agamic generation
In the life cycle of some species (e.g. many members of the Cynipidae) a generation that comprises parthenogenetically reproducing females that are genetically diploid. The unfertilized eggs laid may ...
allele
Common shortening of the term ‘allelomorph’. One of two or more forms of a gene arising by mutation and occupying the same relative position (locus) on homologous chromosomes.
alternation of generations
Reproductive cycles in which a haploid phase alternates with a diploid phase. In mosses and vascular plants, the haploid phase is the gametophyte, the diploid the sporophyte.
ameiosis
Meiosis in which the nucleus divides only once, so the number of chromosomes is not reduced.
amphidiploid
An allopolyploid in which the genetic behaviour of the constitutive genomes is diploid, such that bivalents between chromosomes originating from different genomes do not form during meiosis.
anaphase
One of several stages of cell division. In mitosis the chromatids of each chromosome move apart to opposite ends of the spindle. In the first anaphase of meiosis, the paired homologous chromosomes ...
apomixis
Reproduction in which meiosis and fertilization are altered, so that only one parent contributes genes to the offspring (e.g., agamospermy or thelytoky). Contrast with amixis, amphimixis.
apospory
The development in plants of a diploid embryo sac by the somatic division of a nucellus or integument cell; a form of agamospermy (q.v.).
bivalent
n. (in genetics) a structure consisting of homologous chromosomes attached to each other by chiasmata during the first division of meiosis. —bivalent adj.
bouquet configuration
A polarized arrangement of chromosome ends at the periphery while the remaining chromatin fills the volume of the nucleus. This is the result of telomeres (q.v.) moving along the inner surface of the ...
breakage and reunion
The classical and generally accepted model of crossing over by physical breakage and crossways reunion of broken chromatids during meiosis. See Holliday model.
cell division
Reproduction of cells by division first of the chromosomes (karyokinesis) and then of the cytoplasm (cytokinesis). Cell division to produce more body (somatic) cells is by mitosis; cell division ...
centromere
The point at which the two halves of a chromosome, the chromatids, are joined. See kinetochore; spindle attachment.
chiasma
(pl. chiasmata)The point at which paired homologous chromosomes remain in contact as they begin to separate during the first prophase of meiosis, forming a cross shape. A number of chiasmata can ...
chiasma interference
The non-random frequency of more than one chiasma in a bivalent segment during meiosis. If the frequency of occurrence is higher than that expected from purely chance events then it is termed ...
chromatid
The two daughter strands of a duplicated chromosome that are joined by a single centromere. Upon the division of the centromere, the sister chromatids become separate chromosomes. See Chronology, ...
chromatid interference
The non-random participation of non-sister chromatids of a tetrad in successive crossings-over of meiosis. It results in a deviation from the expected 1:2:1 ratio for the frequencies of two-, three-, ...
chromomere
A small beadlike structure visible in a chromosome during prophase of meiosis and mitosis, when it is relatively uncoiled (in particular at the leptotene and zygotene stages of meiosis). Chromomeres ...
chromosome
1. in prokaryotes, the circular DNA molecule containing the entire set of genetic instructions essential for life of the cell. See genophore. 2. in the eukaryotic nucleus, one of the threadlike ...
cohesin
A multiprotein complex that binds sister chromatids together during the initial stages of nuclear division in cells. It includes members of the SMC (structural maintenance of chromosomes) family of ...
