Adult female carriers of balanced X; autosome translocations (118 cases) and of balanced X inversions (31 cases) have been collected from the literature. Forty-five of the 118 translocation carriers in whom the break was in the critical region (Xq13-q22, Xq22-q26, separated by a narrow region within Xq22) showed gonadal dysgenesis. Seven of the 31 inversion carriers in whom the break was in the same region also had gonadal dysgenesis, whereas the remaining 24 were normal in this respect. The critical region consists mainly of Q-bright material, and is the fifth brightest segment in the human genome. The region contains relatively few genes. It is possible that meiotic crossing-over, rarely, if ever, takes place in it. The critical region may therefore consist of two "supergenes" whose integrity must be maintained to allow normal ovarian development. The effect exerted by this region differs from other known position effects, in that it is independent of the breakpoint within the region and of the chromosome bands to which the broken ends are attached. One possible mechanism causing this effect might be a change in the replication order of the chromosome bands, which, in turn, might affect their function.
The present study explores the origin of human Robertsonian translocations (RT) and the causes of the nonrandom participation of the different acrocentrics in them. Satellite associations have been analysed in 966 cells from 8 persons, and 1266 RT with ascertainment have been collected from the literature. The observation that the chromosomes preferentially taking part in satellite associations vary between individuals is confirmed. However, since a preferred chromosome appears to associate at random with the others, this phenomenon should not add to the nonrandomness of the RT. Most RT presumably arise through adjacent chromatid exchanges corresponding to mitotic chiasmata, in the pericentric regions of the acrocentrics. Our working hypothesis is that there is a basic exchange rate between any two acrocentrics. The surplus of t(14q21q) is presumed to depend on these two chromosomes having a homologous pericentric region. The 10-20 times higher incidence of t(13q14q) as compared with other RT is best explained by crossing-over between homologous, but relatively inverted, segments in these chromosomes. Of the 246 RT ascertained through repeated abortions or infertility, 56 were found through the latter. Of these, chromosome 14 was involved in 51. The infertility may be caused by a small deletion of 14q, as is often the case in 15q in Prader-Willi syndrome. In all RT ascertained through 21 or 13 trisomy, respectively, the relevant chromosome is one of the participants. Our data thus do not give any support to the idea of interchromosomal effects exerted by RT.
Enamel thickness of the maxillary permanent central incisors and canines in seven Finnish 47,XXX females, their first-degree male and female relatives, and control males and females from the general population were determined from radiographs. The results showed that enamel in teeth of 47,XXX females was clearly thicker than that of normal controls. On the other hand, the thickness of "dentin" (distance between mesial and distal dentinoenamel junctions) in 47,XXX females' teeth was about the same as that in normal control females, but clearly reduced as compared with that in control males. It is therefore obvious that in the triple-X chromosome complement the extra X chromosome is active in amelogenesis, whereas it has practically no influence on the growth of dentin. The calculations based on present and previous results in 45,X females and 47,XYY males indicate that the X chromosome increases metric enamel growth somewhat more effectively than the Y chromosome. Possibly, halfway states exist between active and repressed enamel genes on the X chromosome. The Y chromosome seems to promote dental growth in a holistic fashion.
The origin and behavior of human dicentric chromosomes are reviewed. Most dicentrics between two nonhomologous or two homologous chromosomes (isodicentrics), which are permanent members of a chromosome complement, probably originate from segregation of an adjacent quadriradial; such configurations are the result of a chromatid translocation between two nonhomologous chromosomes, or they represent an adjacent counterpart of a mitotic chiasma. The segregation of such a quadriradial may also give rise to a cell line monosomic for the chromosome concerned (e.g., a 45, X line). Contrary to the generally held opinion, isodicentrics rarely result from an isolocal break in two chromatids followed by rejoining of sister chromatids. In this case the daughter centromeres go to opposite poles in the next anaphase, and the resulting bridge breaks at a random point. This mechanism, therefore, leads to the formation of an isodicentric chromosome only if the two centromeres are close together, or if one centromere is immediately inactivated. Observations on the origin of dicentrics in Bloom syndrome support these conclusions. One centromere is permanently inactivated in most dicentric chromosomes, and even when the dicentric breaks into two chromosomes, the centromere is not reactivated. The appearance and behavior of the "acentric" X chromosomes show that their centromeres are similarly inactivated and not prematurely divided. Two Bloom syndrome lymphocytes, one with an extra chromosome 2 and the other with an extra chromosome 7, each having an inactivated centromere, show that this can also happen in monocentric autosomes.
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