Meiotic disjunction in mouse translocations and the determination of centromere position

Technical advances contribute to the study of genomic imprinting

Genomic imprinting in mammals was discovered over 30 years ago through elegant embryological and genetic experiments in mice. Imprinted genes show a monoallelic and parent of origin-specific expression pattern; the development of techniques that can distinguish between expression from maternal and paternal chromosomes in mice, combined with high-throughput strategies, has allowed for identification of many more imprinted genes, most of which are conserved in humans. Undoubtedly, technical progress has greatly promoted progress in the field of genomic imprinting. Here, we summarize the techniques used to discover imprinted genes, identify new imprinted genes, define imprinting regulation mechanisms, and study imprinting functions.

Gene Dosage Effects at the Imprinted Gnas Cluster

Genomic imprinting results in parent-of-origin-dependent monoallelic gene expression. Early work showed that distal mouse chromosome 2 is imprinted, as maternal and paternal duplications of the region (with corresponding paternal and maternal deficiencies) give rise to different anomalous phenotypes with early postnatal lethalities. Newborns with maternal duplication (MatDp(dist2)) are long, thin and hypoactive whereas those with paternal duplication (PatDp(dist2)) are chunky, oedematous, and hyperactive. Here we focus on PatDp(dist2). Loss of expression of the maternally expressed Gnas transcript at the Gnas cluster has been thought to account for the PatDp(dist2) phenotype. But PatDp(dist2) also have two expressed doses of the paternally expressed Gnasxl transcript. Through the use of targeted mutations, we have generated PatDp(dist2) mice predicted to have 1 or 2 expressed doses of Gnasxl, and 0, 1 or 2 expressed doses of Gnas. We confirm that oedema is due to lack of expression of imprinted Gnas alone. We show that it is the combination of a double dose of Gnasxl, with no dose of imprinted Gnas, that gives rise to the characteristic hyperactive, chunky, oedematous, lethal PatDp(dist2) phenotype, which is also hypoglycaemic. However PatDp(dist2) mice in which the dosage of the Gnasxl and Gnas is balanced (either 2∶2 or 1∶1) are neither dysmorphic nor hyperactive, have normal glucose levels, and are fully viable. But PatDp(dist2) with biallelic expression of both Gnasxl and Gnas show a marked postnatal growth retardation. Our results show that most of the PatDp(dist2) phenotype is due to overexpression of Gnasxl combined with loss of expression of Gnas, and suggest that Gnasxl and Gnas may act antagonistically in a number of tissues and to cause a wide range of phenotypic effects. It can be concluded that monoallelic expression of both Gnasxl and Gnas is a requirement for normal postnatal growth and development.

Analysis of mouse conceptuses with uniparental duplication/deficiency for distal chromosome 12: comparison with chromosome 12 uniparental disomy and implications for genomic imprinting

Distal mouse chromosome 12 is imprinted. Phenotypic analysis of mouse embryos with maternal or paternal uniparental disomy for the whole of chromosome 12 has characterized the developmental defects associated with the altered dosage of imprinted genes on this chromosome. Here we conduct a characterization of maternal and paternal Dp(dist12) mice using the reciprocal translocation T(4;12)47H. This limits the region analysed to the chromosomal domain distal to the T47H breakpoint in B3 on mouse chromosome 12. Both MatDp(dist12)T47H and PatDp(dist12)T47H conceptuses are non-viable and the frequency of recovery of Dp(dist12) conceptuses by 10.5 days post coitum (dpc) was lower than expected after normal adjacent-1 disjunction. A subset of MatDp(dist12) embryos can survive up to one day post partum. In contrast to paternal uniparental disomy 12 embryos, no live PatDp (dist12) embryos were recovered after 16.5 days of gestation. Other phenotypes observed in maternal and paternal chromosome 12 uniparental disomy mice are recapitulated in the Dp(dist12) mice and include placental, muscle and skeletal defects. Additional defects were also noted in the skin of both MatDp(dist12) and maternal uniparental disomy 12 embryos. This study shows that the developmental abnormalities associated with the altered parent of origin for mouse chromosome 12 can be attributed to the genomic region distal to the T47H breakpoint.

An Application of Molecular Genotyping in Mice

Microsatellite markers are simple sequence repeats within the mammalian genome that can be used for identifying disease loci, mapping genes of interest as well as studying segregation patterns related to meiotic nondisjunction. Different strains of mice have variable CA repeat lengths and PCR based methods can be used to identify them, thus allowing for specific genotypes to be assigned. Molecular genotyping offers such identification at any developmental stage, which allows for a broad range of anomalies to be studied. We studied chromosomal segregation in relation to nondisjunction in early-gestation mouse embryos using molecular genotyping. Information on the parental origin as well as the number of chromosomes a given progeny carried was obtained in our analysis.

Genetic and functional analysis of neuronatin in mice with maternal or paternal duplication of distal Chr 2

Functional differences between parental genomes are due to differential expression of parental alleles of imprinted genes. Neuronatin (Nnat) is a recently identified paternally expressed imprinted gene that is initially expressed in the rhombomeres and pituitary gland and later more widely in the central and peripheral nervous system mainly in postmitotic and differentiating neuroepithelial cells. Nnat maps to distal chromosome (Chr) 2, which contains an imprinting region that causes morphological abnormalities and early neonatal lethality. More detailed mapping analysis of Nnat showed that it is located between the T26H and T2Wa translocation breakpoints which is, surprisingly, proximal to the reported imprinting region between the T2Wa and T28H translocation breakpoints, suggesting that there may be two distinct imprinting regions on distal chromosome 2. To investigate the potential role of Nnat, we compared normal embryos with those which were PatDp.dist2.T26H (paternal duplication/maternal deficiency of chromosome 2 distal to the translocation breakpoint T26H) and MatDp.dist2.T26H. Expression of Nnat was detected in the PatDp.dist2.T26H embryos, where both copies of Nnat are paternally inherited, and normal embryos but no expression was detected in the MatDp.dist2.T26H embryos with the two maternally inherited copies. The differential expression of Nnat was supported by DNA methylation analysis with the paternally inherited alleles being unmethylated and the maternal alleles fully methylated. Although experimental embryos appeared grossly similar phenotypically in the structures where expression of Nnat was detected, differences in folding of the cerebellum were observed in neonates, and other more subtle developmental or behavioral effects due to gain or loss of Nnat cannot be ruled out.

A candidate model for Angelman syndrome in the mouse

Prader-Willi syndrome (PWS) and Angelman syndrome (AS) are well-recognized examples of imprinting in humans. They occur most commonly with paternal and maternal 15q11-13 deletions, but also with maternal and paternal disomy. Both syndromes have also occurred more rarely in association with smaller deletions seemingly causing abnormal imprinting. A putative mouse model of PWS, occurring with maternal duplication (partial maternal disomy) for the homologous region, has been described in a previous paper but, although a second imprinting effect that could have provided a mouse model of AS was found, it appeared to be associated with a slightly different region of the chromosome. Here, we provide evidence that the same region is in fact involved and further demonstrate that animals with paternal duplication for the region exhibit characteristics of AS patients. A mouse model of AS is, therefore, strongly indicated.

The mouse chromosome 7 distal imprinting domain maps to G-bands F4/F5

Mouse Chromosome (Chr) 7 distal to band F3 on the physical map is known to be subject to imprinting, maternal duplication (MatDp) of the region leading to a late embryonic lethality, while paternal duplication (PatDp) causes death in utero before 11.5 dpc. Using a new mouse reciprocal translocation T(7;11)65H to produce MatDp for distal Chr 7, we have mapped the region subject to imprinting more precisely to bands 7F4/F5 on the cytogenetic map. Fluorescence in situ hybridization (FISH) studies on mitotic and meiotic chromosomes of a T65H heterozygote show that the imprinted gene Igf2 is located in the same region. This was confirmed by the finding that embryos with MatDp of bands 7F4/F5 did not express Igf2. We suggest that other members of the imprinted domain containing Igf2, namely Mash2, H19, Ins2, and p57(K1P2), are also located in 7F4/F5 and that some or all of these genes may be responsible for the two imprinting lethalities seen with MatDp and PatDp for this region.

Embryological and molecular investigations of parental imprinting on mouse chromosome 7

Mouse embryos with duplications of whole maternal (parthenogenetic and gynogenetic) or paternal (androgenetic) genomes show reciprocal phenotypes and do not develop to term. Genetic complementation has identified the distal region of chromosome 7 (Chr 7) as one of the regions for which both a maternal and paternal chromosome copy are essential for normal development, presumably because of the presence of imprinted genes whose expression is dependent on their parental origin. Embryos with the maternal duplication and paternal deficiency of distal Chr 7 are growth retarded and die around day 16 of gestation; the reciprocal paternal duplication embryos die at an unidentified earlier stage. We report here the incorporation of cells with the paternal duplication into chimaeras, resulting in a striking growth enhancement of the embryos. One gene located on mouse distal Chr 7 (ref. 5) is the insulin-like growth factor 2 (Igf2) gene, an embryonic mitogen. In embryos with the maternal duplication of distal Chr 7, the two maternal alleles of the Igf2 gene are repressed. The presence of two paternal alleles of this gene in many cells is probably responsible for the growth enhancement observed in chimaeras. We propose that there are other imprinted genes in this Chr 7 region. We also compare the imprinting of this subgenomic region with phenotypes resulting from the duplication of the whole parental genome in parthenogenones and androgenones.

Genome imprinting phenomena on mouse chromosome 7

Heterozygotes for the reciprocal translocation T(7;15)9H were intercrossed, with albino (c) and underwhite (uw) as genetic markers, in order to study genetic complementation in mouse chromosome 7. Chromosome 15 is known to show normal complementation. Neither reciprocal cross in which one parent was c/c and the other wild type yielded albino progeny at birth although about 17% would be expected, but albino foetuses were recovered when the mother was c/c and father wild type. These products of maternal duplication/paternal deficiency for distal 7 were markedly retarded with small placentae. No albino foetuses were found when the father was c/c and mother wild type, which suggested earlier lethality. Equivalent crosses with uw (chromosome 15) as proximal marker gave normal underwhite progeny when the mother was uw/uw but small placentae, retardation and neonatal death of presumptive underwhites in the reciprocal cross. These abnormal newborn would have had a maternal duplication/paternal deficiency for proximal 7. These and other findings indicate that one region of defective complementation probably lies distal to the breakpoint of T(7;18)50H at 7E2-F2, while another is between the centromere and 7B3. Examination of man-mouse homologies suggests that the loci for three pathological human conditions (Beckwith-Weidemann syndrome, dystrophia myotonia and rhabdomyosarcoma) with differential parental transmission may be located in homologous regions to those affected by imprinting phenomena on mouse chromosome 7.

Genomic imprinting: normal complementation of murine chromosome 16

Parental imprinting effects for chromosome 16 were investigated using disomic animals which were obtained by mating (Rb32Lub x Rb2H) F1 mice. Two allelic forms of the enzyme CuZn-superoxide dismutase, Sod-1a and Sod-1c, were used to identify maternally or paternally disomic animals. Both types of disomic animals were found with the expected frequencies and did not visibly differ from one another or from non-disomic animals. These results indicate that the genomic imprinting mechanism either does not act on chromosome 16, or, if it does, does not do so in a manner which affects normal development.

Chromosome segregation at meiosis I in female T(2;4)1Gö/+ mice: no evidence for a decreased crossover frequency with maternal age

The influence of age and hormones on chromosome segregation at meiosis I was studied in female mice heterozygous for the T(2;4)1Gö translocation. Females of two age groups (18-22 and 40-56 weeks old) were stimulated for ovulation with different doses of gonadotropins (1.5 IU PMS/1.0 IU HCG or 10 IU PMS/10 IU HCG). Analysis of metaphase II oocytes revealed the highest level of hyperhaploidy (1.8%) and presegregation (4.4%) in the young females receiving the low dose. Presegregation preferentially affected the small 4(2) marker chromosome. There was no significant interference of the tetravalent with disjunction of the nontranslocated normal bivalents. Moreover, no remarkable difference in the mode of segregation (adjacent I, II or alternate) was observed. Recombination within the interstitial pairing segments of the chromosomes involved in the translocation allowed us to calculate cross-over frequencies in ovulated oocytes. For both the large 2(4) and the small 4(2) marker chromosomes, this frequency was higher in old than in young T(2;4)1Gö/+ females. Our data do not support the production line hypothesis of Henderson and Edwards (1968) which claims that chiasma frequency in oocytes decreases with maternal age.

Transmission of X-ray-induced reciprocal translocations in normal male mice and in male mice with a reduced sperm count due to translocation homozygosity

Normal (+/+) and translocation T(1; 11.13S)70H homozygous (T/T) male mice received 2 X 2.5 Gy X-rays with a 24-h interval. After 120 days, the frequency of late diplotene-metaphase I spermatocytes with translocation multivalents was 14.1% for +/+ and 13.7% for T/T males, respectively, in one group of animals of each type. The difference is not significant. A second group was allowed to sire progeny for 60 days with 2 normal females per week. Reciprocal translocations detectable at diakinesis/metaphase I were observed in 2.5% of the 395 male progeny from the irradiated +/+ fathers, and in 2.9% of the 489 male progeny from the irradiated T/T fathers. This leads to a pooled estimated transmission of 0.81 +/- 0.19. Translocations induced in the long 11.13 metacentric chromosome were not transmitted with a different frequency. The rate of heritable induced translocations in this study was 5.4 X 10(-5)/rad/gamete. On the basis of the data of Generoso et al. (1984) for the frequency of the heritable spontaneous translocations in male mice, it is concluded that, because of their low doubling dose (3.3-4.6 rad), the spontaneous translocations are probably of postmeiotic origin.

Autosomal aneuploidy in mice: generation and developmental consequences

Spontaneous aneuploidy in the mouse is uncommon, but specific mating schemes have been developed that produce aneuploid conceptuses at high frequencies. The most commonly reported aneuploid condition in the mouse is autosomal trisomy, in which there is an extra copy (in whole or in part) of a chromosome. In this review, we present several of the schemes used in producing trisomic, partially (tertiary) trisomic, and monosomic conceptuses and summarize the developmental consequences that are associated with each of the autosomal trisomies of the mouse.

Cytological localization of adenosine kinase, nucleoside phosphorylase-1, and esterase-10 genes on mouse chromosome 14

We have determined the regional locations on mouse chromosome 14 of the genes for mouse adenosine kinase (ADK), nucleoside phosphorylase-1 (NP-1), and esterase-10 (ES-10) by analysis of rearranged mouse chromosomes in gamma-irradiated Chinese hamster X mouse hybrid cell lines. Irradiated clones were screened for expression of the murine forms of these enzymes; segregant clones that expressed only one or two of the three markers were karyotyped. The patterns of enzyme expression in these segregants were correlated with the presence of rearranged chromosomes. The Adk gene was localized to bands A2 to B, Np-1 to bands B to C1, and Es-10 to bands D2 to E2.

Noncomplementation phenomena and their bearing on nondisjunctional effects

In the mouse, unbalanced gametes with major gains and/or losses of chromosomal material seem just as capable of forming a zygote as normal, fully balanced gametes. This is shown by the results of intercrossing genetically marked translocation heterozygotes, in which complementary unbalanced gametes usually fuse to form fully viable zygotes. However, there are some notable exceptions to this. Studies on a number of reciprocal translocations have shown that gametes with maternal duplication of particular chromosome regions may fail to complement those with a corresponding paternal deficiency, but produce lethal zygotes instead, whereas the reciprocal combination of a paternal duplication with a maternal deficiency produces fully viable offspring. For a particular distal region on chromosome 7 the reverse situation holds. More recent studies on genetic methods of detecting nondisjunction with Robertsonian translocations have revealed the same phenomenon. Mouse chromosomes affected include numbers 2, 6, 7, and 8. There is also defective complementation on chromosome 11 and related phenomena on chromosome 17. These findings help to explain why diploid embryos with 2 male or 2 female pronuclei fail to come to term and may be connected with genetic imprinting of gametes. It seems probable that the same phenomenon occurs in homologous regions of human chromosomes and may mean that the severity of a trisomic effect will depend sometimes on the parental source of the extra chromosome. The phenomenon also affects the efficiency of certain genetic tests for nondisjunction which depend on full complementation.

Double translocation t(7;12),t(2;6) heterozygosity in one family. A contribution to the trisomy 12p syndrome

Double translocation heterozygosity t(2;6),t(7;12) in three generations of a Dutch family is described: the segregation of a double translocation in more than one generation has not been previously published. The index case was a 16-year-old mentally retarded boy with partial trisomy 12p who showed several dysmorphic features such as high prominent forehead, flat face, flat and short nose bridge, short nose, dysplastic ears, prominent lower lip, and several skeletal abnormalities. Based on the findings in this patient and those in nine other cases, the existence of a specific trisomy 12p syndrome is postulated.

The spermatozoan genome and fertility

The strategy for effective reproduction by eliminating genetically unbalanced gametes during spermatogenesis and transport varies in degree of success within as well as among species, but in no animal has it been reported to be completely effective. In the human subject, for example, it is estimated that one in every 50 ejaculated spermatozoa is genetically abnormal. The causal basis of these anomalies is poorly understood. Meiotic accidents, environmental mutagens, and gamete senescence in utero are all implicated. However, many of these abnormal cells are fertile. This fact plus the weight of the evidence reviewed suggest that fertility differences among males which cannot be ascribed to measurable differences in semen characteristics reflect, in large part, the increased opportunity of nuclear defective gametes in the semen of some males to effect fertilization. The elimination of embryos arising from eggs fertilized by genetically defective spermatozoa through spontaneous abortions, although biologically costly, must be viewed as the final check for the elimination of genetic detritus of the species.

Chromosomal radiosensitivity and karyotype in mice using cultured peripheral blood lymphocytes, and comparison with this system in man

The frequencies were studied X-ray-induced dicentric chromosomes and deletions in peripheral blood lymphocytes of mouse and man, cultured in vitro. After doses of 100 and 200 rad, (a) the mouse was equally sensitive as man to the induction of dicentrics, and (b) the frequency of deletions was higher in the mouse, reaching statistical significance at the 200 rad level only. At the 200 rad level, the mouse with normal karyotype was compared with the T(1;13)70H translocation heterozygote and the Ts(1(13))7OH tertiary trisomic of normal appearance. No differences were found either with respect to dicentrics or to deletions. At the 100 rad level, the normal mouse was compared with the tertiary trisomic mouse of the affected phenotype and with the tobacco mouse. The frequency of dicentrics was significantly higher in the phenotypically abnormal trisomics, whereas the deletion frequency was higher in the tobacco mice. C-banding of the slides enabled the locating of breaks in constitutive hetero-chromatin and euchromatin. When exchanges were classified into three categories, i.e. those between eu- and euchromatin, eu- and hetero-, and hetero- and heterochromatin, there was a preference for the first and the last whereas only few occurred between chromatins of contrasting type. Differences between previous determinations of the chromosomal radiosensitivity of mouse and man, using peripheral blood lymphocytes (i.e. man is twice as sensitive as mouse), and the one presented here could be attributed to differences in harvest time of the mouse peripheral blood lymphocytes. Thus the so-called "arm number" hypothesis of Brewen et al. [5] is not confirmed by the present results.

Factors affecting the observed number of young resulting from adjacent-2 disjunction in mice carrying a translocation

The frequency of adjacent-2 disjunction in mice carrying the reciprocal translocation T(9; 17)138Ca was studied by mating together animals heterozygous for the translocation and carrying different recessive marker genes, usingTtfor chromosome 17 andcwcwfor chromosome 9. The proportion of marked young arising from adjacent-2 disjunction varied according to the markers carried in the two parents. When the female carriedTtthe frequencies of marked young were always higher than when non-Tfemales were used, and whenTtandcwcwwere carried in the same parent there was a shortage of marked young obtaining both copies of the proximal region of chromosome 17 from the father. Both these effects were regarded as probably another example of the phenomenon discovered by Johnson, of inviability of young lacking a maternal homologue of a certain region of chromosome 17. There were other variations in frequency of marked young, among crosses using non-Tfemales, which may have been due to differences in transmission ratio of male gametes carrying varioust-haplotypes or to true variations in frequency of adjacent-2 disjunction.

Parental age and recombination frequency in the house mouse

The relationship between parental age and recombination frequency in the offspring of mice has been studied using backcrosses to heterozygous males or females. The mice were allowed to breed until they became infertile. Data for several chromosomes were analysed, mostly from a stock carrying five markers covering approximately three-quarters of the length of chromosome 2.

Analyses of two-point recombination ratios or of multi-point interference ratios revealed no consistent age-related heterogeneity in offspring of male or female heterozygotes. A significant age-related heterogeneity was detected in the offspring of female heterozygotes but not of males, when the number of offspring with no detectable recombination was compared to those with one or more. This difference between the sexes could be related to earlier cytological observations on chiasma frequency in oocytes and spermatocytes. The significance of these findings for the analysis of follicular growth in mammals and in the origin of human trisomic conditions is discussed.

Male meiotic behaviour and male and female litter size in mice with the T(2;8)26H and T(1;13)70H reciprocal translocations

Two reciprocal mouse translocations T(2; 8)26H and T(1; 13)70H, heterozygous in a Swiss random-bred background, show differences in the spectrum of multivalent configurations and in the segregational behaviour of these multivalent configurations. T26H/+ males mostly contained rings of four (R IV, 53·15%) and T70H/+ males chains of four, missing a chiasma in the shortest interstitial segment (C IV 11, 61·55%). The adjacent II frequency, estimated from metaphase II observations, was 8·47% in T26H/+ and 25·22% in T70H/+. Univalents of the shorter translocation chromosome of T70H are able to divide equationally at first anaphase. The hypothesis is advanced that time differences in chiasma terminalization during metaphase I-anaphase are important for explaining the difference in segregation observed between the two translocations. Translocation-caused non-disjunction is probably low in T26H/+ and 4–5% in T70H/+. Univalents involving T70H/+ are usually capable of co-orientation with the other chromosomes of the translocation complex. The summed percentages of adjacent II disjunction and non-disjunction caused by the translocations were estimated from the relative fertility scores of T/+ males and females versus +/+ males and females as 9·8% and 29·0% for T26H/+ and T70H/+ males, respectively, and 9·4% and 27·8% for T26H/+ females and T70H/+ females. For both translocations, the agreement between the various estimates is good. Chiasma frequencies are much higher in telomeric segments than in proximal segments containing centric heterochromatin.

A genetic method for measuring non-disjunction in mice with Robertsonian translocations

A high frequency of chromosomal non-disjunction occurs spontaneously in mice heterozygous for some Robertsonian translocations. If animals heterozygous for the translocation and homozygous for different alleles of a marker gene are mated together a few young homozygous for the marker arise through non-disjunction, and their frequency can be used as a measure. This method has been used with the Robertsonian translocation Rb(9.19)163H and the marker ruby ru (chr. 19); Rb(4.6)-2Bnr with brown (b) and misty (m) (chr. 4); and Rb(9.14)6Bnr with hairless (hr) and piebald (s) (chr. 14) respectively. The frequencies of marked young were: Rb163 0/5260 ruru; Rb2 21/1997 mm bb; and Rb6 19/1702 hrhr ss, and the corresponding calculated non-disjunction frequencies in each arm of the translocation were Rb163, <5 %; Rb2, 15%; Rb6, 15%. These figures show reasonably good agreement with values obtained by other methods. A search for genetic or environmental factors affecting the frequency of marked young in Rb2 and Rb6 revealed that in Rb2 the frequency increased with maternal age, whereas in Rb6 the maternal age of the marked young was non-significantly below that of the total progeny. The reasons for this discrepancy are not clear.

Chromosomal abnormalities and the morphology of mouse sperm heads

In the mouse, numerous mutagens, teratogens and carcinogens have been shown to induce marked elevations in the fraction of sperm with head shape abnormalities. Since carcinogens and teratogens may act by causing genetic damage, a likely explanation of these results is that the sperm abnormalities are also caused by genetic damage. There are two more or less distinct classes of genetic damage, chromosomal aberrations and point mutations. In this paper, we provide evidence, that in general, chromosomal aberrations are not responsible for causing abnormally shaped sperm. Chromosomal aberrations could have caused abnormal sperm morphology in a number of ways. One possibility was that the mere presence of a translocated chromosome within the germ cell led to the malformation of the sperm head. A second possibility was that chromosomal imbalance, i.e., aneuploidy, duplications or deficiencies, within the spermatid or haploid cells caused abnormalities in shape. We tested these hypotheses by measuring the level of abnormally shaped sperm in mice homozygous and heterozygous for 24 various reciprocal and Robertsonian translocations. The diploid cells of these mice are known to be chromosomally balanced, containing translocated chromosomes. A predictable proportion of their gametes are, however, chromosomally unbalanced and carry translocated chromosomes. It was found that the levels of sperm abnormalities in these mice were convincingly unrelated to the levels predicted by any of the above hypotheses. Based on these results it seems that sperm abnormalities in mice are not due to the mere presence of translocated chromosomes in germ cells and also not due to chromosomal aneuploidy or duplication-deficiencies of chromosomal segments in the spermatid during development of the sperm.

The location of the positions of the breakpoints involved in the T26H and T70H mouse translocations with the aid of Giemsa-banding

The positions of the breakpoints involved in the T(2;8)26H and T(1;13)70H mouse translocations have been located to specific minor bands using a trypsin-Giemsa banding method and a nomenclature system for band patterns as developed by Nesbitt and Francke (1973). The breakpoint positions are 2H1 and 8A4 for T26H and 1A4 and 13D1 for T70H. The interstitial segments occupy 80.9% of chromosome 2, 30.1% of chromosome 8, 14.4% of chromosome 1 and 88.0% of chromosome 13. It is concluded that the variation of the location of the breakpoint positions is mainly caused by differential chromosome contraction and measuring errors and only to a small extent by the resolving power of the G-banding technique.

Chromosome mapping in the mouse, fluorescence banding techniques permit assignment of most genetic linkage groups

Chromosome banding techniques have permitted the identification of every normal chromosome in the mouse, Mus musculus, and the demonstration of strain differences. By identifying the chromosomes involved in a series of translocations, it has been possible to assign 14 of the 19 known linkage groups to 14 different chromosomes. These powerful cytological methods promise to revolutionize cytogenetic studies in higher organisms.