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Modern Genetic Analysis Glossary
B
BAC Bacterial artificial chromosome; an F plasmid engineered to act as a cloning vector that can carry large inserts.
back mutation See reversion.
bacteriophage (phage) A virus that infects bacteria.
balanced polymorphism Stable genetic polymorphism maintained by natural selection.
balancer A chromosome with multiple inversions, used to retain favorable allele combinations in the uninverted homolog.
Balbiani ring A large chromosome puff.
Barr body A densely staining mass that represents an inactivated X chromosome.
base analog A chemical whose molecular structure mimics that of a DNA base; because of the mimicry, the analog may act as a mutagen.
bead theory The disproved hypothesis that genes are arranged on the chromosome like beads on a necklace, indivisible into smaller units of mutation and recombination.
bHLH protein See HLH proteins.
bimodal distribution A statistical distribution having two modes.
binary fission The process in which a parent cell splits into two daughter cells of approximately equal size.
biparental zygote A Chlamydomonas zygote that contains cpDNA from both parents; such cells are generally rare.
blast cell A cell that divides, generally asymmetrically, to give rise to two different progeny cells. One is a blast cell just like the parental cell and the other is a cell that enters a differentiation pathway. In this manner, a continuously propagating cell population can maintain itself and spin off differentiating cells.
blastoderm In an insect embryo, the stage in which there is a single layer of nuclei (syncitial blastoderm) or cells (cellular blastoderm) surrounding the central yolk.
blastula An early developmental stage of lower vertebrate embryos, in which the embryo consists of a single layer of cells surrounding the central yolk.
blending inheritance A discredited model of inheritance suggesting that the characteristics of an individual result from the smooth blending of fluidlike influences from its parents.
brachydactyly A human phenotype of unusually short digits, generally inherited as an autosomal dominant.
branch migration The process by which a single "invading" DNA strand extends its partial pairing with its complementary strand as it displaces the resident strand.
bridging cross A cross made to transfer alleles between two sexually isolated species by first transferring the alleles to an intermediate species that is sexually compatible with both.
broad heritability (H2) The proportion of total phenotypic variance at the population level that is contributed by genetic variance.
bud A daughter cell formed by mitosis in yeast; one daughter cell retains the cell wall of the parent, and the other (the bud) forms a new cell wall.
buoyant density A measure of the tendency of a substance to float in some other substance; large molecules are distinguished by their differing buoyant densities in some standard fluid. Measured by density-gradient ultracentrifugation.
Burkitt lymphoma A cancer of the lymphatic system manifested by tumors in the jaw, associated with a chromosomal translocation bringing a specific oncogene next to regulatory elements of one of the immunoglobulin genes.

© 1999 by W. H. Freeman and Company. All rights reserved.