mann Pinscher. MISCELLANEOUS WORKING DOGS: St. Bernard, Welsh Corgi (Cardigan), Welsh Corgi (Pembroke), Newfoundland, Great Pyrenees. TOYS: Maltese, Pug, Japanese Spaniel, English Toy Spaniel (King Charles), Pekingese, Pomeranian, Yorkshire Terrier, Griffon, Chihuahua, Papillon, Poodle (Toy), Mexican hairless.
NONSPORTING BREEDS: Lhasa Apso, Poodle (Standard), Poodle (Miniature), Dal- matian, Chow Chow, Keeshond, Schipperke, En- glish Bulldog, French Bulldog, Boston Terrier. Dollo law the proposition that evolution along any specific lineage is essentially irreversible. For exam- ple, no modern mammal can de-evolve back to a form identical in all respects to the mammal-like, reptilian ancestor from which it was derived.
This biological principle was formulated about 1890 by Louis Dollo, a Belgian paleontologist. See rachet. Dolly a sheep (q.v.) born in Scotland in 1996 and the first mammal to be experimentally cloned. This was done by fusing the nucleus of an adult somatic cell from one sheep with an enucleated egg from an- other, followed by implantation into a surrogate mother. Dolly’s chromosomes were therefore genet- ically identical to those of the somatic cell that pro- vided the nucleus.
When Dolly was two and a half years old, the lengths of her telomeres were deter- mined. The lengths corresponded to telomeres the age of the nuclear donor, not to telomeres of her chronological age. It was later found that the cloned adult sheep contained mtDNA derived solely from the recipient egg.
So Dolly was actually a genetic chimera (q.v.). Her cells contained nuclear DNA of somatic origin, while her mitochondria were derived from ooplasm. Dolly was euthanized in February of 2003 after developing progressive lung disease. Dol- ly’s skin was used in a taxidermic mount currently on display at the Royal Museum of Edinburgh. See Appendix C, 1997, Wilmut et al.; cloning, mitochon- drial DNA (mtDNA), nuclear reprogramming, nuclear transfer, telomere. domain
1. a homology unit; i.e., any of the three or four homologous regions of an immunoglobulin heavy chain that apparently evolved by duplication and diverged by mutation.
2. any discrete, continu- ous part of a polypeptide sequence that can be equated with a particular function.
3. a relatively short sequence of about 100 amino acids that adopts a defined three-dimensional structure within a pro- tein. Also known as a module.
4. any region of a chromosome within which supercoiling is indepen- dent of other domains. 5. an extensive region of DNA including an expressed gene that exhibits pro- nounced sensitivity to degradation by endonucleases.
See immunoglobulin domain superfamily, otu domain, SH domain. domesticated species See Appendix B. dominance referring to alleles that fully manifest their phenotype when present in the heterozygous, heterokaryotic, or heterogenotic state. The alleles whose phenotypic expressions are masked by domi- nant alleles are termed recessive alleles. Sometimes the dominant allele expresses itself late in develop- ment (e.g., Huntington disease, q.v.), in which case the allele is said to show delayed dominance. See co- dominant, incomplete dominance, semidominance.
dominance variance genetic variance for a poly- genic trait in a given population attributed to the dominance effects of contributory genes. dominant complementarily See complementary genes. dominant gene See recessive gene. dominant negative mutation a mutation which produces a product that binds to the product of the normal allele. The heteropolymer that results dam- ages the cell.
A dominant negative mutation there- fore has a more severe effect than the deletion of the same gene. Several hereditary human diseases are caused by dominant negative mutations in genes that encode collagens and keratins (both of which see). donkey Equus asinus, a close relative of the horse. The female is referred to as a jennet, the male as a jack. See Equus, horse-donkey hybrids. donor splicing site See left splicing junction. DOPA the abbreviation for dihydroxyphenylala- nine, a compound derived from the amino acid tyro- sine, by the addition of a second hydroxyl group. See albino, amino acids (page 21), melanism, tyrosinase.
Dopamine the compound derived from DOPA (q.v.) by removal of the carboxyl group. See Parkin- sonism.
dorsoventral genes genes that specify the dorsal or ventral patterning program of the embryonic cells in which they are expressed. In Drosophila, the gene decapentaplegic specifies dorsal development, whereas its mouse homolog, BMP4, specifies ventral develop- ment. In Drosophila, the gene short gastrulation spec- ifies ventral development, whereas the homologous gene in the mouse, chordin, specifies the dorsal de- velopment pattern.
See Saint Hilaire hypothesis. dosage compensation a mechanism that regulates the expression of sex-linked genes that differ in dose between females and males in species with an XX- XY method of sex determination. In Drosophilia melanogaster, dosage compensation is accomplished by raising the rate of transcription of genes on the single X chromosome of males to double that of genes on either X chromosome in females. In mam- mals, the compensation is made by inactivating at random one of the two X chromosomes in all so- matic cells of the female. The inactivated X forms the Barr body or sex chromatin.
In cases where mul- tiple X chromosomes are present all but one are in- activated. See Appendix C, 1948, Muller; Lyon, Russell; 1962, Beutler, et al.; Fabry disease, glucose-6-phosphate dehydrogenase deficiency, Lesch- Nyhan syndrome, Lyon hypothesis, Lyonization, mo- saic, MSL proteins, ocular albinism, Ohno hypothesis. dose 1. gene dose—the number of times a given gene is present in the nucleus of a cell. 2. radiation dose—the radiation delivered to a specific tissue area or to the whole body. Units for dose specifications are the gray, roentgen, red, rep, and sievert. dose-action curve dose-response curve (q.v.).
dose fractionation the administration of radiation in small doses at regular intervals. dose-response curve the curve showing the rela- tion between some biological response and the ad- ministered dose of radiation. See extrapolation number. dosimeter an instrument used to detect and mea- sure an accumulated dosage of radiation. dot blot See dot hybridization. dot hybridization a semiquantitative technique for evaluating the relative abundance of nucleic acid sequences in a mixture or the extent of similarity between homologous sequences. In this technique, multiple samples of cloned DNAs, identical in amount, are spotted on a single nitrocellulose filter in dots of uniform diameter. The filter is then hy- bridized with a radioactive probe (e.g., an RNA or DNA mixture) containing the corresponding se- quences in unknown amounts.
The extent of hybrid- ization is estimated semiquantitatively by visual comparison to radioactive standards similarly spotted. dot-matrix analysis a graphical method of com- paring the nucleotide sequences or amino acid se- quences along sections of two polymeric molecules that may or may not be homologous. For example, a comparison could be made of the exons from a gene known for two different animal species (A and B). A dot plot diagram is generated with the A gene on the vertical axis and the B gene on the horizontal axis.
Dots are placed within this rectangular array at every place where sequences from the two species match. The technique allows all pairs to be compared simultaneously, and regions of sequence similarity are seen as a series of dots. If there is no homology, the dots form a random pattern. If the dots form a diago- nal line, the exons of the two genes have similar se- quences and are arrayed in the same order.
Dotted a gene, symbolized by Dt, residing on chromosome 9 of maize, that influences the rate at which a mutates to A. A is on chromosome 3 and the gene controls the ability of the cells of the aleu- rone (q.v.) layer of the kernel (q.v.) to produce col- ored pigments. Clones of cells with restored pigment production generate spots on the kernel, as shown in the illustration. Dt was the first genetic element to be called a mutator gene (q.v.). However, it is now clear that it is a transposable element (q.v.) be- longing to a different transposon family than Ac or Ds. See Appendix C, 1938, Rhoades; Activator-Disso- ciation system, genetic instability.