Human chromosomal heteromorphisms: nature and clinical significance

History and evolution of cytogenetic techniques: Current and future applications in basic and clinical research

Chromosomes are the vehicles of genes, which are the functional units of a cell's nucleus. In humans, there are more than 20,000 genes that are distributed among 46 chromosomes in somatic cells. The study of chromosome structure and function is known as cytogenetics which is historically a field of hybrid science encompassing cytology and genetics. The field of cytogenetics has undergone rapid developments over the last several decades from classical Giemsa staining of chromosomes to 3-dimensional spatial organization of chromosomes with a high resolution mapping of gene structure at the nucleotide level. Improved molecular cytogenetic techniques have opened up exciting possibilities for understanding the chromosomal/molecular basis of various human diseases including cancer and tissue degeneration. This review summaries the history and evolution of various cytogenetic techniques and their current and future applications in diverse areas of basic research and medical diagnostics.

Prenatal diagnosis of a rare de novo centromeric chromosome 6 variant

Cytogenetic heteromorphisms are described as heritable variations at specific chromosomal regions without phenotypic effect. Polymorphisms of the size of heterochromatic centromeric regions of chromosomes 1, 9 and 16 have been well documented in humans but only four previous reports described centromeric polymorphism of chromosome 6. We present a prenatal diagnosis of a rare de novo centromeric chromosome 6 variant. Cytogenetic and molecular techniques were used to characterize this variant and confirm the de novo nature of this event. This case illustrates the importance of reporting unusual variant chromosomes for genetic counseling and for determination of the frequency of variant chromosomes in the general population.

Molecular cytogenetics and cytogenomics of brain diseases

Molecular cytogenetics is a promising field of biomedical research that has recently revolutionized our thinking on genome structure and behavior. This is in part due to discoveries of human genomic variations and their contribution to biodiversity and disease. Since these studies were primarily targeted at variation of the genome structure, it appears apposite to cover them by molecular cytogenomics. Human brain diseases, which encompass pathogenic conditions from severe neurodegenerative diseases and major psychiatric disorders to brain tumors, are a heavy burden for the patients and their relatives. It has been suggested that most of them, if not all, are of genetic nature and several recent studies have supported the hypothesis assuming them to be associated with genomic instabilities (i.e. single-gene mutations, gross and subtle chromosome imbalances, aneuploidy). The present review is focused on the intriguing relationship between genomic instability and human brain diseases. Looking through the data, we were able to conclude that both interindividual and intercellular genomic variations could be pathogenic representing, therefore, a possible mechanism for human brain malfunctioning. Nevertheless, there are still numerous gaps in our knowledge concerning the link between genomic variations and brain diseases, which, hopefully, will be filled by forthcoming studies. In this light, the present review considers perspectives of this dynamically developing field of neurogenetics and genomics.

Prenatal diagnosis of a rare inherited heterochromatic variant chromosome 4

Heterochromatic chromosome polymorphisms have been extensively reported. Most are associated with C-band positive regions located on chromosomes 1, 9, 16, and Y. We report a prenatal case of a rare heterochromatic variant on chromosome 4. Amniocentesis was performed on a 35-year-old white female for AMA. The karyotype was 46,XY,add(4)(q35)?. One chromosome 4 homolog had an additional dark band at the terminus of the long arm. Parental chromosome analyses revealed that the chromosome 4 was maternally inherited. The mother and fetus were both Q and C-band positive and NOR and DAPI Distamycin staining negative. FISH using Y, 4, and 9 whole chromosome paint (WCP), centromere probes for all chromosomes (Cytocell, Chromoprobe Multiprobe-I System, Rainbow Scientific, Inc., Windsor, CT), alpha-satellite probes for 13/21, 14/22 (D13Z1/D21Z1; D14Z1/D22Z1, Oncor, Gaithersburg, MD), and the 15 PWS/Angelman probe (LSI SNRPN, D15Z1, PML, Vysis, Inc., Downers Grove, IL) were negative. The TelVysion 4q telomere probe (D4S2930, Vysis, Inc.) was positive. A phenotypically normal male was born at 37 weeks. Follow up studies on placenta, cord, cord blood, and foreskin confirmed the prenatal results. Based on these findings, it appears that this chromosome 4 was a rare heterochromatic variant. Heterochromatic variants have been demonstrated to have no phenotypic effect on carriers. This case illustrates the importance of reporting unusual variant chromosomes for genetic counseling purposes. To the best of our knowledge, this is the first report of a heterochromatic variant involving part of the long arm of chromosome 4 in a phenotypically normal mother and child.

Preferential fluorescent staining of heterochromatic regions in human chromosomes 9, 15, and the Y by D 287/170

The utility of a newly synthesized chemical variation of DAPI (4'-6-diamidino-2-phenyl-indole), D 287/170, for differential staining of constitutive heterochromatin in man is demonstrated. Direct staining of human chromosomes with D 287/170 results in brilliant fluorescence of the paracentromeric C-band of chromosome 9, of a proximal short-arm segment of chromosome 15, and of certain heterochromatic regions in the Y. Bright, but less conspicuous fluorescence is occasionally seen at the centromeres of other chromosomes. The staining differentiation obtained by D 287/170 is very distinct, and the intensity of the fluorescent light is unusually high. The new fluorochrome should prove particularly useful for detecting and analyzing human chromosome 9 heterochromatin at various stages of the cell cycle in normal and structurally altered chromosomes.

Role of chromosome heteromorphism in early recurrent miscarriages in the Middle East

To identify the role of chromosome polymorphism in miscarriages, an analysis was carried out on 774 couples; 391 of them with two or more miscarriages and 383 serving as control. All couples were taken from two separate ethnic groups residing in the United Arab Emirates (UAE): 199 cases and 194 controls came from the indigenous population who are UAE nationals (Arabs) whilst 192 cases and 189 controls came from the Asian expatriate population residing in the UAE (Asians). Out of 391 cases, six Arab and three Asian couples were excluded because at least one partner had a chromosomal aberration. From the control group four Arab and four Asians were excluded for similar reasons. The remaining 757 couples were analysed to detect the presence of chromosomal heteromorphism in one or both partners. The results suggest that chromosomal heteromorphism was significantly increased in the Arab study population (P < 0.05). Two heteromorphisms, 9qh and inversion (9)(p11q13) seem to be marginally related to recurrent miscarriages and only in the Arab study population. No differences were found in the distribution of chromosomal heteromorphism in couples analysed in relation to the number of miscarriages (two or more).

New variant of chromosome 11

An additional C-positive band in the centromeric region (p11) was observed in a man. By GTG- and RBA-techniques it was positively stained but by QFQ-technique the staining intensity was negative. Although he was identified through fetal loss in his wife, it apparently represents a familial variant whose clinical significance is unknown.

Molecular characterization of the secondary constriction region (qh) of human chromosome 9 with pericentric inversion

Pericentric inversion of the secondary constriction region (qh) of human chromosome 9 is a frequent occurrence. This structural alteration is regarded as a normal familial variant, termed heteromorphism, and is inherited in a Mendelian fashion without any apparent phenotypic consequences. We characterized the qh region of chromosome 9 from five individuals using a series of molecular cytogenetic techniques. Four out of the five individuals have an additional area composed of alphoid DNA sequences on the inverted chromosome 9 while one case was found to have an apparently intact alphoid DNA sequence. Although the direct function(s) of alphoid DNA sequences remain unclear, the centromeric breakage involving these sequences in inverted chromosome 9 raises a series of questions pertaining to the monocentric, dicentric and pseudodicentric nature of pericentric inversions. Nevertheless, these findings have prompted us to suggest that the structural organization of alphoid DNA sequences of the centromeric region of chromosome 9 are apparently “breakage prone” and may be associated with a higher incidence of pericentric inversions. Furthermore, the hierarchical organization of various satellite DNA families (alpha-satellite, beta-satellite and satellite III) within the primary and secondary constriction regions of chromosomes 9 are elucidated here.

Deciphering the fluorescent variability of human genomic heterochromatin by DA/DAPI technique

A number of selective staining techniques have been utilized to decipher the variability of pericentromeric heterochromatin. One such technique is called DA/DAPI and it is believed to be stain specific. However, we demonstrate otherwise and suggest that pericentromeric regions of all human chromosomes stain positive by DA/DAPI-technique. It must be emphasized that the incidence of DA/DAPI positive stained chromosomes, other than 1, 9, 15, 16 and Y, is a rare occurrence and only a small portion of the pericentromeric region is DA/DAPI positive, as reported here using 50 normal individuals.

Molecular characterization of the smallest secondary constriction region (qh) of human chromosome 16

We report the smallest secondary constriction region (h) in human chromosome 16. The cytochemical, cytogenetic, and molecular techniques revealed the complex heterogeneity of heterochromatin observed in this region. The heteromorphisms can be found due to the variation in centromeric (c) region alone or in combination with the h region. Routine selective staining techniques fail to differentiate the C region from the h region. However, the fluorescence in situ hybridization technique clearly demonstrated that the centromere of chromosome 16, which is composed of 340-base-pair dimers arranged in a tandem array of 1.7-kb higher-order repeat units, is not heteromorphic in the present case, but other molecular cytogenetic techniques demonstrated the presence of a very small h region. The evolution of heterochromatin of this region is discussed.

Detection of the heteromorphic spectrum of heterochromatin in the human genome by in situ digestion using restriction endonuclease AluI

A battery of selective banding techniques has been utilized to identify the heteromorphic markers in the human genome. The recent addition of the AluI/Giemsa (G)-technique has helped not only in identifying the variable sites, but in characterizing their heteromorphic spectra. In the present investigation, we classified the pericentromeric heterochromatin by the AluI/G-technique by its size and position using 50 normal individuals and suggested the potential uses of this banding technique over earlier methods.

Evolution of pericentromeric heterochromatin of human X chromosome

An unusual large heterochromatic segment around the pericentromeric region of the X-chromosome is reported. In normal circumstances, the pericentromeric region of the X-chromosome is negative by the restriction endonuclease AluI/Giemsa technique. However, this unusual X-chromosome was found to have AluI resistant (positive) chromatin. The evolution of extra heterochromatin is a postzygotic event as substantiated by the presence of a normal cell line.

Inversion-duplication of bands q13----q21 of human chromosome 9

Structural abnormalities involving heterochromatic regions of the human genome are difficult to characterize because these segments are G-band negative by GTG technique, a routinely used procedure in clinical cytogenetic laboratories. Chromosome abnormalities of such cases have gone undetected or were incorrectly characterized because these regions are so-called heteromorphisms or variants. Consequently, much anxiety has been aroused by the confusion between a chromosome abnormality and a normal heteromorphic variant. We report the first documented case with a so-called highly unusual h region of chromosome 9 which is not a variation but a structural rearrangement involving a paracentric inversion and a duplication. The major clinical features were psychomotor retardation, microcephaly, narrow palpebral fissures, renal and genital anomalies, vertebral anomalies, protruding tongue, and learning and behavioral problems. A concise review of variable duplicated segments of 9q is also provided.

Chromosome imbalance, normal phenotype, and imprinting

A duplication of the sub-bands 1q42.11 and 1q42.12 was found in a boy and his mother. The proband has short stature (around the 10th centile) but a normal phenotype and psychomotor development. His mother is also asymptomatic. We found 30 published cases of normal subjects with an imbalance of autosomal euchromatic material. In these cases the imbalance involved either only one G positive band or a G positive and a G negative band. Thus the absence of a phenotypic effect cannot always be ascribed to the deficiency in the G positive bands of coding DNA. Moreover, in some cases, the method of transmission of the chromosome abnormality was such that an imprinting effect could be postulated.

Restriction endonuclease AluI resistant chromatin of "aged" slides

We present a modified method for digesting the AluI resistant chromatin of old slides. The older slides require the longer treatment with much higher concentration.

Cytogenetic studies using Q-band polymorphisms in patients with AML receiving marrow from like-sex donors

After bone marrow transplantation (BMT), it is important to monitor the bone marrow and lymphoid cell populations of the recipient to document engraftment. When donor and recipient are of unlike sex, the sex chromosomes serve as a useful marker to determine cellular origin. When donor and recipient are of like sex, autosomal heteromorphisms can be used to identify the origin of cells in metaphase. Using Q-banding, we found that 17 of 20 patient/donor pairs (85%) examined showed at least one chromosome heteromorphism that distinguished between recipient and donor cells with certainty. Five of the patients were followed up after BMT in order to document engraftment. Donor metaphases could be detected in the marrow within two weeks of BMT when the graft was successful. Chimaerism was detected in the lymphocyte population even when the graft persisted. In a case of graft failure, donor cells did not persist in the marrow, and the lymphocyte population did not convert to donor type. These studies demonstrate that autosomal heteromorphisms are useful in the study of myeloid and lymphoid chimaeric states after BMT.

AluI-resistant chromatin of chromosome 18: classification, frequencies and implications

The pericentric chromatin of chromosome 18 was found to be far more heterogeneous for restriction endonuclease AluI (5'...AG decrease CT...3') than previously thought. The extent of such heterogeneity was characterized using 50 normal Caucasians and 5 cases of trisomy 18 or Edwards' Syndrome. The AluI-resistant chromatin can arbitrarily be classified into at least five sizes by comparison with the length of the short arm (p) of chromosome 18. They are: negative (1), small (2), medium (3), large (4) and very large (5) with incidences of 11.30%, 19.13%, 29.57%, 29.57% and 10.43%, respectively. In addition the location of the chromatin can be classified into four types depending upon the position relative to the primary constriction. For example: Type I (absent); Type II (present on p arm only); Type III (present on q arm only); Type IV (present on centromere and extending into both p and q arms). The incidences of types I, II, III, and IV were 11.30%, 62.61%, 0.87%, and 25.22%, respectively. Based on limited data, AluI-resistant chromatin was found to be predominantly "large" and "very large" in Edwards' Syndrome samples. In addition, no case with negative Alu-resistant chromatin was noted. Therefore, it is tempting to speculate that the amount of chromatin present on the centromere might play a role in non-disjunction in Edwards' Syndrome cases. Although the variation observed in the present study is continuous, the proposed classification has some important implications for future investigations.

Non-random distribution of various sizes of human Y chromosomes in different ethnic groups

One hundred and ninety normal males from three ethnically different populations (Caucasians, American blacks and East Indians) were selected to study the frequency distribution of length of the Y chromosome, fluorescent (f) and non-fluorescent (nf) segments as a function of Y/F, f/F and nf/F indices. The QFQ techniques was performed to identify the f and nf segments. The frequency distribution of the total length of Y chromosome was not normally distributed in all three populations (P less than 0.05). Extensive statistical analysis revealed that the non-random distribution of the total length was due to skewness and kurtosis for both the n and nf segments. The clinical implications as well as evolutionary aspects of such variation in ethnic anthropology are discussed.

A new variant of chromosome 16

A new variant of chromosome 16 with an additional C-band negative segment in the proximal region of the short arm is described. This variant chromosome was found in four cases, two of which were detected prenatally. In three probands the variant chromosome 16 was inherited from a phenotypically normal father.

Length polymorphisms of the human Y chromosome in patients with chronic myelogenous leukemia

The extent of length heteromorphisms of the human Y chromosomes was measured in 50 patients with chronic myelogenous leukemia and was compared with 50 normal controls. No significant difference (p greater than 0.05) in the size of the Y chromosome was noted between these two groups. These findings differ from an earlier investigation, where a longer heterochromatic segment of Y chromosome was noted in malignant tumors and it was stressed that constitutive heterochromatin plays a role in susceptibility of cancer. Present investigation, however, suggests that this is not the case in patients with chronic myelogenous leukemia.

Duchenne muscular dystrophy in a girl with an (X;15) translocation

This is a report of a girl with Duchenne muscular dystrophy (DMD) associated with an 46,X,t (X;15) (p21; q 26) chromosome constitution. Although in the eight published cases of girls with DMD and a t(X;aut) different autosomes were involved in the translocation, the breakpoint was always at Xp21. The present case supports the hypothesis that the DMD gene must be located at Xp21. In this study, involvement of the father's chromosomes in the translocation was detected.

Intensity heteromorphisms of human chromosome 15p by DA/DAPI technique

We suggest that the short arms of human chromosome 15 (15p) exhibit intensity heteromorphisms by DA/DAPI technique. A method for classification of variable intensities is proposed. The different intensities can be classified into at least five classes. They are: negative, pale, medium, intense, and brilliant. Therefore we suggest that 15p is not always positive by DA/DAPI technique. The present findings reveal that the heteromorphism on 15p is far greater than previously thought.

A possible cause of non-disjunction of additional chromosome 21 in Down syndrome

A possible cause of non-disjunction of chromosome 21 in Down Syndromes has been cytogenetically evaluated by examining the parents by Ag-staining technique. In all the cases studied so far, the contributing parents have active ribosomal cistrons on both chromosomes 21 i.e. both chromosomes are stained positively by silver staining. These results show that the active NORs might play an essential role in meiotic non-disjunction. Furthermore, the preliminary results demonstrate that the acrocentric associations of homologous and non-homologous nature involving chromosome 21 are the most frequent in the contributing parent which may further indicate the role of multiple cellular factors affecting the associations in promoting the nondisjunction in addition to active NORs. The possible mechanisms regarding the non-disjunction of chromosome 21 have been described.

Investigation of human chromosome polymorphisms by scanning electron microscopy

Human chromosome polymorphisms were investigated by scanning electron microscopy (SEM). Centromeric heterochromatin was of a constricted morphology. The extent of the C banded region was demarcated by a prominent circumferential groove in G banded chromosomes. Circumferential grooves were observed within the heterochromatin of chromosome 9, and the number of grooves present reflected the size of the region. Three dimensional viewing of satellites and short arms of acrocentric chromosomes, from different angles in the SEM, provided the opportunity for accurate assessment of the size of satellites to be made. Also, small morphological variations were defined in the SEM when definition was uncertain in the light microscope (LM).

Spontaneous in vitro malignant transformation in a xeroderma pigmentosum fibroblast line

This paper deals with a spontaneous malignant transformation in one of our XP fibroblast lines. This cell line, designated XP29MA, was derived from a 14-year-old boy who did not show skin tumors or precancerous alterations either at the time of clinical examination or when the biopsy was taken. We have compared the following features in both the malignant and the benign cell line from which the malignant line developed: tumor formation in nude mice, repair capacity, cytogenetic status, light and electron microscopic characteristics. The benign cell line XP29MA had a doubling time of 4.3 d, did not form tumors in nude mice, showed a very low repair capacity (as determined by colony-forming ability, unscheduled DNA synthesis and alkaline elution) but exhibited a normal cytogenetic and ultrastructural status. In contrast, the transformed cell line XP29MAmal grew three times faster, formed colonies in methyl cellulose, gave rise to fibrosarcomas in nude mice, showed a drastically higher repair capacity, and was characterized by an extreme genetic imbalance, resulting from numerical and structural chromosome alterations of Nos. 1, 3, 4, 8, 12, 16, 17, 18, 20 and 21. Ultrastructural examination revealed fusiform and polygonal cells, the latter exhibiting large indented nuclei, vesicular dilatations of the endoplasmatic reticulum and numerous lysosomes. The higher repair capacity in XP29MAmal cells is tentatively explained in terms of reversion, enhancement of post-replication repair and/or expression of SOS-type functions.

Human chromosomal polymorphism. VII. The distribution of chromosomal Q-polymorphic bands in different human populations

The distribution of chromosomal Q-polymorphic bands was studied in different human populations. The populations studied showed no differences in the relative amount of Q bands in all the 12 polymorphic loci of seven autosomes, but interpopulation differences did exist in the absolute amount of Q bands in all the 12 potentially polymorphic loci of seven autosomes, these differences consisting of uniform increases or decreases in this absolute amount. Comparisons of the mean number of Q-heterochromatin bands with fluorescence levels 4 and 5 per individual showed a consistent prevalence of this quantitative parameter of chromosomal Q polymorphism in females as compared to males in all the national groups. It is suggested that there is some dosage compensation of chromosomal Q-heterochromatin material in females due to the absence of a chromosome in their genome, which is able to "compensate" for the large Q band in chromosome Y which is present only in the karyotype of males.

Chromosome heteromorphism analysis in cases of disputed paternity

Blood test results, using standard procedures, failed to exclude the alleged father as the biological father of a child in a case of disputed paternity. Using 21 different systems, the probability of exclusion for the man was 98.19%, and the probability of paternity was only 93.90% with a paternity index of 15.48. Chromosome heteromorphisms of all three individuals were studied. By comparison of fluorescent markers of chromosomes 13, 14, 15, 21, and 22, the child was shown to inherit one homologue of each of these chromosomes from the mother, but none were like the heteromorphisms of the alleged father. This excluded him as the biological father.

Racial variation of a non-fluorescent segment of the Y chromosome in East Indians

Seventy normal male East Indians were selected to study the length of the fluorescent (f), non-fluorescent (nf), and total length of the Y chromosome by the QFQ technique. The length of the Y chromosome was classified into five groups using subjectively defined criteria, namely, very small, small, average, large, and very large. The frequencies of Y/F indices for these groups were 0.0%, 1.42%, 15.71%, 58.57%, and 24.28%, respectively. The variation in the total length of the Y chromosome was accounted for by variations in the length of the nf as well as the f segments. The mean Y/F, f/F, and nf/F indices for East Indians were 1.20, 0.47, and 0.73, respectively. A longer Y was noted in East Indians primarily owing to an increase in size of the nf segment in comparison to normal black and Caucasian populations. Furthermore, it is emphasised that the definition of 'average' or 'large' Y varies from race to race.

Polymorphism in chromosome 4

Chromosomal analyses were performed on 302 individuals, using QFQ banding techniques to determine the occurrence of heteromorphism of chromosome 4. Two type of heteromorphism were observed, one showing an intensely fluorescent band in the centromere region and the other an intensely fluorescent band in the proximal area of the short arm. No one individual possessed both type of heteromorphism. A higher frequency of chromosome 4 heteromorphism was found in patients with schizophrenia, mental retardation, hyperactivity, developmental delay and speech impediments than in normal individuals.

Population heteromorphisms of Ag-stained nucleolus organizer regions (NORs) in the acrocentric chromosomes of East Indians

The nucleolar organizer regions (NORs) of acrocentric chromosomes in 70 normal East Indians were examined by Ag-staining (NSG) and acridine orange reverse banding (RFA) techniques. The Ag-stainability of NORs was variable from one individual to another but characteristics were constant within each individual. The average modal number of Ag-positive NORs per individual was eight. A racial difference in the expression of NORs is suggested. To study the heteromorphism of NORs, the NSG technique was found to be more useful than RFA. Furthermore, it is concluded that there is no direct relationship between a heteromorphism of NORs identified by NSG and that identified by the RFA technique. Quantitative data on these differences is provided. In addition NOR-regions are classified into five sizes namely; very large, large, medium, small, and very small using subjectively defined criteria.

Human chromosomal heteromorphisms in American blacks. IV. Intensity variation in centromeric regions of chromosomes 3 and 4

One hundred normal American Blacks were studied by q-banding technique in order to estimate the type and frequency of heteromorphisms in the centromeric regions of chromosomes 3 and 4. Heteromorphisms were classified using the scale of five intensity levels namely, negative, pale, medium, intense and brilliant fluorescence. QFQ intensity heteromorphisms (greater than or equal to level 3) for chromosomes 3 and 4 were 54.5 and 7%, respectively. The distribution of different intensities was significantly different in American Blacks compared with Caucasians.

Human chromosomal heteromorphisms in American blacks. III. Evidence for racial differences in RFA color and QFQ intensity heteromorphisms

One hundred normal American Blacks (B) were studied by sequential QFQ and RFA banding techniques in order to estimate the type and frequency of heteromorphisms. Color heteromorphisms were classified into one of six colors by RFA and intensity variation into one of five levels by QFQ. The data are compared with a previously studied Caucasian population (C). The frequencies of QFQ and RFA heteromorphisms were significantly higher in the Black than in the Caucasian population. No racial difference was noted for chromosome 21 by QFQ, while RFA demonstrated a clear difference. It is concluded that the maximum characterization of racial differences of human chromosomal heteromorphisms was far greater by RFA than with QFQ. The present study suggests differences in QFQ and RFA heteromorphisms among the two races.

Human chromosome heteromorphisms in Americans Blacks: II. Higher incidence of pericentric inversions of secondary constriction regions (h)

Eighty normal American blacks were studied by the CBG technique (C-bands by barium hydroxide using Giemsa) for estimation of size and inversion heteromorphism of chromosomes 1, 9, and 16, and the data were compared to those of whites using subjectively defined criteria. Size and inversion heteromorphisms were classified into 5 levels. The frequencies of size hetromorphisms of chromosomes 1 and 16 were 10.63% and 6.88%, respectively, which are not significantly different from those of a normal population of whites. A higher incidence of size heteromorphisms for chromosome 9 was noted in whites (47.5% vs 30%). The frequencies of inversion heteromorphism of chromosomes 1, 9, and 16 were 17.5%, 21.9%, and 0.0%, respectively. Overall, 61 chromosomes were found to have an inversion. Of these, 28 were in chromosome 1, and 33 were in chromosome 9. A higher incidence of inversion heteromorphisms of chromosomes 1 and 9 was noted in American blacks, while no inversions were found in chromosome 16 in either population. A significant association of increased size of the h region with inversion (r = 0.99 P less than 0.01) is demonstrated, ie, enlarged h regions have a higher frequency of inversions.

On the structure and polymorphism of the human chromosome no. 15

Chromosome preparations from 67 randomly selected healthy individuals were consecutively stained with silver, mithramycin, pentamidine-DAPI, and quinacrine mustard. In the short arm of the chromosome 15 up to six regions, which were highly polymorphic, apparently independently of each other, could be detected by the given methods. The frequency of occurrence of each individual polymorphic feature in the population investigated ranges from 70% to 87%. They appear randomly and independently of each other.