Showing papers on "X chromosome published in 1979"

Sequential X chromosome inactivation coupled with cellular differentiation in early mouse embryos.

Abstract: Only one of the two X chromosomes is active in each somatic cell of adult female eutherian mammals, making the females (XX) equivalent to the males (XY) with respect to X chromosome dosage1–4. Biochemical analyses showing that both X chromosomes are active in female mouse embryos in midcleavage stage4–8 indicate that X chromosome differentiation involves inactivation. This event occurs in most or all cells of the embryo at the blastocyst stage4,7–9, when there are two cell types, the outer sphere of trophectoderm cells and the inner cell mass (ICM). Because there is genetic evidence that both X chromosomes are potentially active in ICM cells10, it has been suggested that X chromosome inactivation has occurred in only the trophectoderm cells9. Further, one of us (M.M.)4 has proposed that X chromosome differentiation is linked to cellular differentiation, occurring at different times in different cell populations as they ‘depart’ or terminally differentiate from a pluripotent fetal ‘stem line’ (Fig. 1). Analysis of a large number of inner cell masses isolated immunosurgically from female blastocysts has yielded data consistent with the presence of two active X chromosomes11, but ICMs are so small that the biochemical assay used was at the limit of its accuracy. (Nevertheless, a computer analysis of the data8 indicated two ICM populations differing twofold with respect to X chromosome activity.) More tissue is available for analysis in post-implantation embryos, in which, on the above hypothesis, we would expect two active X chromosomes in the pluripotent epiblast region before gastrulation, but only one in the corresponding extra-embryonic ectoderm (a trophectoderm-derived tissue12) and primary endoderm (ICM-derived12, see Fig. 1). We report here that this is the case; we also show that inactivation is complete in the epiblast (fetal precursor) cells between 6.0 and 6.5 d of gestation at the onset of gastrulation.

Non-inactivation of an x-chromosome locus in man

Abstract: Cloned fibroblasts from women heterozygous for X-linked ichthyosis (steroid sulfatase deficiency) were examined to see whether or not this locus is subject to X-inactivation. Of 103 clones examined, all had normal levels of steroid sulfatase activity. Two of the women studied were also heterozygous for glucose-6-phosphate dehydrogenase deficiency. This allowed the demonstration that both X chromosomes were represented as the active X in various clones and that selection did not account for these findings. Thus, the steroid sulfatase locus, like the Xga locus to which it is linked, appears to escape X-inactivation in man.

Muscular dystrophy in an X; 1 translocation female suggests that Duchenne locus is on X chromosome short arm.

Abstract: A unique combination of a Duchenne-like muscular dystrophy in a girl with a translocation-inversion rearrangement involving an X chromosome and a no 1 chromosome appeared as a result of both gene mutation and chromosome mutation in the mother. The X-autosome rearrangement would permit full expression of an X-linked recessive gene, such as that for Duchenne muscular dystrophy, in a female, and this would satisfactorily explain the characteristic Duchenne-like course of our patient's illness. The simultaneous de novo appearance of the Duchenne mutation and the X;1 rearrange suggests possible sites for the Duchenne locus on the X chromosome short arm (at Xp1106 or Xp2107).

Regional assignment of the steroid sulfatase-X-linked ichthyosis locus: implications for a noninactivated region on the short arm of human X chromosome.

Abstract: The expression of steroid sulfatase (SS; sterol-sulfatase; sterol-sulfate sulfohydrolase, EC 3.1.6.2), a microsomal enzyme that catalyzes the hydrolysis of a variety of 3β-hydroxysteroid sulfates, was evaluated in mouse—human hybrid clones. The mouse parental line, A9, was found to have little SS as determined by activity measurements. Human SS can be separated from mouse SS by electrophoresis. Two human fibroblast lines, one carrying an X/13 translocation [46,X,t(X;13)(p22;q12)] and the other carrying an X/20 translocation [46,X,t(X;20)(Xp20q;Xq20p)] were used as the human parental lines. Several independently derived hybrid clones from each of the two fusion experiments were analyzed for the expression of human SS by activity measurements and electrophoresis. Cytogenetic analyses were done on these hybrid clones at the same passage level. The results showed that the expression of human SS in these cell hybrids was concordant only with the presence of the distal half (p22→pter) of the short arm of the human X chromosome, thus assigning the locus for SS to Xp22→Xpter. Earlier studies have shown that the deficiency of SS is the basis for the dermatologic condition X-linked ichthyosis, the gene for which is known to be located approximately 10 centimorgans from the Xg blood group locus. The localization of SS on the X chromosome indicates that Xg locus may be on the short arm of X and possibly on its distal half. The Xg locus is thought to escape X-inactivation in man, and recent investigations suggest that the SS locus also escapes X-inactivation. Our results thus provide evidence for the location of an apparently noninactivated site on the distal half of the short arm of the human X-chromosome that contains the locus for SS and possibly the Xg locus.

Immunological bases for superior survival of females.

Abstract: Evolutionary selection has equipped females with immunoregulatory genes on the X chromosome for coping with life-threatening illness. Five immunodeficiency syndromes occur solely in males, suggesting that they arise from mutant immunoregulatory genes located on the X chromosome. These syndromes, although rare, could contribute to poorer survival of males. Females have higher serum IgM concentrations, superior ability to form antibodies to infectious agents, and experience a lower incidence of viral and bacterial infectious diseases. Preponderance of autoimmune disorders in females could arise from modified immune responses owing to estrogens. Clinical and animal studies indicate that male hormones suppress autoantibody production whereas female hormones support their production. Superior immunocompetence and survival of females is based, in part, on their being protected from mutant immunoregulatory genes located on the X chromosome.

A new characteristic karyotypic anomaly in lymphoproliferative disorders

Abstract: A new characteristic chromosome anomaly t(11;14)(q14;q32?) in lymphoproliferative disorders (LPD) is described in 4 cases. The extra material was found on a "14 chromosome (14q+) and belonged to the long arm of one "11 chromosome in 3 cases and to the long arm of a "14 in the other case. These cases confirm that the distal end of chromosome 14q may function as a "receptor site," according to the hypothesis of Kaiser-McCaw et al. and also tend to indicate that chromosome "14 may not be unique in showing so-called "donor" and "receptor sites," and that other chromosomes, in casu chromosome "11, may behave similarly.

Heteromorphic X chromosomes in 46,XX males: Evidence for the involvement of X-Y interchange

Abstract: G- and R-banded chromosome preparations from eight of twelve 46,XX males, with no evidence of mosaicism or a free Y chromosome, were distinguished in blind trials from preparations from normal 46,XX females by virtue of heteromorphism of the short arm of one X chromosome. Photographic measurements on X chromosomes and on chromosome pair 7 in cells from twelve 46,XX males, eight 46,XX females, and four 46,XY males revealed a significant increase in the size of the p arm of one X chromosome in the group of XX males, independently characterised as being heteromorphic for Xp. No such differences were observed between X chromosomes of normal males and females or between homologues of chromosome pair 7 in all groups. The heteromorphism in XX males is a consequence of an alteration in shape (banding profile) and length of the tip of the short arm of one X chromosome, and the difference in size of the two Xp arms in these 46,XXp+ males ranged from 0.4% to 22.9%. From various considerations, including the demonstration of a Y-specific DNA fragment in DNA digests from nuclei of one of three XX males tested, it is concluded that the Xp+ chromosome is a product of Xp-Yp exchange. These exchanges are assumed to originate at meiosis in the male parent and may involve an exchange of different amounts of material. The consequences of such unequal exchange are considered in terms of the inheritance of genes located on Yp and distal Xp. No obvious phenotypic difference was associated with the presence or absence of Xp+. Thus, some males diagnosed as 46,XX are mosaic for a cryptic Y-containing cell line, and there is now excellent evidence that maleness in others may be a consequence of an autosomal recessive gene. The present data imply that in around 70% of 46,XX males, maleness is a consequence of the inheritance of a paternal X-Y interchange product.

Duplications in Caenorhabditis elegans.

Abstract: Thirteen chromosomal duplications, all unlinked to their linkage group of origin, have been identified following X-irradiation. Ten are X -chromosome duplications, of which six are half-translocations on three autosomomal linkage groups and four are free fragments. Five of the half-translocations are homozygous fertile and two are recognizable cytologically as chromosome satellites, both of which show some mitotic instability. The free- X duplications show varying tendencies for loss. Three appear not to overlap in extent previously identified free- X duplications. The fourth carries genes from linkage group V , as well as X . Three duplications of a portion of linkage group II were identified and found to be free and quite stable in hyperploids. Some of the free duplications tend to disjoin from the X chromosome in males. New X -chromosome map data are presented.

Assignment of the gene for cytoplasmic superoxide dismutase (Sod-1) to a region of chromosome 16 and of Hprt to a region of the X chromosome in the mouse.

Abstract: In the search for homologous chromosome regions in man and mouse, the locus for cytoplasmic superoxide dismutase (SOD-1; superoxide:superoxide oxidoreductase, EC 1.15.1.1) is of particular interest. In man, the SOD-1 gene occupies the same subregion of chromosome 21 that causes Down syndrome when present in triplicate. Although not obviously implicated in the pathogenesis, SOD-1 is considered to be a biochemical marker for this aneuploid condition. Using a set of 29 mouse—Chinese hamster somatic cell hybrids, we assign Sod-1 to mouse chromosome 16. Isoelectric focusing permits distinction between mouse and Chinese hamster isozymes, and trypsin/Giemsa banding distinguishes mouse from Chinese hamster chromosomes. The mouse fibroblasts used were derived from a male mouse carrying Searle's T(X;16)16H reciprocal translocation in which chromosomes X and 16 have exchanged parts. Analysis of informative hybrids leads to regional assignment of Sod-1 to the distal half of mouse chromosome 16 (16B4 → ter). Because the Chinese hamster cell line (380) used for cell hybridization is deficient in hypoxanthine phosphoribosyltransferase (HPRT; IMP: pyrophosphate phosphoribosyltransferase, EC 2.4.2.8), that part of the mouse X chromosome carrying the complementing Hprt gene can be identified by selection in hypoxanthine/aminopterin/thymidine medium and counterselection in 8-azaguanine. Mouse Hprt is on the XT translocation product containing the proximal region X cen → XD.

Genetic control of chromosome breakage and rejoining in Drosophila melanogaster: spontaneous chromosome aberrations in X-linked mutants defective in DNA metabolism

Abstract: Eight X-linked recombination-defective meiotic mutants (representing five loci) and 12 X-linked mutagen-sensitive mutants (representing seven loci) of Drosophila melanogaster have been examined cytologically in neuroblast metaphases for their effects on the frequencies and types of spontaneous chromosome aberrations. Twelve mutants, representing five loci, significantly increase the frequency of chromosomal aberrations. The mutants at these five loci, however, differ markedly both in the types of aberrations produced and the localization of their effects along the chromosome. According to these criteria, the mutants can be assigned to four groups: (i) mutants producing almost exclusively chromatid breaks in both euchromatin and heterochromatin; (ii) mutants producing chromatid and isochromatid breaks in both euchromatin and heterochromatin; (iii) mutants producing chromatid mutants producing chromatid and isochromatid breaks clustered in the heterochromatin.

DNA restriction endonuclease analysis for localization of human beta- and delta-globin genes on chromosome 11.

Abstract: DNA from a clone of a mouse-human hybrid that retained a human chromosome consisting of the major part of chromosome 11 and region q25-26-qter of the X chromosome was digested with various restriction endonucleases, subjected to electrophoresis in agarose gels, and transferred to nitrocellulose filters. The restriction digest pattern of the clone, when hybridized with a 32P-labeled plasmid fragment containing human beta-globin gene sequences, was a composite of the normal human and mouse (A9) patterns. When back-selected in 6-thioguanine to eliminate the 11 translocation chromosome, the hybrid cells showed only the A9 restriction pattern. These results substantiate the localization of beta- and delta-globin genes to human chromosome 11 and exclude the region 11q23-qter as the site.

Genetic mosaics and chimeras in mammals

Abstract: Use of Chimeras and Cell Aggregations to Study Developmental Potency, Gene Expression, Control of Phenotype, and Tumor Origin.- Reversion of malignancy and normalized differentiation of teratocarcinoma cells in chimeric mice.- Viable chimeras produced from normal and parthenogenetic mouse embryos.- Ovarian teratomas in mice are derived from oocytes that have completed the first meiotic division.- Growth control in chimeras.- Attempts at locating the site of action of genes affecting behavior.- Fine structure of cells in embryos chimeric for mutant genes at the T/t locus.- An analysis of differentiation in coaggregation cultures between mouse neuronal and tumor cells.- The use of mosaics and experimental chimeras to study the pathogenesis of neoplasia.- Antibodies to allozymes as potential cell markers for chimeric mouse studies.- Use of Chimeras and Mutational or Disjunctional Mosaics to Study Gamete Differentiation, Sexual Development, and Origin of the Germline.- Reproduction in single-sex chimeras.- Germ cells in XX ? XY mouse chimeras.- Use of the sex reversed gene (Sxr) to investigate functional sex reversal and gonadal determination in mammals.- Why not androgynes among mammals?.- Cell interactions in the sex organs of sex reversed mice heterozygous for testicular feminization.- Spontaneous sex mosaicism in BALB/cWt mice.- Evidence from mutable genes concerning the origin of the germ line.- X Chromosome Inactivation and Derepression.- Bimodal distribution of ?-galactosidase activities in mouse embryos.- Biochemical studies on X-chromosome activity in preimplantation mouse embryos.- Hypoxanthine guanine phosphoribosyl transferase expression in early mouse development.- The direct demonstration of an X-chromosome dosage effect prior to inactivation.- Use of teratocarcinoma stem cells as a model system for the study of X-chromosome inactivation in vitro.- The stability of X-chromosome inactivation: studies with mouse-human cell hybrids and mouse teratocarcinomas.- Studies of human-mouse cell hybrids with respect to X-chromosome inactivation.- X-Chromosome Nonrandomness: Imprinting, Selection.- Preferential inactivation of the paternally derived X chromosome in mice.- Preferential expression of the maternally derived X chromosome in extraembryonic tissues of the mouse.- In search of nonrandom X inactivation: studies of the placenta from newborns heterozygous for glucose-6-phosphate dehydrogenase.- The use of mouse X-autosome translocations in the study of X-inactivation pathways and nonrandomness.- Selection and cell communication as determinants of female phenotype.- Mathematical and Statistical Analyses of Mosaic Patterns.- Clonal growth versus cell mingling.- Combinatorial and computer analysis of random mosaics.- Data analysis of four X-chromosome inactivation experiments.

Sexual and somatic determinants of the human Y chromosome: studies in a 46,XYp- phenotypic female.

Abstract: A case of a 46,XYp- phenotypic female provided an opportunity to evaluate both sexual and somatic determinants for the Y chromosome. The patient had multiple stigmata of Turner syndrome, but normal stature. Laparotomy revealed a normal uterus and tubes, with 1.5 cm undifferentiated gonads. Serological tests for H-Y antigen (ostensibly the product of Y-chromosomal testis-determining genes) indicated absence of the H-Y+ phenotype normally associated with the intact Y chromosome. We conclude that genes exist on the short arm of the human Y chromosome which both suppress some of the somatic stigmata of Turner syndrome and determine normal expression of H-Y antigen and testicular differentiation of the primitive gonad. Our data are consistent with the view that H-Y genes comprise a family of testis-determinants, and that loss of a critical moiety is inconsistent with normal development of the male gonad.

The genetics of a small autosomal region of Drosophila melanogaster containing the structural gene for alcohol dehydrogenase. II. Lethal mutations in the region.

Abstract: Forty-seven lethal mutations and alleles of nine visible loci (including alcohol dehydrogenase) have been mapped by both deficiency mapping and, in most cases, by recombination mapping to a small region (34D-35C) of chromosome arm 2L of Drosophila melanogaster. The lethals fall into approximately 21 complementation groups, and we estimate that the total number of lethal plus visible complementation groups within the 34-band deficiency, Df(2L)64j, is approximately 34, a remarkable numerical coincidence. The possible genetic significance of this coincidence is discussed. Lethals mapping close to the structural gene for alcohol dehydrogenase, both distally and proximally, have been identified and will be used for the construction of selective crosses for the study of exchange within this locus. Despite many abnormal cytological features (e.g., ectopic pairing, weak points) region 35 of chromosome arm 2L does not display any unusual genetic features; indeed, in terms of the amount of recombination per band and the average map distance between adjacent loci, this region is similar to that between zeste and white on the X chromosome.

Late replication in an X-autosome translocation in the mouse: correlation with genetic inactivation and evidence for selective effects during embryogenesis.

Abstract: A technique involving 5-bromodeoxyuridine, 33258 Hoechst, and fluorescence microscopy has been used to analyze replication kinetics in cells from embryonic and adult mice bearing the Cattanach [T(X;7)ICt] translocation in a balanced or an unbalanced form. In balanced 9- and 13-day female embryos, the translocated X was late replicating in 28 and 22% of the cells, respectively, whereas it was late replicating in only 13% of adult cells. In contrast, in unbalanced females, the translocated X was late replicating in 62 and 70% of 9- and 13-day embryos and in 70% of adult cells. Such divergent late replication frequencies suggest the operation, during development, of selection against cells with extreme genetic imbalance. Within a late-replicating translocated X chromosome, the autosomal segment itself replicated late approximately half of the time, regardless of karyotypic balance. The late replication data are consistent with the measurements of levels of mitochondrial malic enzyme (MOD-2, whose locus is on the autosomal segment) activity in these mice [Eicher E. & Coleman, D. (1977) Genetics 85, 647-658]. The present study also shows a dissociation between the replication timing in X chromatin distal and proximal to the autosomal segment, supporting the hypothesis of at least two inactivation centers in the X chromosome.

X-linked mental retardation with macro-orchidism and marker X chromosomes.

Abstract: A family with X-linked mental retardation and a marker X chromosome was ascertained by the presence of macro-orchidism in the three institutionalized probands. Verbal evaluation revealed a generalized language disability with commonly occurring articulation errors. The heterozygous females in this family exhibited some reduction in mental ability; the marker X chromosome was demonstrated in both sexes.

Regional localization of the gene for human phosphoribosylpyrophosphate synthetase on the X chromosome.

Abstract: Sixty-eight independent hybrid clones were isolated after irradiated normal human lymphocytes were fused with Chinese hamster fibroblasts lacking hypoxanthine-guanine phosphoribosyltransferase activity. The cells were grown under selective conditions requiring retention of the X chromosome-linked locus for human hypoxanthine-guanine phosphoribosyltransferase. The frequency and patterns of cotransference of human phosphoribosylpyrophosphate synthetase with the selected marker and with additional X-linked enzymatic markers confirm X linkage of the structural gene for human phosphoribosylpyrophosphate synthetase and support assignment of this gene to a position on the long arm of the X, between the loci for alpha-galactosidase and hypoxanthine-guanine phosphoribosyltransferase.

Meiotic studies of translocations causing male sterility in the mouse. II. Double heterozygotes for Robertsonian translocations.

Abstract: Unusual meiotic behavior of the XY chromosome pair was observed in sterile male mice doubly heterozygous for two Robertsonian translocations, Rb(16.17)7Bnr and Rb(8.17)1Iem. Nonrandom association between the X chromosome and the translocation configuration, ascertained from the frequencies of relevant C-band contacts, was found in 9 of 10 sterile males. Besides the nonrandom association, the XY chromosomes showed signs of impaired condensation, as judged by measurement of their lengths at diakinesis/MI of the first meiotic division. In contrast, neither nonrandom contact nor decondensation of the XY chromosomes pair was found in fertile males heterozygous for a single Robertsonian translocation, Rb1Iem or Rb7Bnr. The present observations lend indirect support to the working hypothesis advanced previously, the assumption that interference with X-chromosome inactivation is a possible cause of spermatogenic breakdown in carriers of various male-sterile chromosomal transloations. Alternative explanations of the available data, which cannot be ruled out, are briefly discussed.

Regional localization of human gene loci on chromosome 9: studies of somatic cell hybrids containing human translocations.

Abstract: Somatic cell hybrids were derived from the fusion of (1) Chinese hamster cells deficient in hypoxanthine guanine phosphoribosyltransferase (HPRT) and human cells carrying an X/9 translocation and (2) Chinese hamster cells deficient in thymidine kinase (TK) and human cells carrying a 17/9 translocation. Several independent primary hybrid clones from these two series of cell hybrids were analyzed cytogenitically for human chromosome content and electrophoretically for the expression of human markers known to be on human chromosome 9. The results allow the assignment of the loci for the enzymes galactose-1-phosphate uridyltransferase (GALT), soluble aconitase (ACONs), and adenylate kinase-3 (AK3) to the short arm of chromosome 9 (p11 to pter) and the locus for the enzyme adenylate kinase-1 (AK1) to the distal end of the long arm of human chromosome 9 (hand q34). Earlier family studies have shown that the locus for AK1 is closely linked to the ABO blood group locus and to the locus of the nail-patella (Np) syndrome. Thus the regional localization of AK1 locus permits the localization of the AK1-Np-ABO linkage group.

Derived X chromosome in the turtle genus Staurotypus

Abstract: C-banding, G-banding, and silver (Ag-AS) staining techniques reveal a distinctive sex chromosome system in the turtle Staurotypus salvinii. Unlike previously described systems in most other vertebrate groups in which the Y or W is derived and the homogametic sex represents the primitive condition, the reverse is true for S. salvinii. The X chromosome is derived; thus the homogametic sex (female) is more derived than the heterogametic sex. The male is intermediate between the female and the ancestral condition observed in other turtle species. Staurotypus does not confirm to the general model of sex chromosome evolution for diploid dioecious organisms.

Partial deletion of chromosome No. 2 in myelocytic leukemias of irradiated C3H/He and RFM mice.

Abstract: Chromosomes of mouse myelocytic leukemias that developed in 7 irradiated mice, 3 C3H/He males, 1 RFM female, and 3 RFM males were analyzed with chromosome-banding techniques. Chromosomes No. 2 were partially deleted in 6 of the 7 mice. Although the deleted No. 2 chromosomes varied in size in the 6 mice, one common characteristic was noted in all these deletions: A segment lying between a certain band in the region 2C and a band in the region 2E, including the whole region 2D, was missing. Another consistent abnormality was an addition or a loss of the Y-chromosomes in the fraction of cells in all 6 males. In addition to these consistent abnormalities, various chromosomes had structural abnormalities. The RFM female, which did not have the abnormal No. 2 chromosome, had abnormalities in chromosomes No. 3, 4, 11, 12 and 15 and in the X-chromosome. Of the 20 chromosome pairs, only such chromosomes as No. 1, 5, 8, 14, 17, and 19 and the Y-chromosome did not have the structural abnormalities. The possible role of the partial deletion of the No. 2 chromosome was considered in relation to the development of mouse myeloid leukemias.

Inherited interstitial del(Xp) with minimal clinical consequences: with a note on the location of genes controlling phenotypic features.

Abstract: In a routine cytogenetic investigation of the outpatients of a hospital for the mentally retarded, a 26-year-old women with a presumptive interstitial deletion of the short arm of one of the X chromosomes was found. The same aberration was found in her phenotypically normal mother and in one of her four sisters, all phenotypically normal. By GTG- and QFQ-banding methods, the deletion was interpreted to involve the entire band Xp21 and adjacent parts of p11 and p22. The karyotype is written 46,X,del(X)(pter leads to p22::p11 leads to qter). By autoradiography and Bud R acridine orange technique, the deleted X was the late replicating one in all three affected persons. The deletion apparently causes shortness of stature but no other phenotypic symptoms or signs. Hence a gene or genes controlling stature is located in band Xp21 or regions immediately adjacent to this band. Since the absence of this region does not cause streak gonads, it does not contain genes controlling the formation of the ovaries. This appears to be the first example of a heritable chromosome deletion compatible with a normal phenotype and reproduction.

The Genetic and Cytogenetic Localization of the Three Structural Genes Coding for the Major Protein of Drosophila Larval Serum

Abstract: The α, β and γ polypeptides that make up Drosophila Larval Serum Protein-1 seem to be coded for by genes that have evolved by duplication of a common ancestral gene. We have found variants of all three polypeptides, and these are variants of the coding sequences. The α-chain variant mapped to 39.5 on the X chromosome and to the polytene interval 11A7-11B9. The β-chain variant mapped to 1.9 on chromosome 2L and to 21D2-22A1. The γ-chain variant was mapped as 0.13 map units from the tip of chromosome 3L or to —1.41 with respect to ru, which has been defined as 0.0, and to 61A1-61A6.

Imbalance in X-chromosome expression: evidence for a human X-linked gene affecting growth of hemopoietic cells

Abstract: In each of six family members who were heterozygous at the X-linked locus for glucose-6-phosphate dehydrogenase, only one or the other of the two alleles at that locus was almost exclusively expressed. The data are consistent with evidence that X-chromosome inactivation is a random process that may be followed by selection for one of the two resulting cell types on the basis of an unknown gene, which is located on the X chromosome and which can affect the rate of proliferation of hemopoietic cells in humans.

Late replicating X chromosomes in human triploidy.

Abstract: The status of X-chromosome replication was studied in twenty-seven 69,XXY and nine 69,XXX human triploids in which the parental origin of the additional haploid set was known from the study of chromosome heteromorphisms. Among the 69,XXY triploids, fourteen had no late replicating X, two had one late replicating X in all cells examined, and eleven had two populations of cells, one with late replicating X chromosome, and one without any. Among the 69,XXX triploids, four had a single late replicating X, and five had two populations of cells, one with one late replicating X, and one with two late replicating X chromosomes. There was no correlation between the parental origin of the triploidy and the type of X-chromosome inactivation. However the number of late replicating X chromosomes was significantly lower in cultures grown from fetal tissue when compared with those grown from extra-embryonic tissue. In cultures derived from extra-embryonic tissue there was a significant correlation between the gestational age of the sample and the proportion of late replicating X chromosomes. The older the specimen, the greater the number of late replicating X chromosomes.