Gene conversions and crossing over were analyzed along 10 intervals in a 405-kb region comprising nearly all of the left arm of chromosome VII in Saccharomyces cerevisiae. Crossover interference was detected in all intervals as measured by a reduced number of nonparental ditypes. We have evaluated interference between crossovers in adjacent intervals by methods that retain the information contained in tetrads as opposed to single segregants. Interference was seen between intervals when the distance in the region adjacent to a crossover was Ͻ‫53ف‬ cM (90 kb). At the met13 locus, which exhibits ‫%9ف‬ gene conversions, those gene conversions accompanied by crossing over exerted interference in exchanges in an adjacent interval, whereas met13 gene conversions without an accompanying exchange did not show interference. The pattern of exchanges along this chromosome arm can be represented by a counting model in which there are three nonexchange events between adjacent exchanges; however, maximum-likelihood analysis suggests that ‫%21-8ف‬ of the crossovers on chromosome VII arise by a separate, noninterfering mechanism. I N meiosis, recombinational repair of double-strand independence of crossing over in two monitored intervals. Interference can also be measured by a lower-thanbreaks (DSBs) that results in gene conversion is frequently accompanied by crossing over (Fogel and Hurst expected incidence of nonparental ditypes (i.e., fourstrand double crossovers) within a single interval. In 1967), whereas an identical DSB is only rarely crossover associated in mitotic cells (Malkova et al. 1996). In budding yeast, mutations such as zip1, msh4, or mlh1, which reduce crossing over, also appear to eliminate the conmitotic cells, crossovers between homologous chromosomes may lead to disadvantageous loss of heterozygosstraints of interference (Ross-Macdonald and Roeder 1994; Sym and Roeder 1994; Khazanehdari and Borts ity; but in meiosis, crossovers play an important role not only in promoting genetic diversity, but in ensuring 2000). In contrast, mus81 or mms4 mutations reduce exchanges without affecting interference (de los Santos proper chromosome segregation (reviewed by Roeder et al. 2003). These observations support the idea that 1997; Zickler and Kleckner 1999; Walker and Hawbudding yeast have two pathways leading to crossovers, ley 2000).but only one of which imposes interference. Intimately connected to the problem of crossover conHow one recombination event might influence antrol at a single locus is the phenomenon of crossover other, at distances of tens of kilobases in yeast and much or chiasma interference, in which the proportion of greater distances in flies and mammals, is not well unclosely spaced crossovers is lower than what would be derstood. Various models have been proposed to exexpected from a random distribution. Interference is plain interference, ranging from those in which the seen in multifactor crosses that produce fewer double constraint is established at the time the DSB is first recombin...

Turner's syndrome and variations of Turner's syndrome including “mixed” gonadal dysgcnesis may be viewed as members of a single group of disorders, the gonadal dysgeneses. Seven patients are described whose various clinical and cytogenetic features make them represcntative of the many individuals with one of the gonadal dysgeneses reported from other clinics. The developmental abnormalities which occur, including dysgenetic gonadal tissue, may be traced to the presence during embryonic life of a monosomic (45, X) or a genetically deficient (e.g., 46, XXqi) cell line, whereas a degree of normal gonadal development in some individuals may he traced to the presence also of normal female or male cells. A unifying explanation for these disorders is presented in terms of a single erroneous chromosomal event occurring after fertilization. Such an event may well be an intrachromosomal rearrangement affecting either an X or a Y, from which may derive a cell line containing a structurally abnormal sex chromosome, a monosomic cell line, or both. The preponderance of affected individuals showing non‐mosaic monosomy (45, X), or the mosaic combination of monosomic cells with cells having a structurally rearranged X or Y chromosome, points to the zygote as the cell in which the disruptive event occurs most often. The less frequent co‐existence of normal diploid cells with abnormal (monosomic or deficient) cells indicates that the disruptive event sometimes occurs in an early postzygotic cell.

Mexico City, with a population estimated in 19 millions, is part of one of the largest metropolitan areas in the world. Unfortunately, scientific work on urban noise in Mexico City is scarce. A study on people reaction to urban noise carried out in open public spaces of two different zones of Mexico City will be presented. A previous survey in those zones showed that the noise levels exceeded the values recommended by international organizations to protect public health and welfare. The general objective of the study to be presented was to identify the sound sources perceived by the pedestrians, and to evaluate their response to urban noise pollution in the two mentioned areas. The results show that for most of the people a) urban noise is not considered a relevant aspect to improve these zones of the city, although urban noise has a negative effect on the decision of using open public spaces; b) the people who believe that noise pollution has negative effects are more annoyed; c) vehicular traffic is the most annoying noise source, contributing significantly the public transportation; d) one of the principal strategies to confront urban noise is to get used to it.

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