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排序方式: 共有285条查询结果,搜索用时 15 毫秒
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Ariella Oppenheim Avi Yaari Deborah Rund Eliezer A. Rachmilewitz David Nathan Corinne Wong Haig H. Kazazian Jr. Barbara Miller 《Human genetics》1990,86(2):175-180
Summary The mechanism for elevated production of fetal hemoglobin (Hb F) in a Druze patient with °-thalassemia intermedia was investigated. Heterozygous family members exhibited normal Hb F levels, suggesting that the increase in -gene expression in the propositus may be partly due to anemic stress. Erythroid progenitors of these family members cultured in vitro [burst forming units (erythroid); (BFUe)] showed elevated synthesis of Hb F, indicating the existence of a genetically determined intrinsic capacity for high Hb F production in this family. The propositus was found to be homozygous for a IVS2-position 1 mutation, on the background of Mediterranean haplotype I, which is not known to be linked to high Hb F production. Moreover, extensive molecular studies of the -globin gene cluster, including sequence analysis of the promoter regions of the -globin genes, did not reveal any cisacting mechanism that could account for the high Hb F production in the propositus. A young niece of the propositus with °-thalassemia major was recently discovered, who was homozygous for the same -globin allele and haplotype as the propositus. However, unlike her uncle, she does not have a high Hb F level and presents with a severe clinical course. Her inability to produce high Hb F suggests that the genetic determinant for increased -gene expression in the propositus is unlinked to the -globin gene cluster. 相似文献
3.
Seed size,pollination costs and angiosperm success 总被引:2,自引:0,他引:2
Summary Seed plants capture pollen before seeds are dispersed and abort unpollinated ovules. As a result, each seed is associated with an accessory cost that represents the costs of pollen capture and the costs of aborted ovules. Accessory costs may explain the minimum seed size among species, because these costs are likely to comprise a greater proportion of total reproductive allocation in species with smaller seeds. For very small propagules, the costs of pollination may not be worth the benefits, perhaps explaining the persistence of pteridophytic reproduction at small propagule sizes. The smallest angiosperm seeds are much smaller than the smallest gymnosperm seeds, both in the fossil record and in the modern flora. This suggests that angiosperms can produce pollinated ovules more cheaply than gymnosperms. Pollination becomes less efficient as a species decreases in abundance, and this loss of efficiency is greater for species with a higher accessory cost per seed. We propose that the greater reproductive efficiency of angiosperms when rare can explain why angiosperm-dominated floras were more speciose than the gymnosperm-dominated floras they replaced. 相似文献
4.
5.
David Haig 《Journal of evolutionary biology》1993,6(1):69-77
Coccoids (scale insects) exhibit a wide variety of chromosomal systems. In many species, paternal chromosomes are eliminated from the male germline such that all of a male's sperm transmit an identical set of maternal chromosomes. In such species, an offspring's sex is determined by whether or not paternal chromosomes are inactivated in the egg's cytoplasm after fertilization. This paper presents a model of the evolution of paternal genome loss in coccoids from an ancestral system of XX-XO sex determination. The model is based on Hamilton's (1967) theory that different genetic elements within the genome have different unbeatable sex ratios. In this model (1) meiotic drive by the X chromosome in XO males causes female-biased sex ratios; (2) the maternal set of autosomes in males evolves effective sex linkage to exploit X-drive; and (3) genes expressed in mothers are selected to convert some of their XX daughters into sons. A similar model may explain the evolution of haplodiploidy. 相似文献
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7.
We examined the genetic structure of snowy plovers (Charadrius alexandrinus) in North America, the Caribbean, and the west coast of South America to quantify variation within and among breeding areas
and to test the validity of three previously recognized subspecies. Sequences (676 bp) from domains I and II of the mitochondrial
control region were analyzed for 166 snowy plovers from 20 breeding areas. Variation was also examined at 10 microsatellite
loci for 144 snowy plovers from 14 breeding areas. The mtDNA and microsatellite data provided strong evidence that the Puerto
Rican breeding group is genetically divergent from sites in the continental U.S. (net sequence divergence = 0.38%; F
ST for microsatellites = 0.190). Our data also revealed high levels of differentiation between sites from South America and
North America (net sequence divergence = 0.81%; F
ST for microsatellites = 0.253). In contrast, there was little genetic structure among breeding sites within the continental
U.S. Our results suggest that snowy plovers in Florida should be considered part of C. a. nivosus (rather than part of C. a. tenuirostris, where they are currently placed), whereas snowy plovers from Puerto Rico should be considered part of C. a. tenuirostris. Snowy plovers in South America should remain a separate subspecies (C. a. occidentalis). Although U.S. Pacific and Gulf Coast breeding areas were not genetically distinct from other continental U.S. sites, demographic
isolation, unique coastal habitats, and recent population declines suggest they warrant special concern. 相似文献
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9.
Selig Hecht Charles D. Hendley Sylvia R. Frank Charles Haig 《The Journal of general physiology》1946,29(5):335-351
1. Brightness discrimination has been studied with individuals breathing oxygen concentrations corresponding to 7 altitudes between sea level and 17,000 feet. The brightnesses were 0.1, 0.01, and 0.001 millilambert involving only daylight (cone) vision. 2. At these light intensities, brightness discrimination begins to deteriorate at fairly low altitudes. The deterioration is obvious at 8,000 feet, and becomes marked at 15,000 feet, where at low brightness, the contrast must be increased 100 per cent over the sea level value before it can be recognized. 3. The impairment of brightness discrimination with increase in altitude is greater at higher altitudes than at lower. The impairment starts slowly and becomes increasingly rapid the higher the altitude. 4. Impairment of brightness discrimination varies inversely with the light intensity. It is most evident under the lowest light intensities studied, but shows in all of them. However, it decreases in such a way that the deterioration is negligible in full daylight and sunlight. 5. The thresholds of night (rod) vision and day (cone) vision are equally affected by anoxia. 6. The quantitative form of the relation between brightness discrimination ΔI/I and the prevailing brightness I remains the same at all oxygen concentrations. The curve merely shifts along the log I axis, and the extent of the shift indicates the visual deterioration. 7. The data are described in terms of retinal chemistry. Since anoxia causes only a shift in log I it is shown that the photochemical receptor system cannot be affected. Instead the conversion of photochemical change into visual function is impaired in such a way that the conversion factor varies as the fourth power of the arterial oxygen saturation. 相似文献
10.
Many commercial chrysanthemum cultivars display unusual somatic variability. The ‘Indianapolis’ family of chrysanthemum sports was analyzed for the genetic potential for color of each of the three layers in the apical meristem of their shoots. Populations of each cultivar were grown and sectors and off-color plants recorded. The location of the pigment within cells and between tissues was determined by microscopic examination of free-hand sections of fresh petals. Adventitious buds were forced from the stems of each cultivar by excising all normal lateral buds. These observations, showed 12 of the 16 ‘Indianapolis’ cultivars to be periclinal chimeras. Adventitious shoots often originated from two or more cells, derived from at least two different apical layers, and thus were themselves periclinal chimeras. While somatic mutation is the ultimate source of the variability in ‘Indianapolis’ chrysanthemums, the most frequent type of sporting resulted from the loss in mitosis of a chromosome carrying a supressor for the formation of yellow chromoplasts, giving a yellow sector or shoot. Sectors resulting from rearrangement of layers in the periclinal chimeras were less frequent than the sectors from chromosome loss. 相似文献