Discordance

Di sj unc ti on 537 functions in meiosis as a diploid. Meiotic pairing will usually occur far more readily between the fully homologous chromosomes from the same species than between equivalent chromosomes from different species, and bivalents will then be formed virtually exclusively. The tetraploid will then behave in meiosis like a diploid and will obey ordinary Mendelian rules, though it may well have functional duplication of many genes. Tetraploids of this kind are called allotetraploids, or sometimes amphidiploids, since they have two different diploid chromosome sets. Wheat (Triticum aestivum) is an allohexaploid, with three diploid genomes originating from different species. Functionally, and with minor variations, it has all its genes in triplicate, but it behaves in meiosis like a regular diploid.

Further Reading

Fincham JRS, Day PR and Radford A (1979) Fungal Genetics, 4th edn. Oxford: Blackwell Scientific Publications.

See also: Dominance; Meiosis; Plasmids; Polyploidy; Transduction

Direct Repeats Copyright ß 2001 Academic Press doi: 10.1006/rwgn.2001.1822

Direct repeats are identical DNA sequences present in two or more copies in the same orientation and within the same molecule. See also: Repetitive (DNA) Sequence

Directed Deletion In Developmental Processes

Disassortative Mating See: Assortative Mating

Discontinuous Replication Copyright ß 2001 Academic Press doi: 10.1006/rwgn.2001.1823

Discontinuous replication is the synthesis of DNA in short (Okazaki) fragments that are later joined to form a continuous strand. See also: Okazaki Fragment

Discordance L Silver Copyright ß 2001 Academic Press doi: 10.1006/rwgn.2001.0342

Discordance is the opposite of concordance. This word is used in two different ways by geneticists. In formal genetic studies, it describes the situation where two expressed traits or alleles that are found together in a parent are separated in the offspring of that parent. The level or percent of discordance refers to the fraction of total offspring characterized from an experimental cross that show discordance. The remaining fraction is concordant. Discordance is also used in twin studies to describe twin pairs that differ in their expression of a particular trait under analysis. See also: Concordance

See: Gene Rearrangements, Prokaryotic

Directed Mutagenesis See: Complement Loci

Directed Mutation J H Miller Copyright ß 2001 Academic Press doi: 10.1006/rwgn.2001.0339

Any mutation that is targeted to a specific gene. See also: Mutation

Disequilibrium See: Gametic Disequilibrium; Linkage Disequilibrium

Disjunction L Silver Copyright ß 2001 Academic Press doi: 10.1006/rwgn.2001.0343

During the anaphase I stage of the first meiotic division, the two homologs of every chromosome `disjoin'