The ancestral (left) and melanic (right) form of the peppered moth. The backgrounds are non-polluted bark (upper drawing) and polluted bark (lower drawing).
bithorax a gene residing at 58.8 on the genetic map and within segment 89E of the salivary map of Drosophila melanogaster. The bx gene is one of a cluster of three genes that specify the type of differ- entiation that cells in the segments starting at the posterior portion of the second thoracic segment through the eighth abdominal segment will undergo. Illustration D on page 210 shows a bx mutant with four wings. Therefore the normal function of this gene is to specify that the hind pair of imaginal discs (q.v.) form halteres (q.v.) not wings during meta- morphosis.
The three genes of the bithorax complex encode DNA-binding proteins with homeodomains. See Appendix C, 1978, Lewis; 1983, Bender et al.; floral identity mutations, homeotic mutations, Hox genes, metamerism, Polycomb, proboscipedia, seg- ment identity genes, spineless-aristapedia. Bittner mouse milk virus See mammary tumor agent. bivalent a pair of homologous, synapsed chromo-somes. See meiosis. Bkm sequences a satellite DNA containing repeats of the tetranucleotide sequences GATA and GACA that was first isolated from the banded krait. In this and many other snakes, the sequences are concentrated in the W chromosome. Bkm sequences also occur in the W chromosomes of birds. See W, Z chromosomes. blackwater fever a name for malaria, with refer- ence to the urinary excretion of heme. The malaria parasite uses only the globin fraction of hemoglobin for its metabolism and discards the heme, which is excreted, darkening the urine.
BLAST Basic Local Alignment Search Tool. This algorithm is widely employed for determining simi- larity between nucleic acid or protein sequences, us- ing sequences present in data bases. blast cell transformation the differentiation, when antigenically stimulated, of a T lymphocyte to a larger, cytoplasm-rich lymphoblast. blastema a small protuberance composed of com- petent cells from which an animal organ or append- age begins its regeneration. blastocyst the mammalian embryo at the time of its implantation into the uterine wall. blastoderm the layer of cells in an insect embryo that completely surrounds an internal yolk mass.
The cellular blastoderm develops from a syncytial blastoderm by the partitioning of the cleavage nuclei with membranes derived from infoldings of the oolemma. blastodisc a disc-shaped superficial layer of cells formed by the cleavage of a large yolky egg such as that of a bird or reptile. Mitosis within the blastodisc produces the embryo. blastokinin See uteroglobin. blastomere one of the cells into which the egg di- vides during cleavage. When blastomeres differ in size, the terms macromere and micromere are often used. blastoporal lip the dorsal rim of the amphibian blastopore, which functions as the organizer induc- ing the formation of the neural tube. See chorda- mesoderm, Spemann-Mangold organizer. blastopore the single external opening of the primitive digestive tract in the gastrula of most ani- mals. The fate of the blastopore differs in those spe- cies belonging to the two subdivisions of the Bilat- eria. See Appendix A,
Deuterostomia, Protostomia. blastula an early embryonic stage in animals con- sisting of a hollow sphere of cells. Blatella germanica the German cockroach, a spe- cies found throughout the world in association with humans. It is the hemimetabolous insect for which the most genetic information is available. See Appen- dix A, Arthropoda, Insecta, Dictyoptera. blending inheritance 1. an obsolete theory of he- redity proposing that certain traits of an offspring are an average of those of its parents because of the blending of their fluidlike germinal influences; he- reditary characters transmitted in this way would not segregate in later generations. 2. a term incor- rectly applied to codominant traits, to genes lacking dominance, or to additive gene action. blepharoplast the basal granule of flagellates.
blocked reading frame See reading frame. blood clotting a cascade of enzymatic reactions in blood plasma that produces strands of fibrin to stop bleeding. Fibrinogen, a protein found in the blood plasma, is acted upon by the enzyme thrombin. As a result a negatively charged peptide is split off the fibrinogen molecule, leaving monomeric fibrin, which is capable of rapid polymerization to produce a clot. Active thrombin is formed from an inactive precursor prothrombin, also found in blood plasma. The conversion of prothrombin into thrombin is a very complex process that requires a number of fac- tors, including a lipoprotein factor liberated from rupturing blood platelets, plasma thromboplastin component, a complex of antihemophilic factor and von Willebrand factors, calcium, ions, and others.
See hemophilia, von Willebrand disease. blood coagulation blood clotting (q.v.). blood group a type in a system of classification of blood, based on the occurrence of agglutination of the red blood cells when bloods from incompatible groups are mixed. The classical human blood group- ings were A, B, AB, and O. However, a multitude of more recently identified groups exists. There are 33 blood group genes that have been localized to specific chromosomes. Seventeen of these reside on three chromosomes: the X has 5 genes, and auto- somes 1 and 19 have 6 genes each. Nine other auto- somes each have between 1 and 3 genes.
See Appen- dix C, 1900, Landsteiner; 1925, Bernstein; 1951, Stormont et al.; A, B antigens, Bombay blood group, Colton blood group, Duffy blood group gene, H sub- stance, Kell-Cellano antibodies, Kidd blood group, Lewis blood group, Lutheran blood group, MN blood group, P blood group, Rh factor, Secretor gene, XG. blood group chimerism the phenomenon in which dizygotic twins exchange hematopoietic stem cells while in utero and continue to form blood cells of both types after birth. See also radiation chimera. bloodline in domesticated animals, a line of direct ancestors.
blood plasma the straw-color fluid remaining when the suspended corpuscles have been removed from blood. See plasma lipoproteins, plasma throm- boplastin component, plasma transferrins, plasmin, serum. blood typing determination of antigens on red blood cells, usually for the purpose of matching do- nor and recipient for blood transfusion. Convention-
Bloom syndrome (BS)
ally, only antigens of the ABO and Rh systems are typed for this purpose. Bloom syndrome (BS) children suffering from this rare heriditary disease have short stature, sun-sensi- tive facial erythema, and a high rate of bacterial in- fections due to defects in their immune systems.
The condition was first described in 1954 by the Ameri- can dermatologist D. Bloom. Somatic cells from BS patients are hypermutable and show high frequen- cies of microscopically visible chromatid gaps, breaks, and rearrangements. BS is inherited as an au- tosomal recessive, and homozygotes are prone to de- velop a wide variety of cancers. The syndrome is caused by mutations in a gene on the long arm of chromosome 15 at band 26.1.
The gene product (BLM) is a helicase (q.v.). Men with BS produce no spermatozoa and are sterile. Antibodies against BLM have been shown to localize near synaptomemal complexes (q.v.) in mouse spermatocytes at the zy- gotene and early pachytene stages of meiosis (q.v.). blotting the general name given to methods by which electrophoretically or chromatographically resolved RNAs, DNAs, or proteins can be trans- ferred from the support medium (e.g., gels) to an immobilizing paper or membrane matrix. Blotting can be performed by two major methods: (1) capil- lary blotting involves transfer of molecules by capil- lary action (e.g., Southern blotting, northern blot- ting, both of which see), and (2) electroblotting, which involves transfer of molecules by electropho- resis.
blue-green algae, blue-green bacteria See Cya- nobacteria. blunt end ligation the use of a DNA ligase (q.v.) to join blunt-ended restriction fragments. Compare with cohesive end ligation. blunt ends See restriction endonuclease. B lymphocyte a cell belonging to the class of lym- phocytes that synthesize immunoglobulins. B lym- phocytes mature within a microenvironment of bone marrow (in mammals) or within the bursa of Fabricius (in birds). At this time, the immunoglobu- lins synthesized by B lymphocytes are transferred to the cell surface.
After the binding of an antigen mol- ecule to a B lymphocyte, it goes through a cycle of mitotic divisions during which the immunoglobulins disappear from the cell surface. The plasma cells that result synthesize immunoglobulins and secrete them into the blood. However, some B lymphocytes do not differentiate into plasma cells, but retain membrane-bound immunoglobulins. These “mem- ory” B lymphocytes function to respond to any sub- sequent encounter with the same antigen See lym- phocyte, V(D)J recombination. bobbed a gene (bb) in Drosophila melanogaster producing a small bristle phenotype.
The locus of bb is very near the centromere, and bb is the only gene known to have alleles on both the X and Y chromo- somes. The plus allele of bb is the nucleolus orga- nizer, and the various hypomorphic alleles may rep- resent partial deletions of ribosomal DNA. See Appendix C, 1966, Ritossa, Atwood, and Spiegel- man. Bolwig organs a pair of eyes first described in 1946 by N. Bolwig in larvae of Musca domestica. In Drosophila, Bolwig organs consist of two groups each containing 12 photoreceptors juxtaposed to the lar- val mouth hooks. See Dscam.
Bombay blood group a rare human variant of the ABO blood group system (first discovered in Bom- bay, India) that does not have A, B, or O antigens. Individuals homozygous for an autosomal recessive allele (h/h) cannot make the precursor H substance (q.v.) from which the A and B antigens are formed. This is a classical case of recessive epistasis in human genetics, because without the product of allele H, the products of the ABO locus cannot be formed. Bombay bloods appear to be group O when rou- tinely tested by antibodies against the A or B anti- gens, but an individual with the Bombay phenotype may be carrying unexpressed genes for the A and/or B antigens. However, they make anti-H that is not found in individuals of groups A, B, or O.
Therefore it is possible for a child of group A or B to be pro- duced from parents that appear to be group O, if one of them is a Bombay phenotype and carries the genes for antigens A or B or both. See A, B antigens. Bombyx mori the commercial silkmoth, which was domesticated in China from its wild progenitor B. mandarina about 5,000 years ago. Bombyx is esti- mated to contain about 18,510 genes in its genome of 429 mbp of DNA. There are about 400 visible phenotypes, and 200 of these have been assigned to linkage groups. The diploid chromosome number is 28, and the chromosomes are holocentric (q.v.).
The female is a female-heterogametic species (ZZ in male, ZW in female). Sex is determined by a domi- nant feminizing gene on the W chromosome. See Appendix A, Arthropoda, Insecta, Lepidoptera; Ap- pendix C, 1913, Tanaka; 1933, Hashimoto; Appen- dix F; endopolyploidy, silk. bond energy the energy required to break a given chemical bond. For example, 58.6 kilogram calories per mol are required to break a carbon to carbon (C−C) bond.