WordNet

used of homologous chromosomes associated in pairs in synapsis (同)double
having a valence of two or having two valences (同)divalent
a threadlike strand of DNA in the cell nucleus that carries the genes in a linear order; "humans have 22 chromosome pairs plus two sex chromosomes"

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二価の(二つの原子価を持つ)
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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2016/02/15 05:25:18」(JST)

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A bivalent

A bivalent, sometimes referred to as a tetrad, is a pair of associated homologous chromosomes held together by a complex after chromosome replication. During meiosis (the Prophase I: Pachynema stage of Meiosis I), the process of synapsis occurs in which bivalents are formed. Each replicated chromosome is composed of two sister chromatids.^[1]^[2]

Bivalents are formed as two homologous chromosomes undergo recombination.^[3] Chromosome movement to accommodate recombination places homologous chromosomes close enough to each other for a protein complex (synaptonemal complex), consisting of a protein (ZIP1/ZYP1 depending on species),^[4] to hold them together. This protein's sequence is not well conserved, though its structure is very similar in all species (globular domains on the N and C termini with a well defined core). In total, a tetrad is made up of four chromatids, but only two chromosomes (one paternal and one maternal). In the atypical model species D. melanogaster and C. elegans recombination occurs secondarily to synapsis.

Recombination produces chiasmata which hold chromosomes together after the synaptonemal complex dissolves. (Prophase 1 substage Diplotene) During Anaphase I, the tetrads separate into homologous chromosome pairs which will be placed in separate daughter cells during Telophase I.^[3]

References

^ Lefers, Mark. "Northwestern University Department of Molecular Biosciences". Retrieved 26 September 2015.
^ "University of Arizona Department of Biochemistry and Molecular Biophysics". The Biology Project. Retrieved 26 September 2015.
^ ^a ^b "Meiosis Definition". Scitable. Nature Education. Retrieved 26 September 2015.
^ "The synaptonemal complex protein ZYP1 is required for imposition of meiotic crossovers in barley | Read by QxMD". www.readbyqxmd.com. Retrieved 2015-10-12.

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English Journal

Partial preferential chromosome pairing is genotype dependent in tetraploid rose.

Bourke PM1, Arens P1, Voorrips RE1, Esselink GD1, Koning-Boucoiran CF1,2, van 't Westende WP1, Santos Leonardo T1,3, Wissink P1, Zheng C4, van Geest G1,5, Visser RG1, Krens FA1, Smulders MJ1, Maliepaard C1.
The Plant journal : for cell and molecular biology.Plant J.2017 Jan 31. doi: 10.1111/tpj.13496. [Epub ahead of print]
It has long been recognised that polyploid species do not always neatly fall into the categories of auto- or allopolyploid, leading to the term "segmental allopolyploid" to describe everything in between. The meiotic behaviour of such intermediate species is not fully understood, nor is there consen
PMID 28142191

Cytogenetic study on antlions (Neuroptera, Myrmeleontidae): first data on telomere structure and rDNA location.

Kuznetsova VG1, Khabiev GN2, Anokhin BA1.
Comparative cytogenetics.Comp Cytogenet.2016 Nov 25;10(4):647-656. doi: 10.3897/CompCytogen.v10i4.10775. eCollection 2016.
Myrmeleontidae, commonly known as "antlions", are the most diverse family of the insect order Neuroptera, with over 1700 described species (in 191 genera) of which 37 species (in 21 genera) have so far been studied in respect to standard karyotypes. In the present paper we provide first data on the
PMID 28123685

To Break or Not To Break: Sex Chromosome Hemizygosity During Meiosis in Caenorhabditis.

Van MV1, Larson BJ1, Engebrecht J2.
Genetics.Genetics.2016 Nov;204(3):999-1013. doi: 10.1534/genetics.116.194308. Epub 2016 Sep 7.
Meiotic recombination establishes connections between homologous chromosomes to promote segregation. Hemizygous regions of sex chromosomes have no homologous chromosome to recombine with, yet must be transmitted through meiosis. An extreme case of hemizygosity exists in the genus Caenorhabditis, whe
PMID 27605052

Japanese Journal

Chromosome Studies of Meehania montis-koyae and M. urticifolia in Japan

,
CYTOLOGIA 79(3), 371-377, 2014
… In this study, we confirmed the chromosome forms for M. … The chromosome count of 2n=27 is a new count for this species. … These two counts reveal the basic chromosome number of x=9, which then becomes 2n=18. … The somatic chromosome complement was formulated as 2n=18=7m+9sm+2st. As such, the second longest chromosome pair is heteromorphic with long arms of different lengths. …
NAID 130004713098

Multiple Associations of Chromosomes Due to Structural Heterozygosity in the Wild Plants of Achillea millefolium L. from Northwest Himalayas (India)

, , , , ,
CYTOLOGIA 79(2), 151-159, 2014
… millefolium, a morphogenetically variable species, has been scored from phytogeographically isolated and unexplored areas of the northwest Himalayas for chromosome counts and male meiotic course. … All the studied accessions shared the same meiotic chromosome number of n = 9 and existed at diploid level. … Meiocytes with multivalents depicted relatively higher chiasma frequency than those with normal bivalent formation. …
NAID 130004701653

Cytogenetical Studies of Four Species in Subfamily Peiratinae from North India (Heteroptera: Reduviidae)

,
CYTOLOGIA 78(4), 411-415, 2013
… The diploid chromosome number and male meiosis in Ectomocoris atrox, E. … Single terminal chiasma per bivalent is seen in all except Ectomocoris atrox. …
NAID 130004448256