Behavior Genetics Association was founded. For a time, most behavior genetics research was an effort to show that the term was not itself an oxymoron— that variations in genes do contribute to individual differences in behavior. Now, as a result of that research, the relevance and importance of genetic variation to individual differences in behavior are widely accepted, and the challenging task is to identify specific gene-behavior pathways. In this entry, we will review the methods used to identify such pathways and then focus on one set of behaviors, use and abuse of alcohol, as a model for the study of genetic and environmental influences.

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