is now known to be caused by mutations in a gene at Xq21.3-q22. The gene is 36,740 bp long, and it encodes a protein containing 659 amino acids. The protein is a tyrosine kinase that has been named in Bruton’s honor, and the gene is now symbolized BTK, for its product, the Bruton tyrosine kinase. The enzyme is a key regulator in the development of B lymphocytes. Boys with XLA lack circulating B cells. The bone marrow contains pre-B cells, but they are unable to mature. See antibody.
agamogony the series of cellular or nuclear divi- sions that generates agamonts.
agamont the diploid adult form of a protoctist that also has a haploid adult phase in its life cycle. An agamont undergoes meiosis and produces aga- metes. See gamont.
agamospermy the formation of seeds without fer- tilization. The male gametes, if present, serve only
to stimulate division of the zygote. See apomixis.
agamous See floral identity mutations.
Agapornis a genus of small parrots. The nest building of various species and their hybrids has pro-vided information on the genetic control of behavior patterns.
agar a polysaccharide extract of certain seaweeds used as a solidifying agent in culture media.
agarose a linear polymer of alternating D-galactose and 3,6-anhydrogalactose molecules. The polymer, fractionated from agar, is often used in gel elec-trophoresis because few molecules bind to it, and therefore it does not interfere with electrophoretic movement of molecules through it.
agar plate count the number of bacterial colonies that develop on an agar-containing medium in a pe-tri dish seeded with a known amount of inoculum. From the count, the concentration of bacteria per unit volume of inoculum can be determined.
age-dependent selection selection in which the values for relative fitness of different genotypes vary with the age of the individual.
agglutination the clumping of viruses or cellular components in the presence of a specific immune serum.
agglutinin any antibody capable of causing clump-ing of erythrocytes, or more rarely other types of cells.
agglutinogen an antigen that stimulates the pro- duction of agglutinins.
aggregation chimera a mammalian chimera made through the mingling of cells of two embryos. The resulting composite embryo is then transferred into the uterus of a surrogate mother where it comes to term. See allophenic mice.
aging growing old, a process that has a genetic component. Hereditary diseases are known in hu- mans that cause premature aging, and mutations that speed up or delay aging have been isolated in Saccharomyces, Caenorhabditis, and Drosophila. See Appendix C, 1994, Orr and Sohal; 1995, Feng et al.; antioxidant enzymes, apoptosis, daf-2, free radical hy- pothesis of aging, Indy, methuselah, Podospora anse- rina, progeria, SGSI, telomerase, senescence, Werner syndrome.
agonistic behavior any social interaction between members of the same species that involves aggres- sion or threat and conciliation or retreat.
agouti the grizzled color of the fur of mammals resulting from alternating bands of yellow (phaeo-melanin) and black (eumelanin) pigments in individ-ual hairs. The name is also given to the genes that control the hair color patterns. In the mouse more than 20 alleles have been described at the agouti lo-cus on chromosome 2. The gene encodes a cysteine-rich, 131 amino acid protein that instructs the me-lanocytes in the hair follicle when to switch from making black to yellow pigment. The protein is translated by nearby follicle cells rather than in the melanocytes themselves. Therefore, the agouti pro-tein acts as a paracrine-signaling molecule. See Ap-pendix C, 1905, Cue´not; autocrine, MC1R gene, mel-anin.
agranular reticulum See smooth endoplasmic retic- ulum (SER). agranulocytes white blood cells whose cytoplasm contains few or no granules and that possess an un- lobed nucleus; mononuclear leucocytes including lymphocytes and monocytes.
agriculturally important species See Appendix B. Agrobacterium tumefaciens the bacterium re-sponsible for crown gall disease (q.v.) in a wide range of dicotyledonous plants. The bacterium en- ters only dead, broken plant cells and then may transmit a tumor-inducing plasmid into adjacent liv- ing plant cells. This infective process is a natural form of genetic engineering, since the bacterium transfers part of its DNA to the infected plant This