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Modern Genetic Analysis Glossary
D
Darwinian fitness The relative probability of survival and reproduction for a genotype.
daughter cells Two identical cells formed by asexual division of a cell.
daughter chromatids Two identical chromatids formed by the replication of one chromosome.
default state The developmental state of a cell (or group of cells) in the absence of activation of a developmental regulatory switch.
degenerate code A genetic code in which some amino acids may be encoded by more than one codon each.
deletion Removal of a chromosomal segment from a chromosome set.
denaturation The separation of the two strands of a DNA double helix or the severe disruption of the structure of any complex molecule without breaking the major bonds of its chains.
denaturation map A map of a stretch of DNA showing the locations of local denaturation loops, which correspond to regions of high AT content.
deoxyribonuclease See DNase.
deoxyribonucleic acid See DNA.
determinant A spatially localized molecule that causes cells to adopt a particular fate or set of related fates.
determination The process of commitment of cells to particular fates.
development The process whereby a single cell becomes a differentiated organism.
developmental field A group of developmentally equivalent cells that together organize and form a particular part of the body plan.
developmental pathway The chain of molecular events that take a set of equivalent cells and produce the assignment of different fates among those cells.
dicentric chromosome A chromosome with two centromeres.
differentiation The changes in cell shape and physiology associated with the production of the final cell types of a particular organ or tissue.
dihybrid cross A cross between two individuals identically heterozygous at two locifor example, AB / ab × AB / ab.
dimorphism A "polymorphism" with only two forms.
dioecious plant A plant species in which male and female organs are on separate plants.
diploid A cell having two chromosome sets or an individual organism having two chromosome sets in each of its cells.
directed mutagenesis Altering some specific part of a cloned gene and reintroducing the modified gene back into the organism.
directional selection Selection that changes the frequency of an allele in a constant direction, either toward or away from fixation for that allele.
discontinuous variation Variation having distinct classes of phenotypes for a particular character.
dispersive replication Disproved model of DNA synthesis suggesting more-or-less random interspersion of parental and new segments in daughter DNA molecules.
distribution See statistical distribution.
distribution function A graph of some precise quantitative measure of a character against its frequency of occurrence.
DNA (deoxyribonucleic acid) A double chain of linked nucleotides (having deoxyribose as their sugars); the fundamental substance of which genes are composed.
DNA-binding domain The site on a DNA-binding protein that directly interacts with specific DNA sequences.
DNA clone A section of DNA that has been inserted into a vector molecule, such as a plasmid or a phage chromosome, and then replicated to form many copies.
DNA fingerprint The autoradiographic banding pattern produced when DNA is digested with a restriction enzyme that cuts outside a family of VNTRs and a Southern blot of the elctrophoretic gel is probed with a VNTR-specific probe. Unlike true fingerprints, these patterns are not unique to each individual.
DNA polymerase An enzyme that can synthesize new DNA strands from a DNA template; several such enzymes exist.
DNase (deoxyribonuclease) An enzyme that degrades DNA to nucleotides.
dominance variance Genetic variance at a single locus attributable to dominance of one allele over another.
dominant allele An allele that expresses its phenotypic effect even when heterozygous with a recessive allele; thus if A is a dominant over a, then A / A and A / a have the same phenotype.
dominant phenotype The phenotype of a genotype containing the dominant allele; the parental phenotype that is expressed in a heterozygote.
donor DNA Any DNA to be used in cloning.
dosage compensation The process in organisms using a chromosomal sex determination mechanism (such as XX versus XY) that allows standard structural genes on the sex chromosome to be expressed at the same levels in females and males, regardless of the number of sex chromosomes. In mammals, dosage compensation operates by maintaining only a single active X chromosome in each cell; in Drosophila, it operates by hyperactivating the male X chromosome.
dose See gene dose.
double crossover Two crossovers in a chromosomal region under study.
double helix The structure of DNA first proposed by Watson and Crick, with two interlocking helices joined by hydrogen bonds between paired bases.
double infection Infection of a bacterium with two genetically different phages.
Down syndrome An abnormal human phenotype, including mental retardation, due to a trisomy of chromosome 21; more common in babies born to older mothers.
drift See random genetic drift.
Duchenne muscular dystrophy A lethal muscle disease in humans caused by mutation in a huge gene coding for the muscle protein dystrophin; inherited as an X-linked recessive phenotype.
duplicate genes Two identical allele pairs in one diploid individual.
duplication More than one copy of a particular chromosomal segment in a chromosome set.
dyad A pair of sister chromatids joined at the centromere, as in the first division of meiosis.

© 1999 by W. H. Freeman and Company. All rights reserved.