Zip 1 a gene in Saccharomyces cerevisiae that is a homolog of the Gowen crossover suppressor (q.v.) of Drosophila melanogaster. Zip 1 encodes a protein that functions both in centromeric coupling (q.v.) and the construction of synaptomemal complexes (q.v.). zonal electrophoresis a technique that makes possible the separation of charged macromolecules and the characterization of each molecule in terms of its electrophoretic mobility. See electrophoresis. zona pellucida one of the envelopes surrounding the mammalian ovum that produces various sub- stances that attract homologous sperm, prevent en- try of foreign sperm, and prevent polyspermy (q.v.) zoogeographic realms the divisions of the land masses of the world according to their distinctive faunas.
See biogeographic realms. zoogeography the study of the geographical dis- tribution of animals. zoonotic viruses viruses that can undergo trans- formations which allow them to cross various spe- cies boundaries to infect humans. For example, the influenza A virus left its original host, the duck, in- fected pigs, and finally humans. ZPG zero population growth; a population status in which birth and death rates are equivalent.
ZR515 a synthetic juvenile hormone analog that mimics the effects of JH and is more resistant to breakdown by the esterases normally found in insect hemolymph. See allatum hormone. Z, W chromosomes See W, Z chromosomes. zwitterion a dipolar ion. For example, amino acids in solution at neutral pH are in dipolar form, with the amino group protonated (−NH3) and the car- boxyl group dissociated (−COO−). zygosity testing the testing of individuals born from a multiple gestation (twins, triplets, etc.) to see if they are homozygotic or dizygotic. The process generally involves comparisons of DNA profiles.
The purpose may be to identify a suitable donor for or- gan transplantation or to determine the risks of other siblings developing a genetic disease, if one is diag- nosed with the condition. See DNA forensics, twins. zygotene (zyg) DNA DNA replicated during the zygotene stage of meiosis. During the premeiotic S phase, only about 99.7% of the DNA is replicated. The remainder replicates during zygotene and is inti- mately involved in synapsis.
See Appendix C, 1971, Hotta and Stern. zygonema See meiosis. zygote the diploid cell resulting from the union of the haploid male and female gametes. zygotene stage See meiosis. zygotic gene a gene expressed in the early em- bryo. Embryos with mutations in zygotic genes are phenotypically abnormal, and this phenotype is de- pendent on genetic contributions from both parents, i.e., upon the genotype of the zygote, rather than the genotype of the mother or the father alone. Also called zygotic effect gene. Compare with maternal ef- fect gene, paternal effect gene.
See Appendix C, 1987, Nu¨sslein-Volhard et al.; 1988, 1989, Driever and Nu¨sslein-Volhard; 1995, Lewis et al.; bicoid, hunchback, zygotic segmentation mutants. zygotic induction the induction of vegetative rep- lication in a prophage that is transferred during con- jugation to a nonlysogenic F− bacterium. zygotic lethal in Drosophila, a lethal gene whose ef- fect is apparent in the embryo, larva, or adult, but that does not render inviable any gamete that carries it. zygotic meiosis See meiosis. zygotic segmentation mutants mutations in Dro- sophila melanogaster that are zygotically expressed and control the spatial pattern of development of the embryo.
The mutations fall into three classes that are defined by the pattern of cuticular defects they produce.
The gap genes are active in contiguous domains along the anteroposterior axis of the em- bryo and regulate segmentation within each domain.
The pair rule genes are expressed in stripes along the blastoderm with a periodicity that corresponds to every other segment. The segment polarity genes regu- late the spatial pattern within each segment. See Ap- pendix C, 1980, Nu¨sslein-Volhard and Wieschaus; 1989, Driever and Nu¨sslein-Volhard; fushi tarazu, Hox genes, hunchback, maternal polarity mutants, metamerism, selector genes. zymogen the enzymatically inactive precursor of a proteolytic enzyme. Zymogens usually become acti- vated by posttranslational modifications. For exam- ple, the zymogen pepsinogen is converted to the di- gestive enzyme pepsin by cleavage in a particular peptide sequence. zymogen granules enzyme-containing particles elaborated by the cells of the pancreas.