Female Sex Chromosome Research Articles

Posttransplant Lymphoproliferative Disorder in a Kidney-Pancreas Transplanted Recipient

Posttransplant lymphoproliferative disorders (PTLDs) are lymphoid proliferations or lymphomas that develop as a consequence of immunosuppression after solid organ or bone marrow transplantation and are mostly associated with an Epstein-Barr virus infection. The morphologic categories include different types of benign and malignant lymphoid proliferations. The majority of PTLDs is of B-cell origin with clonal rearrangements of the immunoglobulin genes. The PTLDs in solid organ transplants are reported to be either of host or of donor origin. Donor-related PTLDs frequently involve the allograft. We report a case of a 52-year-old woman recipient who developed simultaneously PTLDs in several organs 5 month after receiving a sex-mismatched renal and pancreas allograft. Immunosuppression regimen comprised antithymocyte globulin, tacrolimus, mycophenolate mofetil, and steroids. Pathologic features appeared as polymorphic PTLDs in the renal allograft, liver, and central nervous system (CNS). Molecular genetic studies revealed different clonal immunoglobulin heavy chain gene rearrangements in all 3 organs as determined by polymerase chain reaction (PCR). Epstein-Barr virus were detected by nested PCR and in situ hybridization in all 3 tumors. The PTLDs in liver and CNS were of host origin whereas the allograft kidney PTLD was found to originate from the male donor as shown by the simultaneous detection of female and male sex chromosomes by PCR and fluorescence in situ hybridization. The recipient died in consequence of the CNS involvement, after intracerebral hemorrhage with uncal and tonsillar herniation.

Premature Ovarian Failure (POF) Syndrome: Towards the Molecular Clinical Analysis of its Genetic Complexity

The Premature Ovarian Failure (POF) syndrome is a very heterogeneous clinical disorder due probably to the complex genetic networks controlling human folliculogenesis. Clinical subgroups of POF patients whose aetiology of ovarian failure is based on the same genetic factors are therefore difficult to establish. Some experimental evidence suggests that these genes might be clustered on the female sex chromosome in the POF1 and POF2 loci. This review is aimed to present an overview of the actual structural changes of the X chromosome causing POF, and to present a number of X and autosomal female fertility genes which are probably key genes in human folliculogenesis and are therefore prominent POF candidate genes. Towards the molecular analysis of their functional contribution to the genetic aetiology of POF in the clinic, an interdisciplinary scheme for their diagnostic analysis is presented in a pilot study focussed on chromosome analyses and the expression analysis of some major POF candidate genes (DAZL, DBX, FOXL2, INHalpha, GDF9, USP9X) in the leukocytes of 101 POF patients. It starts with a comprehensive and significantly improved clinical diagnostic program for this large and heterogeneous patient group.

Sex chromatin body as a marker of radiation-induced sex chromosome aberrations in the potato tuber moth, Phthorimaea operculella (Lepidoptera: Gelechiidae)

Genetic sexing technique based on the construction of a balanced lethal strain (BLS) has been proposed for Phthorimaea operculella (Zeller). The isolation of female with T(W;Z) translocation is a fundamental step to develop such a strain. Gamma irradiation was used to induce the requested translocations. The availability of sex-linked morphological marker is required to facilitate the detection of such mutations. Since a visible sex-linked marker has not been found in P. operculella, the main objective of our study was therefore to determine the possibility of using sex heterochromatin body as a marker to identify the required translocated females. The appearance of sex heterochromatin body and the analysis of sex chromosomes in F1 females of irradiated P. operculella females were investigated. The percentage of abnormality in sex heterochromatin body in highly polyploid Malpighian tubule nuclei was positively correlated with the applied dose. Based on the appearance of this body, three mutant lines were isolated: elongated, small and fragmented lines. W chromosome was easily distinguished from Z chromosome when the analysis of pachytene sex chromosome bivalents of P. operculella females was carried out. The aberrations involved in W chromosome directly influenced the appearance of sex heterochromatin body in highly polyploid somatic cells of the isolated mutant lines. The results showed that sex heterochromatin could be used as sex determination and cytogenetic marker in P. operculella.

Gender does not impact the developmental rate of in vitro fertilization-derived human embryos as confirmed by day 3 blastomere biopsy and preimplantation genetic diagnosis (PGD)

Background: An increase in male offspring has been described in different mammalian species following the transfer (ET) of faster-cleaving embryos. An association between ET of more advanced embryos and a higher male to female offspring sex ratio has also been observed by some in vitro fertilization (IVF) programs especially when human embryos are transferred at the blastocyst stage. Objective: To determine if gender has any impact on the developmental rate of IVF-derived human embryos prior to implantation. Materials and Methods: An IRB-approved retrospective review of 500 embryos undergoing PGD (67 IVF cycles) from 2002–2003. Oocyte retrieval and ICSI were performed according to standard protocols. Embryos were biopsied on day 3 of development and gender was determined by fluorescence in situ hybridization (FISH) with probes specific for the X and Y chromosomes. Embryos were classified by FISH as having a normal [female (XX) or male (XY)] or abnormal complement of sex chromosomes. After biopsy, embryos at 6 cells or greater were placed into extended culture and morphologically assessed for ET at the blastocyst stage of development. Embryos were also assessed at 24 hours postinsemination for cleavage status (cleavage, pronuclei/syngamy [PN-S], or pronuclei/non-syngamy [PN-NS]), and on day 2 and day 3 for blastomere number. Statistical analysis was performed using Chi-square analysis. Results: FISH analysis revealed 399 normal embryos (186 male:213 female) and 101 abnormal embryos (35 male: 54 female:12 nullisomic). Cleavage stage development rates between male or female embryos (normal [Tables I-III], abnormal or total) were equivalent. Four hundred and forty-one (441) cleavage stage embryos were available for extended culture of which 357 were normal (167 male:190 female) and 84 were abnormal (32 male:44 female:8 nullisomic). In extended culture, no difference was identified in developmental rates between normal male or female embryos (Table IV) but embryos abnormal for male or female sex chromosomes exhibited a slower developmental rate (p<0.001). Conclusions: Gender does not impact the rate of development through early embryogenesis. Consistent with reported literature, the decrease in development of embryos with abnormal sex chromosome complements in extended culture is most likely related to aneuploidy and not gender. ART- Preimplantation Genetic Diagnosis

Evidence for male XO sex-chromosome system in Pentodon bidens punctatum (Coleoptera Scarabaeoidea: Scarabaeidae) with X-linked 18S-28S rDNA clusters.

In scarab beetle species of the genus Pentodon, the lack of analysis of sex chromosomes in females along with the poor characterization of sex chromosomes in the males, prevented all previous investigations from conclusively stating sex determination system. In this study, somatic chromosomes from females and spermatogonial chromosomes from males of Pentodon bidens punctatum (Coleoptera: Scarabaeoidea: Scarabaeidae) from Sicily have been analyzed using non-differential Giemsa staining. Two modal numbers of chromosomes were obtained: 2n = 20 and 19 in females and males, respectively. This finding along with other karyological characteristics such as the occurrence of one unpaired, heterotypic chromosome at metaphase-I and two types of metaphase-II spreads in spermatocytes demonstrate that a XO male/XX female sex determining mechanism - quite unusual among Scarabaeoidea - operates in the species investigated here. Spermatocyte chromosomes have also been examined after a number of banding techniques and fluorescent in situ hybridization with ribosomal sequences as a probe (rDNA FISH). The results obtained showed that silver and CMA(3) staining were inadequate to localize the chromosome sites of nucleolus organizer regions (NORs) due to the over-all stainability of both constitutive heterochromatin and heterochromatin associated to the NORs. This suggests that heterochromatic DNA of P. b. punctatum is peculiar as compared with other types of heterochromatin studied so far in other invertebrate taxa. By rDNA FISH major ribosomal genes were mapped on the X chromosome.

Estimating Disease Risk with Genetic Screening

It is more than a year since the complete sequence of the human genome was publicly announced (International Human Genome Sequencing Consortium, 2001). The impact of this milestone on medical research is already apparent. Scientists are geared up to adopt new experimental approaches in the race to combat human disease. New information flowing from the human genome project should fuel a revolution in medical research with wide-ranging implications for human health, longevity and employability. There are very good reasons to collect and use all of this information for the benefit of mankind, to improve health care and sustain life. It remains for society to dictate the level of protection placed on personal genetic information. Checks and balances will need to be implemented sooner rather than later. However, the time may come when the genetic blueprint of an individual will be viewed no differently to any other personal information. The potential to predict disease risk by genetic screening is considered here along with the health and medical benefits which should flow from new approaches to medical research fuelled by completion of the human genome project. Genetics Today The science of genetics originated in the mid-1800s with the experiments of Gregor Mendel who investigated the inheritance of characteristics in pea plants. While the classical genetics experiments of the early 20 th century identified many inherited traits, it was the discovery of the DNA double helix in 1953 by Watson and Crick which led to identification of DNA as the carrier of genetic information. All somatic cells in an individual carry genes from both the father and mother and these are inherited en bloc on chromosomes. The human cell has 23 different chromosome pairs including an XX pair of sex chromosomes in females and an XY pair in the male. The approximately 30,000 genes carried by the 46 chromosomes represent the human genome. Each chromosome is made up of a string of four different nucleotides or bases, making up a four-letter code referred to as the sequence. For a given gene, the nucleotide sequence forms the genetic code, represented by triplets of bases. Genes are aligned along this strand, delineated by regions of coding rather than non-coding DNA. Each gene is first transcribed into an RNA intermediate form and then the base triplets are translated into amino acids. The amino acids are

Chromosome banding in Amphibia

The chromosomes of the rare South American marsupial frogs Gastrotheca walkeri and G. ovifera were extensively reexamined with various banding techniques. The karyotypes of both species are distinguished by a new category of XY♀/XX♂ female sex chromosomes. The unusual Y chromosomes are characterized by containing the least amount of constitutive heterochromatin in the karyotypes. This is in contrast to all previously known amphibian Y chromosomes and does not fit the evolutionary model of early XY differentiation in vertebrates. In male meiosis, the heteromorphic XY chromosomes of both species still exhibit the same pairing configurations as the autosomes. DNA flow cytometric measurements show the nuclear DNA amount of G. walkeri to be 10.90 pg. The significance of the XY/XX sex chromosomes of these marsupial frogs, the various classes of constitutive heterochromatin detected, and the data obtained from meiotic analyses are discussed in detail.

Karyotypes found in the population declared at increased risk of Down syndrome following maternal serum screening.

Of the 65 328 pregnancies of South Australian mothers screened by the South Australian Maternal Serum Antenatal Screening (SAMSAS) Programme between 1 January 1991 and 31 December 1997, 3431 (5.25%) were declared at increased risk of fetal Down syndrome. Fetal or neonatal karyotype was determined in 2737/3431 (79.8%) of these pregnancies, including 16 with early fetal loss. Interrogation of the database of the South Australian Neonatal Screening Service showed 643 live-born infants whose phenotype was not subsequently questioned among the 694 pregnancies whose karyotype was not determined. Of the remaining 51/3431 pregnancies, 19 ended in early fetal loss without karyotyping and no newborn screening or other records could be found for 32 cases. The 129 instances of abnormal karyotype found were Down syndrome (84), trisomy 18 (four), trisomy 13 (three), triploidy (two), female sex chromosome aneuploidy (six) and male sex chromosome aneuploidy (five), inherited balanced rearrangements (19), mosaic or de novo balanced abnormalities (four) and unbalanced karyotypes (two). In the pregnancies declared at increased risk of fetal Down syndrome, only the karyotype for Down syndrome occurred with a frequency greater than that expected for the general, pregnant population.

From brain determination to testis determination: evolution of the mammalian sex-determining gene.

In mammals, sex is determined by an XY male:XX female sex chromosome system in which a male-dominant gene on the Y chromosome (SRY) determines testis formation. Sex chromosomes evolved from an ordinary autosome pair as the Y chromosome was progressively degraded. The Y chromosome has lost nearly all of its 1500 original genes, and those that survived did so because they evolved a critical role in male determination or differentiation. SRY is typical of Y-borne genes. Comparative gene mapping and sequencing shows that SRY arose quite recently as a degraded version of the SOX3 gene on the X chromosome. SOX3 is expressed predominantly in brain, and so is more likely to be a brain-determining than a testis-determining gene. The male-dominant action of SRYmay be an illusion, as its structure suggests that it works by interfering with the action of a related gene, which in turn inhibits testis development. This hypothesis can give a good account of how a brain-determining gene acquired a role in testis determination via differential dosage of SOX3. SRYhas no central role in sex determination and it can be replaced as a trigger and loft, as have many other Y-borne genes in recent evolutionary history. The absence of SRY in two species of the mole vole (Ellobius) suggests that its useful life is already running out.

Sex chromosome differentiation revealed by genomic in-situ hybridization.

In this work, genomic in-situ hybridization (GISH) was used to study the sex chromosome molecular differentiation on chromosomes of male and female individuals of the isopod crustacean Asellus aquaticus. As a composite hybridization probe, we contemporaneously used male and female whole genomic DNA differently labelled in the presence of an excess of unlabelled DNA of the female homogametic sex. The karyotype of A. aquaticius normally displays eight homomorphic chromosome pairs, but a heteromorphic sex chromosome pair is present in about a quarter of the males of a natural population previously identified by us. GISH did not reveal any sex chromosome molecular differentiation on the male and female homomorphic sex chromosome pair, and the karyotypes of these individuals were equally labelled by the male- and female-derived probe, while the heteromorphic Y chromosome showed a differentially labelled region only with the male-derived probe. This region evidently contains male-specific sequences but, because no similar hybridized region is observed on the male homomorphic chromosome pair, they are probably not important for sex determination but represent a molecular differentiation acquired from the Y chromosome.

Male-specific restriction of recombination frequency in the sex chromosomes of the medaka, Oryzias latipes

In the medaka, Oryzias latipes, the mechanism of sex determination (XX/XY) can be revealed by genetic crosses using a body-colour gene, though it does not have cytologically recognizable sex chromosomes. The recombination restriction of sex chromosomes in heterogametic (XY) males has been demonstrated. To elucidate whether the recombination is prevented by the heterogamety of the sex chromosomes or by maleness, we examined the recombination frequencies among three loci located on the sex chromosomes (r, SL1 and SL2) in heterogametic males (XY), homogametic males (XX and YY), homogametic females (XX) and heterogametic females (XY). The recombination frequencies between r–SL1 and SL1–SL2 were as follows: 0, 0 (XY males); 0, 1·5 (XX males); 1·6% (YY males; 1·2%, 14·4% (XY females); 0·8%, 21·8% (XX females). These results indicate that the recombination restriction of the sex chromosomes in heterogametic males does not result from heterogametic sex chromosomes, but from maleness. Such sex-chromosome- specific recombination restriction in heterogametic sex may have triggered the differentiation of sex chromosomes in vertebrates.

Inheritance and linkage of isozymes in Yponomeuta padellus (Lepidoptera, Yponomeutidae)

Inheritance and genetic linkage of 29 allozyme loci were studied by single-pair crosses of Yponomeuta padellus (Lepidoptera, Yponomeutidae). All loci segregated as Mendelian genes with codominant alleles except for a null allele at the hbdh locus. The three loci est-2, 6 pgdh, and fudh were sex-linked and occurred in that order along the Z chromosome. Autosomal linkage analysis was facilitated by the lack of crossing-over in females characteristic of Lepidoptera, because linkage in female-informative crosses is all-or-none and observation of ‘forbidden recombinants’ provides conclusive evidence that two loci are not syntenic. Convincing evidence was found for linkage of the autosomal loci me and mpi. This linkage group and aat-1, ca, hbdh, hk-1, pgi and pgm were assigned to seven separate autosomes. Weaker evidence from male-informative families supported four additional linked pairs. Previous cytological studies have shown that the sex chromosomes in heterogametic females are associated in a trivalent, consisting of a W chromosome translocated to an autosome (A w ), paired with the Z chromosome and the homologous autosome A. Any locus located on this autosome and its homologous segment on the A w chromosome should show strict segregation of one allele to sons and the other to daughters. However, none of the female-informative loci in this study satisfied that criterion. Use of the method of forbidden recombinants and its utility in the study of chromosomal evolution in Yponomeuta are discussed.

Differentiation of female chicken primordial germ cells into spermatozoa in male gonads.

In avian species, the developmental fate of different-sex germ cells in the gonads is unclear. The present study attempted to confirm whether genetically female germ cells can differentiate into spermatozoa in male gonads using male germline chimeric chickens produced by the transfer of primordial germ cells (PGC), and employing molecular biological methods. As a result of Southern hybridization, specific sequences of the W chromosome (the female specific sex chromosome in birds) were detected in the genomic DNA extracted from one out of four male germline chimeric chickens. When two-color in situ hybridization was conducted on the spermatozoa of this germline chimera, 0.33% (average) of the nuclei of each semen sample showed the fluorescent signal indicating the presence of the W chromosome. The present study shows that female PGC can differentiate into spermatozoa in male gonads in the chicken. However, the ratio of produced W chromosome-bearing (W-bearing) spermatozoa fell substantially below expectations. It is therefore concluded that most of the W-bearing PGC could not differentiate into spermatozoa because of restricted spermatogenesis.

Chromosome studies of three families of pelagic heteropod molluscs (Atlantidae, Carinariidae, and Pterotracheidae) from Hawaiian waters

Chromosome number and morphology were studied in gonadal tissue of 11 species of Atlantidae, 2 species of Carinariidae, and 3 species of Pterotracheidae, using an air-drying technique and Giemsa staining. In the Atlantidae the diploid chromosome number was the same in males and females and there were no heteromorphic chromosomes. The diploid chromosome number in nine species of Atlanta was 30 and the majority of chromosome pairs were metacentric and submetacentric. In Protatlanta souleyeti the diploid number was 28, and included five metacentric, six submetacentric, and three subtelocentric chromosome pairs. Oxygyrus keraudreni had a diploid chromosome number of 32, with 10 metacentric and 6 submetacentric chromosome pairs. A striking difference between the Atlantidae and the Carinariidae and Pterotracheidae was the presence of heteromorphic sex chromosomes in the latter two families. Male Pterosoma planum (2n = 32) had simple XY sex chromosomes, but males of Carinaria japonica (2n = 33), Pterotrachea scutata (2n = 33), Pterotrachea hippocampus (2n = 31), and Firoloida desmaresti (2n = 31) showed three heteromorphic chromosomes, suggesting a multiple sex-determining mechanism, X1X2Y. The locations of the female sex chromosomes in the karyotypes of female Pterotrachea hippocampus (2n = 32) and Firoloida desmaresti (2n = 32) were tentatively proposed. Cytogenetic features observed among the three families are supportive of previous interpretations of evolutionary trends in the Heteropoda based on morphology, i.e., that the Atlantidae are the most primitive family and gave rise to the Carinariidae and Pterotracheidae.

Sex chromosome loss and non-disjunction in women: analysis of chromosomal segregation in binucleated lymphocytes.

Chromosomal lagging and non-disjunction are the main mechanisms of chromosomal malsegregation at mitosis. To date, the relative importance of these two events in the genesis of spontaneous or induced aneuploidy has not been fully elucidated. A methodology based on in situ hybridization with centromeric probes in binucleated lymphocytes was previously developed to provide some insight into this matter. With this method, both chromosomal loss and non-disjunction can be simultaneously detected by following the distribution of specific chromosomes in the nuclei and micronuclei of binucleated cells. In this study, this approach was used for studying the role of chromosomal loss and non-disjunction in the age-related malsegregation of sex chromosomes in females. For this purpose, cultures of cytokinesis-blocked lymphocytes were established from 12 healthy women ranging in age from 25 to 56. The occurrence of malsegregation of X chromosomes in vitro was estimated in binucleated cells that contained four signals, which originates from the division of normal disomic cells. In this cell population, the frequencies of X chromosome loss and non-disjunction ranged from 0% to 1.69% (mean 0.75%), and from 0.20% to 1.33% (mean 0.57%), respectively. This indicates that both events contribute to malsegregation of X chromosomes in vitro. Moreover, a small but not negligible fraction of binucleated cells with two or six copies of the X chromosome was noticed in all donors. These cells, which are thought to arise from parental monosomic and trisomic types, may indicate the malsegregation of X chromosomes in vivo. The frequency of X chromosome aneuploidy both in vivo and in vitro significantly correlated with the age of donors. Analysis of chromosomal distribution in unbalanced cells demonstrated that both X homologues were frequently involved. The frequency of such multiple events (0.17%) was far greater than that expected by mere chance, indicating a tendency to multiple malsegregation events in the cell population investigated. Finally, parallel analysis of the segregation of chromosomes X and 1 in five of the donors confirmed the greater (about tenfold) susceptibility of X chromosomes to malsegregate compared with autosomes.

Turner syndrome and female sex chromosome aberrations: deduction of the principal factors involved in the development of clinical features

Although clinical features in Turner syndrome have been well defined, underlying genetic factors have not been clarified. To deduce the factors leading to the development of clinical features, we took the following four steps: (1) assessment of clinical features in classic 45,X Turner syndrome; (2) review of clinical features in various female sex chromosome aberrations (karyotype-phenotype correlations); (3) assessment of factors that could lead to Turner features; and (4) correlation of the clinical features with the effects of specific factors. The results indicate that the clinical features in 45,X and in other female sex chromosome aberrations may primarily be determined by: (1) degree of global non-specific developmental defects caused by quantitative alteration of a euchromatic or non-inactivated region; (2) dosage effect of a pseudoautosomal growth gene(s), a Y-specific growth gene(s), and an Xp-Yp homologous lymphogenic gene(s); and (3) degree of chromosome pairing failure in meiocytes that are destined to develop as oocytes in the absence of SRY. 1991; Grumbach and Conte 1992). However, the pertinent factors have not been determined to date. The method to clarify the factors responsible for the development of the Turner phenotype can be broken down into the following steps: (1) assessment of clinical features in classic 45,X Turner syndrome; (2) review of clinical features in various female sex chromosome aberrations (karyotype-phenotype correlations); (3) assessment of factors that could lead to Turner features; and (4) correlation of the clinical features with the effects of specific factors. If the clinical features in 45,X and in other female sex chromosome aberrations are explained by the effects of specific factors, it can be said that such factors contribute to the development of Turner features. In this paper, we take each of the above steps, and propose the principal factors involved in the development of clinical features in Turner syndrome.

Sex chromosome pairing and sex chromatin bodies in W–Z translocation strains of Ephestia kuehniella (Lepidoptera)

Structure and pairing behavior of sex chromosomes in females of four T(W;Z) lines of the Mediterranean flour moth, Ephestia kuehniella, were investigated using light and electron microscopic techniques and compared with the wild type. In light microscopic preparations of pachytene oocytes of wild-type females, the WZ bivalent stands out by its heterochromatic W chromosome strand. In T(W;Z) females, the part of the Z chromosome that was translated onto the W chromosome was demonstrated as a distal segment of the neo-W chromosome, displaying a characteristic non-W chromosomal chromomere-interchromomere pattern. This segment is homologously paired with the corresponding part of a complete Z chromosome. In contrast with the single ball of heterochromatic W chromatin in highly polyploid somatic nuclei of wild-type females, the translocation causes the formation of deformed or fragmented W chromatin bodies, probably owing to opposing tendencies of the Z and W chromosomal parts of the neo-W. In electron microscopic preparations of microspread nuclei, sex chromosome bivalents were identified by the remnants of electron-dense heterochromatin tangles decorating the W chromosome axis, by the different lengths of the Z and W chromosome axes, and by incomplete pairing. No heterochromatin tangles were attached to the translocated segment of the Z chromosome at one end of the neo-W chromosome. Because of the homologous pairing between the translocation and the structurally normal Z chromosome, pairing affinity of sex chromosomes in T(W;Z) females is significantly improved. Specific differences observed among T(W;Z)1-4 translocations are probably due to the different lengths of the translocated segments.

Women with Turner's Syndrome: helping them reach their full potential

ABSTRACT The physical and psychological effects of Turner's Syndrome, a female sex chromosome abnormality, briefly are described. Principles for helping women master the special challenges posed by the disorder are discussed. The authors argue that women with TS must take control of defining their identities and potential in order to attain full status within society.

Presumptive sex chromosomes of a unisexual homomorphic species of lizards, Lepidodactylus lugubris.

Incorporation of 5-bromo-2'-deoxyuridine (BrdU) in prometaphasic chromosomes allows active from inactivated X chromosomes in female mammalian cells to be distinguished. We have applied the technique to the all-female, chromosomally homomorphic gecko Lepidodactylus lugubris. Similar differences to those obtained between the two female sex chromosomes of mammals are observed in the patterns of chromosome pair 1. It is argued that this is more likely to reflect incipient ZW heterogamety than female (XX) homogamety.

Chromosomes of gaur cross domestic cattle hybrids

The chromosomes of five gaur (Bos gaurus hubbacki) domestic cattle (B indicus cross B taurus) hybrids (three females, two males) were studied using the leucocyte culture method and centromeric (C) banding technique. All the hybrids had a diploid chromosome number of 2n = 58, made up of two submetacentric autosomes (different in size) and 54 acrocentric autosomes, most of which could be arranged in pairs in descending order of size. The sex (X) chromosomes in females were a pair of submetacentric chromosomes smaller than the submetacentric autosomes. The Y chromosome in males was a small submetacentric chromosome. The C banding patterns were useful in identifying the X and Y chromosomes and the inherited submetacentric autosomes from the gaur sire. Phenotypically, the hybrids resembled normal B indicus cross B taurus calves except for the presence of a distinct hump-like dorsal ridge containing the spinous processes of the third to 11th thoracic vertebrae, upright 'deer-like' ears and long lean legs. The potential of these hybrids as important genetic resources for meat production is stressed.