dD

dD is a term whose meaning is not fixed and depends on the scientific context. In physics and fusion research, the symbol d commonly denotes a deuteron, the nucleus of deuterium. The expression d–d fusion refers to reactions between two deuterons, which can proceed via channels such as d + d → He-3 + n and d + d → t + p. These reactions release energy and neutrons and have been studied in the contexts of stellar nucleosynthesis and experimental fusion devices. Practical fusion power faces challenges including relatively high Coulomb barriers, the need for high temperatures and confinement, and neutron production.

In chemistry and biochemistry, deuterium labeling is widespread. The letters D or 2H are used to indicate deuterium substitution, for example in isotopically labeled molecules or deuterated solvents. Heavy water with the formula D2O is a well-known example. The specific notation “dD” is not a universally standardized chemical abbreviation; many authors use [2H], D, or a deuterium-prefixed designation to show deuteration at particular positions.

In genetics, uppercase and lowercase letters denote alleles, with D often representing a dominant allele and d a recessive one. A genotype written as dD (or Dd) denotes a heterozygous combination for that gene, with the phenotype determined by the dominance relationship of the alleles in question. The interpretation of dD in genetics depends on the gene being considered.

Overall, dD serves as a context-dependent shorthand. Its precise meaning is defined by the discipline and the surrounding notation within a given document.