Genetic risks for familial reciprocal translocations with special emphasis on those leading to 9p, 10p and 12p trisomies

Exploring the Genetic Causality of Discordant Phenotypes in Familial Apparently Balanced Translocation Cases Using Whole Exome Sequencing

Familial apparently balanced translocations (ABTs) are usually not associated with a phenotype; however, rarely, ABTs segregate with discordant phenotypes in family members carrying identical rearrangements. The current study was a follow-up investigation of four familial ABTs, where whole exome sequencing (WES) was implemented as a diagnostic tool to identify the underlying genetic aetiology of the patients' phenotypes. Data were analysed using an in-house bioinformatics pipeline alongside VarSome Clinical. WES findings were validated with Sanger sequencing, while the impact of splicing and missense variants was assessed by reverse-transcription PCR and in silico tools, respectively. Novel candidate variants were identified in three families. In family 1, it was shown that the de novo pathogenic STXBP1 variant (NM_003165.6:c.1110+2T>G) affected splicing and segregated with the patient's phenotype. In family 2, a likely pathogenic TUBA1A variant (NM_006009.4:c.875C>T, NP_006000.2:p.(Thr292Ile)) could explain the patient's symptoms. In family 3, an SCN1A variant of uncertain significance (NM_006920.6:c.5060A>G, NP_008851.3:p.(Glu1687Gly)) required additional evidence to sufficiently support causality. This first report of WES application in familial ABT carriers with discordant phenotypes supported our previous findings describing such rearrangements as coincidental. Thus, WES can be recommended as a complementary test to find the monogenic cause of aberrant phenotypes in familial ABT carriers.

Goyal-Naqvi Syndrome (Concurrent Trisomy 10p and Terminal 14q Deletion): A Review of the Literature

Goyal-Naqvi syndrome (GNS) is a newly documented clinical entity that comprises trisomy 10p and terminal 14q deletion, though trisomy 10p and terminal 14q deletion have been discovered as distinct conditions in 1974 and 1997, respectively. Nevertheless, to date, the total number of reported cases of each of these conditions is estimated to be in double digits. Both manifest as a constellation of features like craniofacial dysmorphism, hypotonia, intellectual impairment and global developmental delay. Characteristic facies include protruded forehead, hypertelorism, epicanthic folds, down slanting palpebral fissures, flat nasal bridge, long philtrum, thin upper lip, carp-shaped mouth, retro-micrognathia and low set ears. Besides, trisomy 10p is strikingly associated with clinodactyly and camptodactyly which aids in clinical diagnosis, apart from other musculoskeletal deformities like hip dysplasia and pes planus. Intersex conditions have been found to commonly co-exist. As other systems also display involvement frequently, trisomy 10p is a discernible multiple congenital anomalies/mental retardation (MCA/MR) syndrome. On the other hand, with terminal 14q deletion, increased risk of certain types of cancer was predicted as specific tumor suppressor genes are lost in the deletion and thus, screening was recommended. Genetic workup using techniques like fluorescence in situ hybridization (FISH), spectral karyotyping (SKY) and chromosomal microarray-based comparative genomic hybridization (CGH) was found to be helpful in diagnosis of trisomy 10p and 14q deletion. Prenatal diagnosis of these conditions has been well documented too. Intrauterine growth retardation has been observed to be related to trisomy 10p. There is a paucity of literature on the management of children diagnosed with trisomy 10p or with terminal 14q deletion. Although management of a child diagnosed with concomitant occurrence of trisomy 10p and terminal 14q deletion by a multidisciplinary approach emphasizing physiotherapeutic intervention has shown remarkable improvement in motor skills, the care of children diagnosed with these genetic aberrations needs further investigation. Documentation of more such cases will help to expand phenotypic spectrum for early identification and to delineate natural history for a life span approach. Early identification and intervention facilitate tapping of the maximum neuroplastic potential for better neurodevelopmental outcomes. We present a review of current literature on this novel syndrome to identify gaps in knowledge to build future research.

Chromosomal abnormalities as a cause of recurrent abortions in Egypt

BACKGROUND:

In 4%-8% of couples with recurrent abortion, at least one of the partners has chromosomal abnormality. Most spontaneous miscarriages which happen in the first and second trimesters are caused by chromosomal abnormalities. These chromosomal abnormalities may be either numerical or structural.

MATERIAL AND METHODS:

Cytogenetic study was done for 73 Egyptian couples who presented with recurrent abortion at Genetic Unit of Children Hospital, Mansoura University.

RESULTS:

We found that the frequency of chromosomal abnormalities was not significantly different from that reported worldwide. Chromosomal abnormalities were detected in 9 (6.1%) of 73 couples. Seven of chromosomal abnormalities were structural and two of them were numerical.

CONCLUSION:

Our results showed that 6.1% of the couples with recurrent abortion had chromosomal abnormalities, with no other abnormalities. We suggest that it is necessary to perform cytogenetic in vestigation for couples who have recurrent abortion.

Genetic counseling in Robertsonian translocations der(13;14): frequencies of reproductive outcomes and infertility in 101 pedigrees

Robertsonian translocations 13/14 are the most common chromosome rearrangements in humans. However, most studies aimed at determining risk figures are more than 20 years old. Their results are often contradictory regarding important topics in genetic counseling such as infertility and unfavorable pregnancy outcomes. Here, we present a study on a sample of 101 previously unreported pedigrees of der(13;14)(q10;q10). In order to minimize problems of partial ascertainment, we included families with a wide range of reasons of ascertainment such as birth of a child with congenital anomalies, prenatal diagnosis due to maternal age, fertility problems and recurrent pregnancy loss. No evidence of increased infertility rates of female and male carriers was found. The detected miscarriage frequency of female carriers was higher than previously reported (27.6 +/- 4.0% of all spontaneous pregnancies). This may be explained by an over-correction of earlier studies, which excluded all unkaryotyped miscarriages. In three out of 42 amniocenteses, translocation trisomies 13 were diagnosed (7.1 +/- 4.0% of all amniocenteses). The frequency of stillbirths was 3.3 +/- 1.6% for female carriers and 1.4 +/- 1.4% for male carriers. A low risk for the live birth of translocation trisomy 13 children was confirmed since no live born children with trisomy 13 or Pätau syndrome were detected in the ascertainment-corrected sample.

Cytogenetic study in cases with recurrent abortion in Saudi Arabia

BACKGROUND

A proportion of cases with repeated abortion are caused by chromosomal abnormality in one of the parents. Several studies have been done to determine the role of chromosomal abnormalities in couples with repeated fetal loss in various countries. None of these studies was done in the Arab Peninsula.

MATERIAL AND METHODS

Cytogenetic study was done for 193 consecutive Saudi couples who presented with repeated abortion at the King Khalid University Hospital in Riyadh, Saudi Arabia.

RESULTS

We found that the frequency of chromosomal abnormalities was not significantly different from that reported worldwide. The nature of those abnormalities and their relation to the obstetric history of cases were discussed.

CONCLUSION

This study should help physicians working in the region to realize the contribution of chromosomal abnormalities to cases of repeated fetal loss. It should also help in setting priorities of cytogenetic screening in individual cases.

Intrachromosomal insertion of chromosome 7

A direct intrachromosome insertion of chromosome 7 is described, and previous reports of intrachromosomal insertions are listed. All others were ascertained through a phenotypically abnormal proband. Ours is the first presenting with multiple pregnancy losses. Since the risk of chromosomally abnormal liveborns is appreciable, prenatal diagnosis should be made available to known carriers of chromosome insertions.

Genetic counseling in families with inherited balanced translocations: experience with 36 families

We report on genetic counseling and investigations in 36 families with inherited balanced translocations ascertained in different ways, with special regard to the completeness and reasons for incompleteness of family investigation. Quantitative evaluation of the results of cytogenetic investigations shows that non-directive genetic counseling was very effective in many families. Yet, in most of the families (34) genetic counseling and investigation remained incomplete in the sense that not all living potential translocation carriers could be counseled or investigated or that the origin of a fresh mutation could not be established by a normal karyotype in the parents of a carrier. Only in seven families could nearly all living potential carriers be counseled and investigated. The most frequent reason for incompleteness was the impossibility of transmitting or refusal to transmit information about the genetic risks to relatives (21 families), whereas direct rejection of investigation by a counseled individual was a rather rare event (18 adults). Families ascertained because of an unbalanced child seem to be more willing to transmit genetic information to relatives than families ascertained in other ways. Non-directive genetic counseling gave us an insight into the emotional problems arising during counseling of translocation families.

Segregation of a t(3;20) translocation through three generations resulting in unbalanced karyotypes in six persons

With the aid of high resolution chromosome banding, a t(3;20) translocation was discovered in a large family with six retarded members. The translocation involved very small terminal segments. The unbalanced products resulting in partial trisomy 20q and monosomy 3p were observed in the retarded subjects. Gene localisation studies of the ADA gene, with a presumed locus on the long arm of chromosome 20, were also carried out and seem to exclude this gene from the distal part of 20q (20q13.1----qter).

Prospective risk in reciprocal translocation heterozygotes at amniocentesis as determined by potential chromosome imbalance sizes. Data of the European Collaborative Prenatal Diagnosis Centres

The Date of the European Cooperative Prenatal Diagnosis Laboratories (Boué and Gallano, 1984) of 596 prenatal (amniocyte) diagnoses of familial rcp was examined as to relationships between balanced/unbalanced result and ascertainment, carrier parent and chromosome imbalance size (percentage haploid autosome length). Each rearrangement was graphed once with actual (unbalanced result) or potential (normal or balanced result) imbalances plotted with trisomy as the ordinate and monosomy as the abscissa. The graphed data was divided into 15 regions, each of 2.0 per cent trisomy and 0.75 per cent monosomy and the rate of unbalanced pregnancies determined for each region. The highest rates of chromosomally unbalanced progeny (excluding regions with inadequate data) were found closest to the origin (i.e. associated with the smallest imbalances) and these were for ascertainment category 1 (previous rcp unbalanced child) 22.3 per cent for maternal carriers and 39 per cent for paternal carriers. Overall in pooled data for this ascertainment category (without reference to the imbalance graphs) there were for paternal carriers 28.6 per cent unbalanced pregnancies and for maternal carriers 18.1 per cent. The graphed data, therefore, revealed the higher rates associated with some of the rcp with small potential (combined duplication/deficiency) imbalances. Lesser rates were observed for ascertainment category 2 (carrier parent with a history of recurrent miscarriage) with overall percentages of imbalanced progeny ranging from 2.7 (paternal carriers) to 4.7 (maternal carriers). Again, higher rates were revealed in graphed data for small potential imbalances. All unbalanced results for this group (ascertainment category 2) plotted in the region closest to the origin with rates of 16 per cent (maternal carriers) and 9.5 per cent (paternal carriers) in this region. Remarkably in both ascertainment groups 1 and 2 there was no significant difference in the size of the imbalanced segments for unbalanced progeny. In ascertainment group 1 this was (dup/def; mean +/- S.D.): 1.09 +/- 0.77/0.47 +/- 0.45 and in ascertainment group 2: 1.09 +/- 0.80/0.66 +/- 0.71. From the graphed data which arguably denote viability relationships, a trisomy was approximately 2.7 times as likely to survive until amniocentesis as a monosomy of equivalent size. It is proposed that given further data, risk estimates could be determined for rcp heterozygotes using the present approach where empiric data (from the family history or an analysed series of similar rcp) is not available.

Comments on methods and results in: Sherman et al., "Segregation analysis of balanced pericentric inversions in pedigree data"

The analysis by Sherman et al. (1986), its basis and results have been examined. The analysis relies on general methods, which may give acceptable results under the special conditions considered by the authors, but will usually produce more or less misleading results. The program POINTER (Lalouel & Morton 1981) is shown to be based partly on a chain of irrelevant arguments for the actual context. The so-called "conventional ascertainment rules" (Morton et al. 1983) are shown to produce misleading results in cases where their assumptions are not satisfied. The mean risk for unbalanced offspring is underestimated because of an erroneous ascertainment correction. The segregation frequencies are found to be different in three national samples, contrary to the claim by Sherman et al. Only a small proportion of all information available in the data has been utilized. Alternative and more appropriate models, hypotheses and procedures have been suggested. The frequent use of packages with computer programmes of standard statistical procedures in nonstandard situations with data from collaborative studies in human cytogenetics is discussed.

Role of chromosome aberrations in recurrent abortion: a study of 269 balanced translocations

We have studied a sample of 5,445 couples in which the woman was ascertained to have had two or more spontaneous abortions: 396 from our Cytogenetics Unit (present series) and 5,049 from the literature (literature series). In approximately 5% of these couples one of the members was a carrier of a balanced translocation, either reciprocal (2/3 of cases) or Robertsonian (1/3). In 1% of the couples there were other chromosome anomalies, mostly gonosomal aneuploidies or mosaicisms. A pericentric inversion of the heterochromatic region of chromosome 9 was present in 3% of the couples of the present series and in 1% of the literature series. The number of female carriers exceeded significantly that of males. The probability for one member of the couple to be a carrier increased with the number of abortions at the time of ascertainment, but it does not seem modified by the concomitant presence of term pregnancies. The analysis of the cytogenetic findings in 80 cases of Robertsonian and 156 cases of reciprocal translocations suggests that some chromosomes are preferentially involved, and that in reciprocal translocations the breakpoints are not distributed at random on the chromosome arms. There is an excess of breakpoints on chromosomes 6, 7, and 22 and a dearth on chromosome 12. This distribution is significantly different from that of a sample of reciprocal translocations ascertained for a malformed child. In both samples the breakpoints seem associated with fragile sites more frequently than expected by chance. An analysis of the potential and effective chromosome imbalance suggests that in subjects with unbalanced chromosomes survival is correlated with a minimum imbalance.

Segregation patterns and phenotypes of unbalanced offspring in a large family with (10;18) chromosome translocation

We describe a large family in whom a balanced 10;18 chromosome translocation is segregating through five generations. Six severely mentally retarded relatives and an abnormal fetus further define the phenotypic expression of dup (18q21----qter). Other segregants detected prenatally included a fetus with deletion 18q21----qter and two fetuses with dup(18pter----q21) owing to tertiary trisomy. One of the latter also had an extra X chromosome; this might be another example of possible nonhomologous pairing in man.

Transmitting balanced translocation carrier information within families: a follow-up study

Approximately 1 of 500 individuals is a carrier of a balanced chromosome translocation. Since many translocations are inherited, many (but not all) relatives of carriers have a need to be informed of their potential carrier status. Presently, no data are available as to what extent individuals identified as balanced carriers inform at-risk relatives of the problem. We interviewed 12 balanced translocation carriers to learn whether such information had been transmitted to relatives. The 12 propositi had 36 surviving sibs and 21 surviving parents. Of the 36 sibs, 32 were informed of their risk. The four sibs not informed were from two families. Only 16 of the 32 informed sibs had subsequent carrier testing. Of the 21 surviving parents, 14 were told by their children of their carrier status; subsequently, three parent couples were tested. This survey provides data showing that individuals do not always disclose genetic risk information to relatives. Therefore, genetic professionals need to determine if they have a duty to transmit such information to at-risk relatives in light of the harm that may occur when information is withheld.

Balanced reciprocal translocations: risk factors for aneuploid segregant viability

Reciprocal translocations were studied in two groups of balanced carrier couples: 202 had 210 translocation aneuploid between (LB) infants, and 95 couples had repetitive abortions (AB) without liveborn aneuploids. The observed translocation aneuploidies in the LB group were compared to predicted potential aneuploidies in AB by frequency of chromosome involvement, meiotic segregation mode, and mean trisomic, monosomic and combined genomic imbalances. Qualitative and quantitative differences identified genomic regions and chromosomes possibly vital for in utero survivability. LB aneuploidies indicate non-random chromosome involvement, selection of least detrimental segregants and segments, and predominant transmission from maternal balanced carriers (especially in 3:1 tertiary segregation, 93.5%). For an individual with a balanced reciprocal translocation and untested reproductive capability, an approach is given for predicting whether a translocation aneuploid conceptus will be liveborn or aborted.

Balanced rearrangement of chromosomes 2, 5, and 13 in a family with duplication 5q and fetal loss

We have studied a family in which a mother and daughter (the proposita) had the karyotype 46,XX,ins(2;5),t(5;13). The mother had four spontaneous abortions, a mentally retarded son with duplication (5q), and a daughter who died at 3 months. The proposita had a phenotypically abnormal abortus. Rearrangements involving several chromosomes are very rare. Observations on this family are consistent with the predicted high likelihood of reproductive loss.

Risk for chromosome abnormality at amniocentesis following a child with a non-inherited chromosome aberration. A European Collaborative Study on Prenatal Diagnoses 1981

Based on 2890 prenatal diagnoses from 12 European countries the risk for a chromosomally abnormal fetus at amniocentesis after the birth of a child with a chromosome abnormality has been estimated to be 1.3 per cent when the mother's age is 34 years or less at amniocentesis and 1.8 per cent if the mother is older. This risk does not depend on paternal age, and it is independent of the type of the chromosome abnormality of the index child. Some geographical heterogeneities were detected. Therefore, the overall risk has to be considered as a rough estimate. The chromosome constitution of the abnormal fetus differed from that of the index patient in 21 of 41 cases. Several explanations for the higher risk have been discussed. If the index child had trisomy 18, 13 or a sex chromosome abnormality, the fetus tended to be a female. If the index child was a trisomy 21, the fetal sex ratio was normal.

Segregation analysis in reciprocal translocation carriers

Segregation analysis of the offspring of balanced translocation carriers was performed on 327 pedigrees collated from published sources and personal communications. Correction was made for bias of ascertainment. Translocations studied involved chromosome arms 1p, 4q, 6p, 6q, 7p, 8p, 10p, 10q, 11q, 14q, 16q, and 17p. Findings included similar rates of occurrence of abnormal liveborn offspring in male and female carriers except for a reduction of risk in male carriers of translocations segregating by 3:1 mode; an elevated risk of fetal loss (spontaneous abortions and stillbirths) in female carriers of 6q, 11q, and 16q translocations compared to male carriers of these translocations; a fetal loss rate exceeding general population estimates in female carriers of 6q and 10q translocations and in male carriers of 6p, 8p, 10q, and 14q translocations including a rate of nearly 50% among female 6q translocation carriers; a higher than expected number of balanced carriers among liveborn offspring; and a low risk of abnormal liveborn children among carriers ascertained by means other than through unbalanced probands. We propose that some translocation carriers may be helped by consideration of more specific empiric risk figures than have traditionally been used.

Unbalanced reciprocal translocations in cases of Prader-Willi syndrome

A case of Prader-Willi syndrome (PWS) associated with a de novo unbalanced 15q;17q reciprocal translocation presumptively resulting from the tertiary monosomic form of 3:1 meiotic disjunction is described. Twenty-three similar unbalanced translocations have been identified from the literature. The 24 karyotypes are characterised by having 45 chromosomes, monosomy for the pericentromeric region of chromosome 15 (range pter----q11 to q21), and little monosomy of the recipient (non-15) chromosome. Two-thirds of the cases with these karyotypes have phenotypic features of PWS. It seems probable that (i) where unbalanced reciprocal translocations are associated with PWS, they will almost invariably be presumptive segregants of the tertiary monosomic form of 3:1 disjunction and (ii) the majority of cases found with this type of karyotype, particularly it appears when de novo in origin, will be associated with phenotypic features of PWS.

Familial Wolf's syndrome with a hidden 4p deletion by translocation of an 8p segment. Unbalanced inheritance from a maternal translocation (4;8)(p15.3;p22). Case report, review and risk estimates

This is the case report of a patient with Wolf's syndrome having a monosomy 4pter----p15.3 and an additional trisomy 8pter----p22, derived from a maternal balanced translocation t(4;8)(p15.3;p22) after 2:2 disjunction and adjacent-1 segregation. The patient's phenotype is presumably slightly modified by the trisomic 8p segment. Literature analyses indicate that phenotypic "hybrids" with traits of monosomy 4p and of other autosomal segment trisomies exist. The dermatoglyphics of the patient were not highly characteristic for Wolf's syndrome. Also the dermatoglyphics of the balanced translocation carriers were unspecific and did not reflect the carrier status. Pedigree analyses of 46 reported families with reciprocal translocations involving the short arm of chromosome 4 show a high risk (20.5% +/- 4.6%) for unbalanced offspring (trisomy or monosomy 4p) after 2:2 disjunction and adjacent-1 segregation, if the breakpoint in the recipient chromosome is terminal and the resulting imbalance concerns the 4p segment only. It is considerably lower (4.5% +/- 2.5%) if the breakpoint in the recipient chromosome is subterminal, as in the reported case, and the resulting imbalance concerns other chromosome segments additionally to the 4p segment. In both instances, the risk decreases with increasing segment length. The risk for unidentified abortions, stillbirths or neonatal deaths is also high in these families (about 40%). The frequency of progeny with balanced compared to progeny with normal karyotype corresponds to the expected 50% for alternate segregation.

The phenotype in partial 13q trisomies, apropos of a familial (13;15)(q22;q26) translocation

A 12 month-old male patient with a karyotype 46,XY,-15,+der(15),t(13;15)(q22;q26)pat is presented. His stillborn sib showed malformations compatible with the 13q deletion syndrome, probably due to a 46,XY,der(13) karyotype. Phenotypic analysis of 41 cases from the literature with partial distal 13q (D13q) trisomies indicate that the segment 13q22----qter in trisomy with or without another concomitant aneusomy is sufficient to produce the majority of the trisomy 13 syndrome features, some of which (cleft palate, increased HbF and projections in PMN) are present in different non-overlapping partial 13q trisomies. About 82% of the D13q trisomies are inherited, more frequently from the mother.

Reproductive risks for translocation carriers: cytogenetic study and analysis of pregnancy outcome in 58 families

Here we report on studies of the reproductive risks for heterozygous carriers of chromosome translocations. Pregnancy outcome, breakpoints, mode of segregation of the translocated chromosomes, and resulting chromosome imbalance were analyzed in 58 families (46 with reciprocal and 12 with Robertsonian translocations) ascertained for birth of a malformed child, recurrent spontaneous abortion, or hypogonadism. These families include a total of 122 informative sibships. The analysis of the data, after correction for ascertainment bias, showed that the incidence of spontaneous abortion is nearly 50% in reciprocal and between 20 and 25% in Robertsonian translocation families ascertained for malformed child or recurrent abortion. The risk of malformed infants with unbalanced genome is approximately 6% among the liveborn offspring of reciprocal translocation carriers and 23% among the liveborn offspring of carrier mothers of t(14q21q). The distribution of the breakpoints on the chromosomes involved in reciprocal translocations ascertained through a malformed child is nonrandom, with an excess on chromosomes 5, 9, 13, and 15. The study of chromosome imbalance, expressed as gain or loss of a portion of genetic information relative to the total haploid autosome length (percent HAL), shows that among the common types of disjunction-segregation leading to unbalanced gametes, adjacent 1 seems to be the one producing on the average the least level of genome imbalance. This explains why it is the most frequently observed type of segregation giving rise to gametes from which subjects with a chromosome imbalance compatible with life can be generated.

Mating between two balanced translocation carriers in two unrelated families

Partial trisomy 9p and a 13/14 translocation occurred in the daughter of a t(5;9)(p15;p12) mother and a t(13;14)(p11;q11) father. Two additional offspring displayed a normal karyotype and a translocation trisomy 13 respectively. Two first cousins, selected for chromosome analysis because of a spontaneous abortion, were found to have an identical translocation t(14;21)(p11;q11). Their second pregnancy was monitored by midtrimester amniocentesis and disclosed a balanced fetus. The different zygotic chromosome constitutions and the counselling problems in the marriages between two balanced translocation carriers are discussed.

Partial trisomy 12q: a clinically recognisable syndrome. Genetic risks associated with translocations of chromosome 12q

A newborn child with an unusual facial appearance and multiple abnormalities was found to be trisomic for a large part of 12q as a result of adjacent 1 segregation of a familial translocation, t(9;12) (p24;q21.2). A combination of cytogenetic analysis, clinical features, and enzyme marker studies allows an accurate assessment of the breakpoints. Although trisomic for a considerably larger area of 12q than other reported cases, there are many similar features suggesting that trisomy 12q is a clinically recognisable syndrome. The frequency and mode of segregation of 12q translocations and their implications for genetic counselling are discussed.

New chromosomal dysmorphic syndromes. 4. Trisomy 12p

This is the report of two independent families in which a balanced maternal translocation led to trisomy 12 p in one of each their offspring. Evaluation of 21 further case reports indicates that this is a phenotypically well defined syndrome which leads to severe developmental retardation. It can be recognized by a characteristic combination of craniofacial anomalies which are summarized in a phantom picture. The gene sequences which produce the typical features in the trisomic state must be localized distally to band 12p12, which is the breakpoint in the partial trisomies. The specific craniofacial anomalies are not visibly modified by the length of the trisomic segment or additional small monosomies or trisomies of recipient chromosomes. However, the frequency and severity of organ malformations and the resulting probability of survival seem to decrease with increasing degrees of chromosomal imbalance. A cytogenetic classification of the 21 inherited translocations and a segregation analysis from the pedigree data was performed. For the different types of translocations the calculated risk figures are given.