Deformation syndrome --- A recognizable pattern of dysmorphic features caused by extrinsic factors that affect the fetus in utero.
Degeneracy of the code --- The genetic code is described as degenerate because most of the 20 amino acids are specified by more than 1 of the 64 codons.
Degree of relationship --- The distance between two individuals in a pedigree. First-degree relatives include parents, siblings, and children. Second-degree relatives are aunts and uncles, nephews and nieces, grandparents and grandchildren.
Deletion --- The loss of a sequence of DNA from a chromosome. The deleted DNA may be of any length, from a single base to a large part of a chromosome.
Denaturation (of DNA) --- The conversion of DNA from the double-stranded to the single-stranded state, usually accomplished by heating to destroy chemical bonds involved in base pairing.
Deoxyribonucleic acid --- See DNA.
Determination --- During development, the second stage of commitment in which a cell follows its developmental program regardless of whether it is transplanted to a different region of the embryo.
Developmental program --- The process by which a cell in an embryo achieves its fate.
Dicentric --- A structurally abnormal chromosome with two centromeres.
Dictyotene --- The stage of the first meiotic division in which a human oocyte remains from late fetal life until ovulation.
Differentiation --- The process whereby a cell acquires a tissue-specific pattern of expression of genes and proteins and a characteristic phenotype.
Diploid --- The number of chromosomes in most somatic cells, which is double the number found in the gametes. In humans, the diploid chromosome number is 46.
Discordance --- The situation in which (1) one member of the pair has a certain qualitative trait and the other does not or (2) the relatives have values of a quantitative trait that are at opposite ends of the distribution. See concordance.
Disomy --- See uniparental disomy.
Disruption --- A birth defect caused by destruction of tissue; may be caused by vascular occlusion, a teratogen, or rupture of the amniotic sac with entrapment.
Dizygotic (DZ) twins --- Twins produced by two separate ova, separately fertilized. Also called fraternal twins.
DNA (deoxyribonucleic acid) --- The molecule that encodes the genes responsible for the structure and function of living organisms and allows the transmission of genetic information from generation to generation.
DNA ligase --- An enzyme that can form a phosphodiester bond between the deoxyribose backbones of two strands of DNA. See ligation.
DNA methylation --- In eukaryotes, the addition of a methyl residue to the 5-position of the pyrimidine ring of a cytosine base in DNA to form 5-methylcytosine.
DNA polymerase --- An enzyme that can synthesize a new DNA strand by use of a previously synthesized DNA strand as a template.
Domain --- A region of the amino acid sequence of a protein that can be equated with a particular function.
Dominant --- A trait is dominant if it is phenotypically expressed in heterozygotes. If heterozygotes and homozygotes for the variant allele have the same phenotype, the disorder is a pure dominant (rare in human genetics). If homozygotes have a more severe phenotype than do heterozygotes, the disorder is termed semidominant or incompletely dominant.
Dominant negative --- A disease-causing allele, or the effect of such an allele, that disrupts the function of a wild-type allele in the same cell.
Donor splice site --- The boundary between the 3’ end of an exon and the 5’ end of the next intron. Also called 5’ splice site.
Dosage compensation --- As a consequence of X inactivation, the amount of product formed by the two copies of an X-linked gene in females is equivalent to the amount formed by the single gene in males. See X inactivation.
Double heterozygote --- An individual who is heterozygous at each of two different loci. Contrast with compound heterozygote.
Double minutes --- Very small accessory chromosomes, a form of gene amplification.
Dyschronic expression --- Expression of a gene at a time when it is not normally expressed.
Dysmorphism --- Morphological developmental abnormalities, as seen in many syndromes of genetic or environmental origin.
Ecogenetic disorder --- A disorder resulting from the interaction of a genetic predisposition to a specific disease with an environmental factor.
Ectoderm --- One of the three primary germ layers of the early embryo. Begins as the layer farthest from the yolk sac and ultimately gives rise to the nervous system, the skin, and neural crest derivatives such as craniofacial structures and melanocytes.
Ectopic expression --- Expression of a gene in places where it is not normally expressed.
Embryonic stem cell --- A cell derived from the inner cell mass that is self-renewing in culture and, when reintroduced into the inner cell mass of a blastocyst, can repopulate all the tissues of the embryo.
Empirical risk --- In human genetics, the probability that a familial trait will occur in a family member, based on observed numbers of affected and unaffected individuals in family studies rather than on knowledge of the causative mechanism.
Endoderm --- One of the three primary germ layers of the early embryo. Ultimately gives rise to the gut, liver, and portions of the urogenital system.
Enhancer --- A DNA sequence that acts in cis (i.e., on the same chromosome) to increase transcription of a gene. The enhancer may be upstream or downstream to the gene and may be in the same or the reverse orientation. Contrast with silencer.
Enzymopathy --- A metabolic disorder resulting from deficiency or abnormality of a specific enzyme.
Epigenetic --- The term that refers to any factor that can affect gene function without change in the genotype. Some typical epigenetic factors involve alterations in DNA methylation, chromatin structure, histone modifications, and transcription factor binding that change genome structure and affect gene expression without changing the primary DNA sequence.
Episome --- A DNA element that either can exist as an autonomously replicating sequence in the cytoplasm or can integrate into chromosomal DNA. Adeno-associated viral vectors, used in gene therapy, are episomes that exist in the cytoplasm for long periods and can, although rarely, be inserted into the nuclear genome.
Euchromatin --- The major component of chromatin. It stains lightly with G banding, decondensing and becoming light-staining during interphase. Contrast with heterochromatin.
Eugenics --- Increasing the prevalence of desirable traits in a population by decreasing the frequency of deleterious alleles at relevant loci through controlled, selective breeding. The opposite is dysgenics.
Eukaryote --- A unicellular or multicellular organism in which the cells have a nucleus with a nuclear membrane and other specialized characteristics. See also prokaryote.
Euploid --- Any chromosome number that is an exact multiple of the number in a haploid gamete (n). Most somatic cells are diploid (2n). Contrast with aneuploid.
Exon --- A transcribed region of a gene that is present in mature messenger RNA.
Expression profile --- A quantitative assessment of the mRNAs present in a cell type, tissue, or tumor. Often used to characterize a cell, tissue, or tumor in comparison to the expression profile of another cell, tissue, or tumor.
Expression vector --- A cloning vector engineered to provide for the transcription and translation of a cloned DNA insert so that the host carrying the vector produces the protein encoded by the insert. See vector.
Expressivity --- The extent to which a genetic defect is expressed. If there is variable expressivity, the trait may vary in expression from mild to severe but is never completely unexpressed in individuals who have the corresponding genotype. Contrast with penetrance.
Familial --- Any trait that is more common in relatives of an affected individual than in the general population, whether the cause is genetic, environmental, or both.
Fate --- The ultimate destination for a cell that has traveled down a developmental pathway. The embryonic fate map is a complete description of all the fates of all the different parts of the embryo.
Fetal phase --- Stage of intrauterine development from weeks 9 to 40.
Fetoscopy --- A technique for direct visualization of the fetus.
FISH --- Fluorescence in situ hybridization. See in situ hybridization.
Fitness (f) --- The probability of transmitting one’s genes to the next generation compared with the average probability for the population.
Flanking sequence --- A region of a gene preceding or following the transcribed region.
Founder effect --- A high frequency of a mutant allele in a population founded by a small ancestral group when one or more of the founders was a carrier of the mutant allele.
Fragile site --- Nonstaining gap in the chromatin of a metaphase chromosome, such as the fragile site at Xq27 in fragile X syndrome.
Frameshift mutation --- A mutation involving a deletion or insertion that is not an exact multiple of three base pairs and thus changes the reading frame of the gene downstream of the mutation.
Gain-of-function mutation --- A mutation associated with an increase in one or more of the normal functions of a protein. To be distinguished from novel property mutation.
Gamete --- A reproductive cell (ovum or sperm) with the haploid chromosome number.
Gatekeeper genes --- Tumor-suppressor genes that directly regulate cell proliferation.
Gene --- A hereditary unit; in molecular terms, a sequence of chromosomal DNA that is required for the production of a functional product.
Gene dosage --- The number of copies of a particular gene in the genome.
Gene-environment interaction --- Combined action of alleles at one or more loci and nongenetic factors, such as environmental exposures or random events, in causing a complex disease.
Gene family --- A set of genes containing related exons, indicating that the genes have evolved from an ancestral gene by duplication and subsequent divergence.