Genetic analysis of the 5S RNA genes in Drosophila melanogaster

The 5S RNA genes of Drosophila melanogaster in either an isogenic wildtype or a multiply inverted (SM1) chromosome 2 increase their multiplicity when opposite a deficiency for the 5S gene site. This is analogous to the compensation phenomenon previously described for the 18S and 28S ribosomal RNA genes of the X chromosome nucleolus organizer region. Molecular hybridization of 5S RNA to DNA containing various doses of the 56F1-9 region of chromosome 2 demonstrates that most, if not all, of the 5S genes reside in or near this region. Also, a deficiency missing approximately one-half of the wild-type number of 5S genes was isolated and genetically localized. This mutant has a phenotype like that of bobbed, a mutant known to be partially deficient in 18S and 28S ribosomal RAN genes. Finally, we report the existence of a chromosomal rearrangement which splits the second chromosome into two segments, each containing 5S DNA.

Chromosomes and causation of human cancer and leukemia. XVI. Banding studies of chronic myelocytic leukemia, including five unusual Ph11 translocations

Forty-two Ph1-positive cases of chronic myelocytic leukemia (CML) were examined with chromosomal banding techniques. Thirty-seven of these cases had the "standard" type of Ph1 translocation between chromosomes No. 9 and No. 22 [t(9;22)(q34;q11)] in the Ph1-positive marrow cells; 5 cases had unusual types of Ph1 translocation. Of the 37 cases, 21 had additional numerical and/or structural chromosomal changes, 2 had a missing Y chromosome, and 1 had an extra Ph1 in the Ph1-positive cells. In the 5 cases with unusual types of Ph1 translocation, chromosomes No. 2, No. 9 No. 10, and No. 13 were involved. The clinical picture in these 5 patients did not differ materially from that of the other Ph1-positive patients with CML, probably indicating that the recipient chromosome, with which the translocation from No. 22 takes place, does not play a crucial role in the course of the CML. In the 21 cases with abnormal karyotypes, nonrandom chromosomal changes were observed. Most of the changes were related to events occurring at the centromeric region. The prognosis of cases with only an extra No. 8 or Ph1 appears to be better than that for cases with an iso-17q [I(17a)] chromosome or other extra chromosomes. The presence of the Ph1 (delected No. 22) in every case points to the essentiality of this karyotypic findings in the diagnosis of CML and possibly in the genesis of the disease.

Non-mendelian segregation in hybrids between chinese hamster cells

Mechanisms of segregation have been examined in hybrids between Chinese hamster cells, where chromosome loss in comparison to other systems is minimal. Hybrid cells were grown in HAT medium and subjected to back selection with bromodeoxyuridine (BUDR) or azaguanine (AZG). In AZG or BUDR at 30 mug/ml, segregation began with a random high frequency event that gave rise to cells capable of growth in both HAT and back selection medium, unlike the precursor hybrid or original parental cell types. BUDR-resistant segregants were propagated serially in the presence of BUDR, and were examined by clonal analysis for changes in plating properties during long term culture. Over a period of 300 days the HAT/BUDR plating ratio for sergregant cells declined continuously. A parallel decrease was observed in the rate of H3-thymidine incorporation, along with a drop in thymidine kinase activity. These shifts took place only in the presence of BUDR, and could be reversed by altered selection in HAT medium. Clonal studies showed that the evolution of segregant properties occurred in most if not all cells of the population, and did not arise from variation and selection of minority cell types. These properties of the segregating system are not consistent with models based on gene mutation, chromosome rearrangements, or chromosome loss. The evolution of segregants resembles more closely a sorting-out progress, taking place by intracellular selection over many generations. The segregating units may conceivably be cytoplasmic determinants linked functionally to nuclear genes, and which serve to modulate the events of phenotypic expression. Several lines of evidence which bear on this concept are discussed.

A case of "g 2 deletion syndrome": ring or partial monosomy? (46,XX,22r or 46,XX,22p- ?)

A case of "G2 Deletion Syndrome" is reported, based on concordant cytogenetic, clinical and dermatoglyphic findings. The definition if the syndrome, as associated with either a ring or a partially deleted chromosome 22, is discussed. The resulting interpretation favours the hypotheses of deletion of the short arm extending into the centromere.

Trisomy 9q-. a variant of the 9p trisomy syndrome

A low-birth-weight near-term male infant was found to have a non-familial 47,XY chromosome complement with an extra medium-sized metacentric chromosome slightly larger than a number 16. By Giemsa-trypsin (G-banding) this extra chromosome was determined to be a number 9 with deletion of approximately half of the long arm at region q 22. Chromosome studies on the clinically normal 38-year-old mother showed a balanced translocation with the deleted portion attached onto the distal end of a number 8 short arm, i.e. 46,XX,t(8;9)(p23;q22). Nondisjunction during meiosis of this woman's normal and deleted number 9 chromosomes is the basis of the child's abnormalities. One half-sibling of the child has a balanced translocation similar to that in the mother. Chromosome analyses on 4 others of the child's maternal half-siblings and on the maternal grandmother all showed normal patterns.

A deletion map of cyc1 mutants and its correspondence to mutationally altered iso-1-cytochromes c of yeast

Mutants arising spontaneously from sporulated cultures of certain strains of yeast, Saccharomyces cerevisiae, contained deletions of the CYC1 gene which controls the primary structure of iso-1-cytochrome c. At least 60 different kinds of deletions were uncovered among the 104 deletions examined and these ranged in length from those encompassing only two adjacent point mutants to those encompassing at least the entire CYC1 gene. X-ray-induced recombination rates of crosses involving these deletions and cyc1 point mutants resulted in the assignment of 211 point mutants to 47 mutational sites and made it possible to unambiguously order 40 of these 47 sites. Except for one mutant, cyc1-15, there was a strict colinear relationship between the deletion map and the positions of 13 sites that were previously determined by amino acid alterations in iso-1-cytochromes c from intragenic revertants.

Localization of human gene loci using spontaneous chromosome rearrangements in human-Chinese hamster somatic cell hybrids

Analysis of human-Chinese hamster somatic cell hybrids with spontaneously derived chromosome structural changes has provided data for the regional and subregional localization of gene loci which have previously been assigned to human chromosomes 2, 12, and X. Correlation of the expression of human gene loci with the human chromosome complements present in somatic cell hybrids indicates that the cytoplasmic malate dehydrogenase (MDH1) locus is in the 2p23yields2pter region, and red cell acid phosphatase (AcP1) is at or adjacent to 2p23. The cytoplasmic isocitrate dehydrogenase (IDH1) locus is at or adjacent to 2q11, peptidase B (Pep B) is at or adjacent to 12q21, lactate dehydrogenase B (LDH B) is in the 12q21yiedls12pter region, glucose-6-phosphate dehydrogenase (G6PD) is in the Xq24yieldsXqter region, and the gene loci for phosphoglycerate kinase (PGK), alpha-galactosidase (alpha-gal), and hypoxanthine guanine phosphoribosyltransferase (GPRT) are in the Xp21yieldsXq24 region.

Concentration-effect studies with MMS, TEB, 2, 4, 6-TriCl-PDMT, and DEN on the induction of dominant and recessive lethals, chromosome loss and translocations in Drosophila sterm

Comparative tests were made with four mutagens, treating male germ cells, particularly mature sperm, of Drosophila melanogaster. Dominant lethals, sex-linked recessive lethals, sex-chromosome loss and partial loss, and in one test translocations were used as genetic and points. The four mutagens, methanesulphonate (MMS), 2,3,5,6-tetraethyleneimino-I,4-benzoquinone (TEB), I(2,4,6-trichlorophenyl)-3,3-dimethyltriazene (2,4,6-triC1-PDMT), and diethyl nitrosamine (DEN) are known to differ in their chemical properties and mode of mutagenic action. An apparent relationship between dominant lethal induction and other genetic damage was found only with TEB. All four mutagens are efficient inducers of sex-linked recessive lethals. At low concentrations there was no direct concentration-frequency relationships. The two direct mutagens, MMS and TEB were effective in the chromosome loss tests. DEN does not induce translocations or any of the other types of damage studied which can be attributed to chromosome breakage. It is concluded that the sex-linked recessive lethal test is a simple and efficient way of preliminary screening chemical mutagens with Drosophila melanogaster.

[Comparison of the karyotype of the orangutan (Pongo pygmaeus) to those of man, chimpazee, and gorilla]

Various banding thechniques were employed to compare the karyotype of the Orang-utan (Pongo pygmaeus) with those of Man, the Chimpanzee, and the Gorilla. The following was concluded: 1) Almost all the bands are common, their analogy being close to 99%. The four species derive, therefore, from a common ancestor. The rearrangements which occurred during evolution were generally balanced; 2) The analysis of rearrangements shows that the Orang-utan, then the Gorilla, then the Chimpanzee, and lastly Man were derived in that order from the common stemline.

Partial losses of sex chromosomes induced by X-rays in spermatozoa of Drosophila melanogaster

The frequency of induced partial and complete losses of sex chromosomes from spermatozoa of Drosophila melanogaster was determined by mating irradiated males to females heterozygous for an attached-X Y chromosome and for a bb-deficient chromosome. The relative involvement of the X- and Y-chromosomes in partial losses was determined by (a) irradiating males with an X-chromosome centromere marked by a y+ translocated to the short right arm, and a Y-chromosome marked by w+, and (b) comparing the frequencies of fragments recovered from irradiated males with a normal, marked Y-chromosome and from irradiated males and a deleted bb Y-chromsome. 50-80% of the induced losses of sex-chromosomes in rod-X/Y males were partial losses, whereas only 25% of those induced in ring-X/Y males were partial losses. More than 70% of the recovered fragments were of Y-chromosome origin. Fragments recovered from attached-X-Y sons of irradiated males wer checked for the presence of the most proximal euchromatic X-chromosome genes. Most fragments of the rod-X were broken more proximally than bb. In the ring-X there was an excess of fragments broken in the proximal euchromatin. It was concluded that many fragments of the Y-chromosome was due to breaks induced by one-hit events, in both arms of the chromosome that were folded upon themselves. A similar mechanism had been postulated earlier for fragments of the ring-X chromosome.

Escape from mating-type incompatibility in bisexual (A + a) Neurospora heterokaryons

In Neurospora crassa, strains of opposite mating type generally do not form stable heterokaryons because the mating type locus acts as a heterokaryon incompatibility locus. However, when one A and one a strain, having complementing auxotrophic mutants, are placed together on minimal medium, growth may occur, although the growth is generally slow. In this study, escape from such slow growth to that at a wild type or near-wild type rate was observed. The escape cultures are stable heterokaryons, mostly having lost the mating type allele function from one component nucleus, so that the nuclear types are heterokaryon compatible. Either A or a mating type can be lost. This loss of function has been attributed to deletion since only one nuclear type could be recovered in all heterokaryons except one, but deletion spanning adjacent loci has been directly demonstrated in a minority of cases. Alternatively when one component strain is tol and the other tol+ (tol being a recessive mutant suppressing the heterokaryon incompatibility associated with mating type), escape may occur by the deletion or mutation of tol+, also resulting in heterokaryon compatibility. An induction mechanism for escape is speculated upon.

Short arm deletion of chromosome 12: report of two new cases

Two boys (W.M. and C.P.) are described, in each of whom a short-arm deleted C chromosome was apparently present. The clinical findings on W.M. are stenosis of the sagittal sutura associated with atrophy of the nervus opticus and mental retardation, and on C.P. osteogenesis imperfecta. An analysis of the G- and Q-banding patterns revealed in each patient a 12p-- chromosome. The deletion involved most of band p12 as shown by length measurements on G-banded chromosomes. Both cases were compared to proven and presumed cases of 12p-- from literature, but no common clinical phenotype could be demonstrated.

The induction of chromatid deletions in accord with the breakage-and-reunion hypothesis

In the present experiments it has been possible to study large numbers of X-ray induced chromatid deletions, or breads, in Chinese hamster chromosomes and to discern whether or not a sister chromatid exchange also occurs at the point of breadage. Chromatid deletions are only infrequently associated with a sister chromatid exchange. This is contrary to the expectations derived from the exchange hypothesis of Revell. Pn the basis of this hypothesis, in which chromatid deletions are considered to be incomplete exchanges that occur in the necks of little loops in the chromosomes, 40% of the chromatid breaks are expected to be associated with sister chromatid exchanges. The present data are in accord with the conclusions drawn from the earlier autoradiographic experiments of HEDDLE AND BODYCOTE, and show that chromatide breaks can be accounted for on the basis of the breakage-and reunion hypothesis, with the majority being simple breaks and some being incomplete exchanges between two such breaks.

A case of ring 18 chromosome in a sibship with multiple spontaneous abortions

We report the study of a female infant with physical stigmata suggestive of 18 chromosomes deletion, in whom cytogenetic studies revealed a 446,XX,r(18) complement. She was the last born of a sibship of seven composed otherwise by three spontaneous abortions, two perinatal deaths and one living female. The chromosome studies of the parents were normal. The cytogenetic finding and the phenotype are discussed in relation to the 18 chromosome deletion syndromes. The phenotype of the propositus would indicate that her ring 18 is significantly deficient of long arm segment. The apparent sporadic occurrence of this chromosomal anomaly in this family is discussed.

6-mercaptopurine, an inducer of cytogenetic and dominant-lethal effects in premeiotic and early meiotic germ cells of male mice

Dominant-lethal effects of 6-mercaptopurine on male mice were studied using eight doses, ranging from 150 to 482 mg/kg. Effects of the 150-mg/kg dose were studied over the entire spermatogenic cycle, and those of the higher doses for matings made between days 28.5 and 41.5 after treatment. It was found that, with low doses, there was only one period in which clearcut increases in induced dominant-lethal mutations were detected, namely in matings that occurred 32.5 to 35.5 days after treatment. With higher doses, effects could be detected beyond that period through day 39.5. Spermatozoa utilized for matings during the period of greatest response were presumably derived from germ cells that were in late differentiating spermatogonial and early meiotic spermatocyte stages at the time of treatment. These results are similar to those of Ray and Hyneck. To date, 6-mercaptopurine is unique in inducing dominant lethality only at these particular stages. A study of chromatid aberration induction in the treated males themselves was carried out for 150 and 250 mg/kg doses of 6-mercaptopurine over the period of 9 to 16 days after treatment. A considerable increase in ischromatid and chromatid deletions was observed in diakinesis-metaphase-I spermatocytes on days 14 and 15 after treatment. For reasons discussed, the cells sampled at this may be assumed to have been in early meiosis (preleptotene), with some in late differentiating spermatogonial stages, at the time of treatment. The rough agreement in sensitive cell type for dominant lethality and chromatid aberration induction suggests that chromatid deletions are the cause of dominant lethality in this study. Conservative estimates of the frequency of dominant lethality expected from the chromatid aberration frequencies tend to substantiate this suggestion.

A patient with congenital anomalies and a deletion of the long arm of the long arm of chromosome 4 [46,XY,del(4)(q31)]

This paper describes the clinical symptoms and cytogenetic findings in a patient previously described in a doctoral thesis (van Kempen, 1969). The patient is a boy with multiple congenital anomalies and a deletion of the long arm of chromosome 4. A recent Giemsa banding study showed absence of the terminal G-band, as was found in the patient described by Golbus et al (1973). The symptoms and other data on the three patients known to have a deletion of the long arm of chromosome 4 are presented to facillitate comparison of these cases. However, the number of cases so far on record is too small to warrant conclusions on the basis of this comparison.

Partial deletion of the short arm of chromosome 12(p11; p13). Report of a case

Chromosome analysis of a 15-day old boy with multiple malformations revealed a partial deletion 12(p11;p13).

Long arm deletion of chromosome no. 6 in a mentally retarded boy with multiple physical malformations

In a severe mentally retarded boy with multiple physical malformations cytogenetic analysis by the method of heat Giemsa staining confirmed the long arm deletion of chromosome No. 6--46, XY, del(6) (q2). Cytogenetic finding in parents were normal.

In support of the telomere concept

The frequency of recovered X-ray-induced (4000R) rearrangements that, in all probability mimic terminal deletions of the X chromosome was only one of, roughly, 10-5 X chromosomes screened for tip deficiencies. Although the single exception looks terminally deleted, it is probably capped by a very short or nonpolytene telomeric segment. It is apparent from these data that the probability of "healing" or stabilization of a terminally deleted X in the zygotic nucleus or developing embryo of Drosophila melanogaster is vanishingly small. The telomeric caps in two obviously interstitial deficiencies that were recovered represent, roughly, 1/500 of the length of a mitotic chromosome. These findings give some indication of the extreme difficulty of detecting short telomeric segments capping either deleted polytene chromosomes or deleted metaphase chromosomes of, for example, humans.

Intragenic deletions and salivary band relationships in Drosophila

In the absence of assumptions pertaining to the organization and function of chromomeric DNA, the cytogenetic analysis of intragenic deletions that start at Notch and spread to the right or left of the locus suggests that the recombinational gene is bilaterally associated with salivary band 3C7. Either there are two genes resolved as a single cistron, or one must seek an alternative interpretation that allows some modicum of independent in the relationship between gene and band. Although we momentarily lean toward the hypothesis that gene and salivary band are separate entities on a binemic chromosome, alternative views can be devised, and the data must remain open to reinterpretation.--The recessive visible allele fa-swb behaves as a point mutant at the left end of the map and seems to be a deletion in the interval 3C6 to 7; we suspect some part of the band is missing. We have used the aberration in fa-swb as a cytological marker, isolated intragenic recombinants, and subjected them to examination. The analysis indicates that the chromosomal interchanges occurred to the right of 3C7.

Complementation analysis of methyl methane-sulfonate-induced recessive lethal mutations in the zeste-white region of the X chromosome of Drosophila melanogaster

Recessive lethan mutations in the 3A1 to 3C2 region of the X-chromosome of Drosophila melanogaster were detected in 113 of 33,544 sperm treated by feeding 5 mM methyl methanesulfonate in 1% sucrose for 22 hours. Seven of the 113 lethans were sterile, leaving 106 for analysis by complementation tests. With only one exception, these mutants were found to have lesions restricted to single loci. One of these single-site mutations was in qt, 2 in tko, 18 in zw-1, 12 in zw-8, 6 in zw-4, 3 in zw-10, 3 in zw-13, 21 in zw-2, 7 in zw-3, 5 in zw-6, 6 in zw-12, 1 in zw-7, 12 in zw-5, 5 in zw-11, and 3 in zw-9. One of the lethals, m69, was non-complementary to two adjacent loci, zw-2 and zw-3, possible indicating a deletion encompassing two loci. The results confirm that there are at least 15 recessive lethal loci in the region and are consistent with the hypothesis of LIM and SYNDER (1968 and 1974) that inability of monofunctional alkylating chemicals to induce deletion associated mutations is a characteristic of the compounds.

Phenotypic-karyotypic localization of the human Rh-locus on chromosome 1

Statistical evaluation of between homologue differences and cytogenetic banding techniques were utilized to identify a small but significant homologue difference in Alp. An inherited chromosome variant characterized by an apparent deletion of the region delimited by the band pia3 and the telomeric end (t1) of chromosome A1, with telomeric conservation, was found to be consistently present in 3 familial pedigree members who expressed the highly infrequent -D-blood group type (-D/-D-). The most likely interpretation is that both the difference in length and the difference in banding characteristics reflect a real difference in DNA content of these homologues.

IS1 is involved in deletion formation in the gal region of E. coli K12

The DNA sequence IS1, which is 800 pairs long, has been shown to integrate into various bacterial and phage operons. The presence of this DNA sequence in the gal operon of E. coli K12 leads to an 30-2000 fold increase in deletion formation in the gal region as compared to wildtype. This high frequency of deletion formation is specific for IS1 and is independent of the cellular recA function. While the frequency of reversion of gal::IS1 mutations, which also is independent of recA, is not affected by the growth temperature of the cells, the formation of deletions in the gal::IS1 system is strongly dependent on the temperature of growth. Mapping experiments showed that one endpoint of the deletions in most cases is at the site of the IS1 mutation and the second endpoint seems to be at various but preferred sites. The formation of the different classes of delections observed is affected differently by the growth temperature of the cells. A model to account for these results is presented.

[Interstitial deletion of the long arm of one 11 chromosome]

A case of in testitial deletion of the long arm of chromosome 11 associated with mild facial deformity, broad skull and mental retardation is reported.

Tetracycline-sensitive mutants of the F-like R factors R100 and R100-1

The majority of tetracycline-sensitive (Tets) mutants of R100 and R100-1 are multisite (deletion) mutants. About 50% of these are also transfer-deficient, indicating that the Tetr locus is closely linked to the transfer genes. Tet(s) mutants with single-site lesions are also described.

Recombination-deficient deletions in bacteriophage lambda and their interaction with chi mutations

We have isolated a new class of deletion mutants of phage lambda that extend from the prophage attachment site, att, into the gam and cIII genes. In this respect they are similar to certain of the lambda pbio transducing phage, but they differ in having a low burst size and in forming minute plaques. Lytically grown stocks of the deletions contain a variable proportion of phage that produce large plaques. These have been shown to carry an additional point mutation. Similar mutations, called chi, have been described by Lam et al. (1974), who showed that they result in a hot-spot for recombination produced by the host recombination system (Rec). We show that chi mutations can occurat several sites in the lambda genome and produce a Rec-dependent increase in the burst size of the one deletion tested.---In addition to reducing burst size, the one deletion tested reduces synthesis of DNA and emdolysin but increases production of serum blocking protein. A chi mutation partially restores DNA synthesis and endolysin production and reduces serum blocking protein to normal levels. Our results are consistent with the hypothesis put forward by Lam et al., that chi enhances the frequency of Rec-promoted recombination, which provides the only pathway for production of maturable DNAin a red gam infection. The mechanism of the differential effect on protein production is, however, unclear.---Chi mutations are found to occur in DNA other than that of lambda. We show that, as has been suggested elsewhere (McMilin, Stahl and Stahy 1974), the lambda pbio transducing phages carry a chi mutation within the E. coli DNA substitution. A chi mutation also arose in a new substitution of unknown origin isolated in the course of this work.

Retinoblastoma. A model of hereditary fragile chromosomal regions

Direct karyotyping of tumour cells from three familial and two sporadic cases of retinoblastoma revealed the existence of a Dq- marker chromosome. The hypothesis is launched that a specific region on the long arm of one of the D chromosomes is the site of a locus which is essential for the sustained differentiation of specialized retinal tissue and may be the site of other loci essential for the maturation of other embryonic tissues. Fragility of this region and its potentiality for breakage under the influence of various environmental insults could be the basic cytological event leading to the development of sporadic retinoblastoma. Mutants at these loci, including those of sustained differentiation, could be a less common operational event whereby some variants could enhance the fragility of their respective chromosomal region and thereby explain the genetic transmission of retinoblastoma in certain families. It is common for the critical functional disruption of the locus to precede the deletion which may then be considered the terminal event in the fragile region.

A boy with 46, X, del, Y, due to a de nove mutation

A newborn male referred for genetic investigation because of a large sized head and dysplastic ears, but with apparently normal male genitalia was found to have a deletion of all of the brightly fluorescent part of the long are of chromosome Y and absence of the Y fluorescent body on buccal smear. His father and his two brothers had normal Y chromosomes. Social and family history as well as marker investigation make illegitimacy most unlikely and leaves an occurrence of a new chromosomal mutation in the father the most probably interpretation. Follow-up of the infant to the age of 9 months revealed a large baby with normal development.

Genetic aspects of childhood tumours

Present concepts on the etiology of childhood tumours are reviewed. The difference in clinical manifestations of the hereditary and nonhereditary types are illustrated with data on retinoblastoma and on nephroblastoma. Notwithstanding these differences it is most likely that the fundamental etiologic process is the same in both and that it consists in successive mutational events. The possible consequences of the association of retinoblastoma with a deletion of chromosome 13 in some cases are discussed. Several explanations for the association of Wilms' tumour and aniridia are also discussed.

Unexpected findings with the new chromosome banding techniques in a patient formerly diagnosed as having G-deletion syndrome II

A girl is described, who, upon her first admission to our pediatric department in 1965, was supposed to have G-deletion syndrome II on the base of the chromosome findings in routine orcein-stained preparations and her clinical aspect. When, however, in 1972 a Q- and G-banding analysis was performed, the patient seemed to be a t(14q+; 22q-) translocation carrier. These findings are discussed in relation to the known heterogeneity of the clinical picture of patients previously reported as having a G-deletion syndrome and in whom no banding studies were done. Finally, the need for more extensive studies with the different techniques is stressed, particularly, in cases of Gq- chromosomes, of which the familial occurrence is suggestive of a hidden reciprocal translocation.

Some experiments of nature with sex

Professor Penrose was a human geneticist but above all a human biologist interested in all aspects of man's adaptation and behaviours. However, I need not remind you of the fact that his interests were wider. They were, among others, with the field of biology itself. This interest was one reason why he was particularly delighted with some of the discoveries made on human chromosomes and their anomalies, and he shared very abundantly in the work and in the excitement of these discoveries.

Determining the repair rate for radiation-induced chromatid deletions during the first 24 hours after irradiation

The number of chromatid plus isochromatid deletions present in the testes and bone marrow of the Chinese hamster was measured as a function of time following acute exposure to cobalt-60 gamma irradiation. The number of breaks remaining scorable at any time after irradiation exposure could be represented by a simple exponential equation (N = Noe-kt). The values of No and k, calculated after 100 roentgens of cobalt-60 gamma irradiation, were 1.15 breaks/cell and 0.16/hour, respectively, for the testes, and 0.53 break/cell and 0.14/hour, respectively, for the bone marrow. The average length of time that the breaks remain scorable, 1/k, was found to be 6.3 hours for the testes and 7.1 hours for the bone marrow. By suitable formulation, it was possible to predict the number of breaks present by using varied radiation exposure rates and times after exposure. This formulation could be used for both single and multiple radiation exposures.

A large deletion of the long arm of chromosome No. 4 in a child with limb abnormalities

Images