Activator, or Ac -- Along with the Dissociator, or Ds, one of two mutable loci able to move in the genome and identified by Barbara McClintock.

Allele -- Any of the alternative forms of a gene that can occupy a particular chromosomal locus.

Breakage-Fusion-Bridge (BFB) cycle -- Damage occurring to a dicentric chromosome--a chromosome with two centromeres--during each cell cycle.

Centromere -- The constricted region of a nuclear chromosome, to which the spindle fibers, the microtubule apparatuses that control chromosome movement during mitosis and meiosis, attach during division.

Chemical change -- Rearrangement of a substance's atomic matter resulting in a change of physical properties.

Chromatid -- The subunit of a chromosome after replication and prior to anaphase of meiosis II or mitosis. At anaphase of meiosis II or mitosis, when the centromeres divide and the sister chromatids separate, each chromatid becomes a chromosome.

Chromosome arms -- During cell division, the portions of the chromosome that extend outward from the centromere.

Controlling elements -- A term used by maize geneticists to indicate a mobile genetic element capable of producing an unstable mutant target gene. There are two types of controlling elements, the regulator and the receptor.

Crossover, or crossing over -- A process in which homologous chromosomes exchange parts, normally reciprocally but sometimes unequally.

Cytology -- The branch of biology that deals with the formation, structure, and function of cells.

Diploid -- The state of having two copies of each chromosome per nucleus or cell.

Endosperm -- Triploid tissue in a seed, formed from fusion of two haploid female and one haploid male nuclei, that contains reserve food materials and is formed within the embryo sac of plants.

Gene expression -- The process of producing a protein from its DNA-coding, or mRNA-coding, sequences.

Genetics -- The branch of biology that deals with heredity, especially the mechanisms of hereditary transmission of inherited characteristics among similar or related organisms.

Germ cells -- Sperm and egg cells and their precursors. Germ cells are haploid.

Haploid -- An organism or cell, such as a sperm or egg, which has one chromosome set, or an organism composed of such cells.

Heterozygous translocation -- The relocation of a chromosomal segment to a different position in the genome of an individual with a gene pair having different alleles in the two chromosome sets of the diploid individual.

Hidden mutations, or recessive mutations -- An allele that is not expressed in the heterozygous condition, or the phenotype of the homozygote of a recessive allele.

Knob -- A small beadlike structure visible on a chromosome during prophase of meiosis and mitosis that can serve as a landmark, allowing certain chromosomes to be identified readily in the nucleus.

Meiosis -- Two successive nuclear divisions--with corresponding cell divisions--that produce haploid sexual spores in plants and fungi that have one-half of the genetic material of the original cell.

Mitosis -- The nuclear division that produces two daughter nuclei identical to the original nucleus, normally just before cell division.

Mutability -- Tending to undergo genetic mutation.

Mutation -- The process that produces a gene or a chromosome that differs from the wild-type. This process, either through an alteration in the nucleotide sequence of the DNA coding for a gene or through a change in the physical arrangement of a chromosome, results in the creation of a new character or trait not found in the parental type.

Ring chromosome -- A structurally abnormal chromosome in which the end of each chromosome arm has been lost and the broken arms have been reunited in a formation resembling a ring.

Semisterile -- The phenotype of individuals heterozygotic for certain types of chromosome aberration, expressed as a reduced number of viable gametes and hence reduced fertility. McClintock used it as a term for mutable loci on the maize gene that exhibited these qualities.

Sister-strand exchanges, or sister chromatid exchange (SCE) -- An event similar to crossing over that can occur between sister chromatids at mitosis or at meiosis.

Spontaneous mutation -- A mutation occurring in the absence of a mutagen, usually due to errors in the normal functioning of cellular enzymes.

Sporophyte -- The diploid stage of the plant life cycle that produces spores by meiosis and alternates with the gametophyte stage, which is the haploid stage of the plant life cycle that produces gametes by mitosis.

Suppressor-mutator -- A gene that, when mutated, appears to restore the wild-type phenotype to a mutation at another locus. McClintock used it as a term for mutable loci on the maize gene that exhibited these qualities.

Transposition -- The movement of a DNA segment within the genome of an organism.

Triploid -- Having three complete sets of chromosomes that often produce severe developmental abnormalities. In animals, many but not all triploid fetuses are usually spontaneously aborted.

Trisomic -- A diploid with an extra chromosome of one type, producing a chromosome number of the form 2n + l. The state of a cell or individual that has three chromosomes instead of two for a given pair.

Twin sectors, or twin spot -- A pair of mutant sectors within wild-type tissue, produced by a mitotic crossover in an individual of appropriate heterozygous genotype.

Variegation -- A type of position effect appearing when particular loci are contiguous with heterochromatin, the densely staining condensed chromosomal regions believed to be mostly genetically inert. In position-effect variegation, genes in some cells are inactivated through their abnormal juxtaposition with heterochromatin.

Wild-type -- The typical form of an organism or gene as it occurs in nature, as distinguished from mutant forms that may result from selective breeding.