g gravity; employed in describing centrifugal forces. Thus, 2,000 × g refers to a sedimenting force 2,000 times that of gravity. G guanine or guanosine. G0, G1, G2 See cell cycle. ga/gigaannum one billion years. The age of the earth is 4.6 ga. See Appendix C, 1953, Patterson. gain of function mutation a genetic lesion that causes a gene to be overexpressed or expressed at the wrong time. Such mutations often affect up- stream elements that control the time in the life cy- cle when a gene is turned on or the specific tissue in which it is expressed. Gain of function mutations are often dominant. Contrast with loss of function muta- tions.
GAL4 a transcriptional activator protein encoded by the gal4 gene of yeast and required for the ex- pression of genes encoding galactose-metabolizing enzymes. The GAL4 protein consists of two separa- ble but essential domains (q.v.): an N-terminal (q.v.) domain which binds to specific DNA sequences up- stream (q.v.) from the various target genes, and a C- terminal (q.v.) domain which is required to activate transcription. These properties of GAL4 have been exploited to develop the yeast two-hybrid system (q.v.) for detecting protein-protein interactions and the Drosophila targeted gene expression technique (q.v.) for studying the functioning of master control genes in various targeted tissues. See Appendix C, 1989, Field and Song; 1995, Halder et al. galactose a six-carbon sugar that forms a compo- nent of the disaccharide lactose and of various cereb- rosides and mucoproteins. See beta galactosidase.
galactosemia a hereditary disease in humans in- herited as an autosomal recessive due to a gene on the short arm of chromosome 9. Homozygotes suf- fer from a congenital deficiency of the enzyme ga- lactosyl-1-phosphate uridyl-transferase, and galac- tose-1-phosphate accumulates in their tissues. They exhibit enlargement of the liver and spleen, cata- racts, and mental retardation.
Symptoms regress if galactose is removed from the diet. Prevalence 1/ 62,000. See Appendix C, 1971, Meril et al. galactosidase See alpha galactosidase, beta galac- tosidase. Galapagos finches See Darwin’s finches. Galapagos Islands a cluster of 14 islands that straddle the equator 650 miles west of Ecuador. Many of its species are found nowhere else in the world, such as marine iguanas, flightless cormorants, giant tortoises, and a special group of finches. The five weeks Darwin spent exploring these islands in 1835 were the crucial weeks of his scientific life.
See Appendix C, 1837, Darwin; Darwin’s finches; hot spot archipelago. Galapagos rift See rift. gall an abnormal growth of plant tissues. gallinaceous resembling domestic fowl. Gallus gallus domesticus the domesticated chicken, the bird for which the most genetic infor- mation is available. Its ancestor is the Asian red jun- gle fowl, from which it was domesticated around 8,000 BC. Its genome size is about 1 gbp and its number of protein coding genes is 23,000. Birds, snakes, and lizards have two classes of chromo- somes: macrochromosomes and microchromosomes. In chickens there are 10 macrochromosomes and 29 microchromosomes. The fifth chromosome in length is the metacentric Z, which occurs in duplicate in males.
The female has one Z and a smaller w chro- mosome. The macrochromosomes including the Z replicate synchronously. All the microchromosomes replicate late and so does the w of females. Roughly 60% of all the protein coding genes in the chicken have human orthologs. The haploid chromosome number is 39. The female is the heterogametic sex (ZW), whereas the male is the homogametic sex (ZZ). There are over 200 genes that have been