MAP, an expert system for multiple pairwise linkage analysis

Chiasma-based genetic maps of chromosome 21

The available cytogenetic data on meiotic chiasmata have been used to construct sex-specific genetic maps, showing the genetic distances and recombination fractions along the length of 21q. The male maps are based on direct observations of spermatocytes, while the female maps are derivations related to the increased chromosome length in oocytes. The male chiasma data have also been used as a frame of reference for ordering and positioning loci on the physical map with D21S110 as a fixed point.

Obligate short-arm exchange in de novo Robertsonian translocation formation influences placement of crossovers in chromosome 21 nondisjunction

Robertsonian translocations (ROBs) involving chromosome 21 are found in approximately 5% of patients with Down syndrome (DS). The most common nonhomologous ROB in DS is rob(14q21q). Aberrant recombination is associated with nondisjunction (NDJ) leading to trisomy 21. Haplotype analysis of 23 patients with DS and de novo rob(14q21q) showed that all translocations and all nondisjoined chromosomes 21 were maternally derived. Meiosis II NDJ occurred in 21 of 23 families. For these, a ROB DS chromosome 21 genetic map was constructed and compared to a normal female map and a published trisomy 21 map derived from meiosis II NDJ. The location of exchanges differed significantly from both maps, with a significant shift to a more distal interval in the ROB DS map. The shift may perturb segregation, leading to the meiosis II NDJ in this study, and is further evidence for crossover interference. More importantly, because the event in the short arms that forms the de novo ROB influences the placement of chiasmata in the long arm, it is most likely that the translocation formation occurs through a recombination pathway in meiosis. Additionally, we have demonstrated that events that occur in meiosis I can influence events, such as chromatid segregation in meiosis II, many decades later.

Linkage analysis in the presence of errors II: marker-locus genotyping errors modeled with hypercomplex recombination fractions

It is well known that genotyping errors lead to loss of power in gene-mapping studies and underestimation of the strength of correlations between trait- and marker-locus genotypes. In two-point linkage analysis, these errors can be absorbed in an inflated recombination-fraction estimate, leaving the test statistic quite robust. In multipoint analysis, however, genotyping errors can easily result in false exclusion of the true location of a disease-predisposing gene. In a companion article, we described a "complex-valued" extension of the recombination fraction to accommodate errors in the assignment of trait-locus genotypes, leading to a multipoint LOD score with the same robustness to errors in trait-locus genotypes that is seen with the conventional two-point LOD score. Here, a further extension of this model to "hypercomplex-valued" recombination fractions (hereafter referred to as "hypercomplex recombination fractions") is presented, to handle random and systematic sources of marker-locus genotyping errors. This leads to a multipoint method (either "model-based" or "model-free") with the same robustness to marker-locus genotyping errors that is seen with conventional two-point analysis but with the advantage that multiple marker loci can be used jointly to increase meiotic informativeness. The cost of this increased robustness is a decrease in fine-scale resolution of the estimated map location of the trait locus, in comparison with traditional multipoint analysis. This probability model further leads to algorithms for the estimation of the lower bounds for the error rates for genomewide and locus-specific genotyping, based on the null-hypothesis distribution of the LOD-score statistic in the presence of such errors. It is argued that those genome scans in which the LOD score is 0 for >50% of the genome are likely to be characterized by high rates of genotyping errors in general.

Multipoint estimation of genetic maps for human trisomies with one parent or other partial data

Centromeric-mapping methods have been used to investigate the association between altered recombination and meiotic nondisjunction in humans. For trisomies, current methods are based on the genotypes from a trisomic offspring and both parents. Because it is sometimes difficult to obtain samples from both parents and because the ability to use sources of DNA previously not available (e.g., stored paraffin-embedded pathological samples) has increased, we have been interested in creating similar maps for trisomic populations in which one of the parents of the trisomic individual is unavailable for genotyping. In this paper, we derive multipoint likelihoods for both missing-parent data and conventional two-parent data. We find that likelihoods for two-parent data and for data generated without a sample from the correctly disjoining parent can be maximized in exactly the same way but also that missing-parent data has a high frequency of partial data of the same sort produced by intercross matings. Previously published centromeric-mapping methods use incorrect likelihoods for intercross matings and thus can perform poorly on missing-parent data. We wrote a FORTRAN program to maximize our multipoint likelihoods and used it in simulation studies to demonstrate the biases in the previous methods.

Linkage analysis in the presence of errors I: complex-valued recombination fractions and complex phenotypes

Linkage is a phenomenon that correlates the genotypes of loci, rather than the phenotypes of one locus to the genotypes of another. It is therefore necessary to convert the observed trait phenotypes into trait-locus genotypes, which can then be analyzed for coinheritance with marker-locus genotypes. However, if the mode of inheritance of the trait is not known accurately, this conversion can often result in errors in the inferred trait-locus genotypes, which, in turn, can lead to the misclassification of the recombination status of meioses. As a result, the recombination fraction can be overestimated in two-point analysis, and false exclusions of the true trait locus can occur in multipoint analysis. We propose a method that increases the robustness of multipoint analysis to errors in the mode of inheritance assumptions of the trait, by explicitly allowing for misclassification of trait-locus genotypes. To this end, the definition of the recombination fraction is extended to the complex plane, as Theta=straight theta+straightepsiloni; theta is the recombination fraction between actual ("real") genotypes of marker and trait loci, and straightepsilon is the probability of apparent but false ("imaginary") recombinations between the actual and inferred trait-locus genotypes. "Complex" multipoint LOD scores are proven to be stochastically equivalent to conventional two-point LOD scores. The greater robustness to modeling errors normally associated with two-point analysis can thus be extended to multiple two-point analysis and multipoint analysis. The use of complex-valued recombination fractions also allows the stochastic equivalence of "model-based" and "model-free" methods to be extended to multipoint analysis.

Susceptible chiasmate configurations of chromosome 21 predispose to non-disjunction in both maternal meiosis I and meiosis II

The cause of non-disjunction of chromosome 21 remains largely unknown. Advanced maternal age is associated with both maternal meiosis I (MI) and meiosis II (MII) non-disjunction events. While reduced genetic recombination has been demonstrated in maternal MI errors, the basis for MII errors remains uncertain. We studied 133 trisomy 21 cases with maternal MII errors to test the hypothesis that segregation at MII may also be influenced by genetic recombination. Our data support a highly significant association: MII non-disjunction involves increased recombination that is largely restricted to proximal 21q. Thus, while absence of a proximal recombination appears to predispose to non-disjunction in MI, the presence of a proximal exchange predisposes to non-disjunction in MII. These findings profoundly affect our understanding of trisomy 21 as they suggest that virtually all maternal non-disjunction results from events occurring in meiosis I.

European Gene Mapping Project (EUROGEM): breakpoint panels for human chromosomes based on the CEPH reference families. Centre d'Etude du Polymorphisme Humain

Meiotic breakpoint panels for human chromosomes 2, 3, 4, 5, 6, 7, 8, 9, 10, 13, 14, 15, 17, 18, 20 and X were constructed from genotypes from the CEPH reference families. Each recombinant chromosome included has a breakpoint well-supported with reference to defined quantitative criteria. The panels were constructed at both a low-resolution, useful for a first-pass localization, and high-resolution, for a more precise placement. The availability of such panels will reduce the number of genotyping experiments necessary to order new polymorphisms with respect to existing genetic markers. This paper shows only a representative sample of the breakpoints detected. The complete data are available on the World Wide Web (URL http:/(/)www.icnet.uk/axp/hgr/eurogem++ +/HTML/data.html) or by anonymous ftp (ftp.gene.ucl.ac.uk in/pub/eurogem/maps/breakpoints).

Pairwise analysis of radiation hybrid mapping data

Two point lod scores are widely used in pedigree analysis as they provide a fast and efficient method of establishing linkage. Groups of markers that lie in close proximity to one another can be formed by admitting any locus that is linked to at least one existing member of the group with lod score greater than some predetermined value. It seems natural to extend this technique to Radiation Hybrid Mapping both for constructing groups of tightly linked loci that may then be analysed using more powerful statistics and as a method of ordering in its own right. A general extension of two point analysis is derived and the problems associated with radiation hybrid data are discussed. In particular, the additional parameters representing the probabilities of different fragments being retained (which have no parallel in classical linkage analysis) lead to a range of estimators of the breakage probability, O, which have equal and maximal likelihood. Ways of circumventing this problem are discussed along with the potential errors they introduce. Importantly the ambiguity in estimation of theta is not carried through to the lod score as this depends only on the maximum value of the likelihood and not on the particular value of theta at which it occurs. Thus even though two point analysis fails to provide robust estimates of either breakage probabilities or the distance between loci, it represents a simple and effective method of constructing linkage groups that may be analysed with more powerful statistical methods. This is particularly important given the large number of microsatellites, ESTs and candidate genes currently being typed on radiation hybrids.

A note on the combination of estimates of a recombination fraction

A number of ways of combining two or more independent estimates of the same recombination fraction can be found in the literature. We revisit this topic in the context of human gene mapping, and explore the value of transforming the recombination fraction to a new parameter whose log-likelihood function is more nearly quadratic. It is shown that the arcsine of the cube-root is one such function. These observations lead naturally to a way of summarizing and combining the summarized set of log-likelihood functions of a common recombination fraction. This idea is illustrated using pedigree data concerning six loci on chromosome 10 from the CEPH consortium. A comparison is also made with the method of summarizing and combining using 'equivalent numbers' of recombinants and informative meioses.

Programs, databases, and expert systems for human geneticists--a survey

We present an overview of the variety of databases and programs that offer substantial aid to medical and molecular geneticists. Databases and expert systems for genetic diseases and birth defects, programs for segregation and linkage analysis, certain DNA and protein sequence databases, and information resources in general for molecular biology are addressed. These systems cannot be used effectively without the newly developed techniques of information exchange based on international computer networks. A short introduction is given to the Internet and to European institutions and organizations that offer help with the acquisition and use of bioinformatic resources.

Software for genetic linkage analysis: an update

Recent developments in human genetic linkage analysis have included the appearance of new software and collections of data and program resources, accessible by means of the Internet. Many of these new programs and collections are described, including their availability, literature background, and specific technical information.

Exclusion from proximal 11q of a common gene with megaphenic effect on atopy

We have typed three markers on proximal 11q in 131 random families with three or more children studied for atopy. A summary map that includes the FCER1B candidate was constructed. Using a 2-locus disease model, we performed combined segregation and linkage analysis of three models, none of which suggested linkage. Nine marker loci on other chromosomes were also negative. In the regions swept by these 12 markers we cannot rule out a rare gene, perhaps of large effect, nor a common gene of small effect. However, a common gene of large effect is excluded. These results and alternative strategies are discussed in the perspective of inconsistent evidence for a major atopy gene.

An integrated map of chromosome 9

An integrated map of 211 loci on chromosome 9 is presented for which 198 loci have genetic locations. The results of the analysis indicate very strong interference for the chromosome and positional variations in recombination rates, most extreme in the male map where there is an excess of recombination near the p telomere and a marked suppression of recombination in a large region that includes the centromere.

Integrated genetic map of human chromosome 2

A framework genetic map of human chromosome 2 is described, integrating data from the Centre d'Etude du Polymorphisme Humain (CEPH) version 6 database, the CEPH chromosome 2 consortium database, the National Institute of Health (NIH)/CEPH Collaborative Mapping group and other laboratories. A comprehensive map is also presented, showing regional locations of a large number of additional loci. The framework map is used to identify an informative set of meiotic breakpoints within the CEPH families, and the utility of this information for mapping new markers is discussed. The degree of typing error within the data set is estimated, as are the sex-specific interference parameters. A location database for these genetic and additional cytogenetic data is constructed using algorithms which map genetic distances on to a physical scale, and the potential for this approach to aid the integration of genetic and physical data is examined.

Integration of gene maps: updating chromosome 1

The first integrated map of chromosome 1 was published in 1992. We present an updated summary map of 371 loci constructed from a location database that includes physical and genetic data. The summary map subsumes a composite physical location, sex-specific genetic location, cytogenetic assignment, mouse homology, rank and references to physical maps. The genetic length is 208 cM for the male map, in close agreement with the chiasma map, and 371 cM for the female map. There is evidence for a high level of interference on chromosome 1. The location database comprising both data and analytical software is discussed in relation to alternative approaches and possible enhancements.

Disomic locus content mapping

A linear genetic map may be constructed from segregation of markers in DNA fragments broken by radiation or shearing and/or incorporated into a vector: this is locus content mapping. Theory for monosomic locus content mapping is extended to disomy, making the phase of heterozygous markers informative for linkage and each clone potentially informative for every chromosome. Because there is no interference among breaks and phase may be reliable inferred for all heterozygous loci, multiple pairwise mapping is especially appropriate.

Combining two-point genetic linkage analyses using mapping functions

A likelihood ratio statistic is proposed for combining two-point genetic linkage analyses when the two-point analyses are between a trait and a well-defined map of markers. It is assumed that the two-point analyses are independent, as in the case of choosing only the most informative marker per family. The asymptotic distribution of the likelihood ratio statistic is derived under the null hypothesis of no linkage of the trait with a map of 2 markers, with intermarker genetic distance delta. This distribution is shown to be a chi-square mixture distribution with mixing probability depending on delta and the assumed mapping function. We use this asymptotic result to approximate the distribution of the likelihood ratio statistic for the more general case of more than 2 markers. Simulation results indicate that this may be reasonable. Power is evaluated by simulations and results indicate that this approach, which constrains the intermarker distances to their known values, tends to be more powerful than other methods proposed in the literature.

Another procedure for the preliminary ordering of loci based on two point lod scores

Because of the difficulty of performing full likelihood analysis over multiple loci and the large numbers of possible orders, a number of methods have been proposed for quickly evaluating orders and, to a lesser extent, for generating good orders. A new method is proposed which uses a function which is moderately laborious to compute, the sum of lod scores between all pairs of loci. This function can be smoothly minimized by initially allowing the loci to be placed anywhere in space, and only subsequently constraining them to lie along a one-dimensional map. Application of this approach to sample data suggests that it has promise and might usefully be combined with other methods when loci need to be ordered.

An RFLP map for 2q33-q37 from multicase mycobacterial and leishmanial disease families: no evidence for an Lsh/Ity/Bcg gene homologue influencing susceptibility to leprosy

The mycobacterial diseases leprosy and tuberculosis (TB) and the leishmaniases are characterized by a wide spectrum of disease phenotypes, and by the fact that the majority of individuals exposed to the causative organisms Mycobacterium leprae, M. tuberculosis and Leishmania sp. become infected but do not present with clinical disease. In order to determine whether a human homologue to the murine macrophage resistance gene Lsh/Ity/Bcg influences susceptibility to human disease, multicase families for all three diseases have been collected, and linkage analysis performed using a panel of markers in the region of human chromosome 2q33-q37 known to be conserved with the Lsh/Ity/Bcg-containing region of murine chromosome 1. Because of the paucity of available polymorphic markers/linkage information for 2q33-q37, data from 35 multicase leprosy, TB and visceral leishmaniasis families (310 individuals) were first pooled to produce a detailed RFLP map of the region. Peak LOD scores well in excess of 3 were observed for linkage between adjacent pairs of a more proximal (2q33-q35) set of markers CRYGP1, MAP2, FN1, TNP1, VIL1 and DES, and between adjacent pairs of a more distal (2q35-q37) set COL6A3, D2S55 and D2S3. These peak LOD scores and the corresponding values for theta were used in the MAP92 program to generate a multiple two-point map with gene order/map intervals (cM) of: CRYGP1-4.65-MAP2-3.45-FN1-5.95-TNP1-3.41-VIL1-3. 01- DES-20.14-COL6A-10.91-D2S55-3.67-D2S3. Although local support for the placement of loci in this order was weak (LOD < 2, except for DES-COL6A3 where LOD = 6.02), the map is consistent with the gene order for those loci (Cryg, Fn-1, Tp-1, Vil, Des, Col6a3) previously mapped in the mouse. Data from 17 multicase leprosy families (149 individuals) were further analysed for linkage between a putative disease susceptibility locus (DSL) controlling susceptibility to leprosy per se and each of the marker loci. Assuming 100% penetrance for the susceptibility allele, no positive LOD score was obtained for linkage between the DSL and any of the marker genes. Instead, the data provide convincing evidence (LOD scores < -2) that a DSL does not fall within 10-20 cM of CRYGP1, MAP2, TNP1, VIL1, DES or D2S55, or within 5-10 cM of FN1, COL6A3 or D2S3. This effectively excludes a putative DSL controlling susceptibility to leprosy per se from the entire region 2q33-q37.(ABSTRACT TRUNCATED AT 400 WORDS)

Integration of gene maps: chromosome 21

Physical, cytogenetic, and genetic data including microsatellite markers and a covering sequence-tagged site (STS) map have been entered into a location database and integrated into a summary map that subsumes a composite physical location, sex-specific genetic locations, cytogenetic and regional assignments, mouse homology, rank, and references. With the omission of 52 loci whose location is known only from cytogenetic assignment to an interval greater than 10 megabases, there are 198 loci in the covering STS map and an additional 145 loci. The physical length is consistent with 11 megabases for 21p and 39 megabases for 21q. With error filtration and allowance for high interference, the genetic length in males corresponds to the chiasma map (54.7 centimorgans), and the genetic length in females is 76.5 centimorgans. The relation between map integration and the STS paradigm is illustrated and discussed.

Emery-Dreifuss muscular dystrophy: linkage to markers in distal Xq28

Emery-Dreifuss muscular dystrophy (EMD) is characterised by (1) early contractures of the Achilles tendons, elbows, and postcervical muscles, (2) slowly progressive muscle wasting and weakness with a predominantly humeroperoneal distribution in the early stages, and (3) cardiomyopathy with conduction defects and risk of sudden death. Inheritance is usually X linked recessive but can be autosomal dominant. Family linkage studies have mapped X linked EMD to the distal long arm of the X chromosome but precise genetic localisation has been hampered by the rarity of this condition. We report three new families with X linked Emery-Dreifuss muscular dystrophy studied with DNA markers from Xq27-qter and three previously published families typed for additional markers. No recombination was observed with the red/green cone pigment locus, RGCP (lod score, Z = 2.46), the factor VIII coagulant gene locus, F8C (Z = 6.39), or with DXS115 (Z = 4.94). Two recombinants were observed which mapped EMD distal to DXS15 (DX13) and DXS52 (St14) respectively. Multipoint linkage analysis gave odds exceeding 200:1 for EMD being distal to these markers. A multipoint analysis incorporating published data gave the map cen-DXS304-9cM-DXS15-3cM-DXS52-2 cM-(RGCP,EMD)-3cM-F8C-2cM-DXS115 with odds of 120:1 in favour of a location for EMD between DXS52 and F8C as compared to the next best position distal to F8C.

The AD1 locus in familial Alzheimer disease

The AD1 locus on chromosome 21 (MIM 104300) maps to the beta-amyloid precursor locus (APP) at approximately 27.7 Mb from pter (10.9 cM in males and 33.9 cM in females), flanked proximally by D21S8 and distally by D21S111, with D21S124 and D21S210 close but of uncertain order. AD1 accounts for 63 +/- 11% of multiplex Alzheimer pedigrees for which lod scores have been reported. Since a much smaller proportion of pedigrees have mutations in the cDNA for beta-amyloid (APP exons 16 and 17), it is likely that the AD1 locus spans controlling elements near those exons. There is no evidence for a second locus on chromosome 21. The remaining pedigrees may include sporadic cases as well as mutations at an AD2 locus on another chromosome.

Algorithms for a location database

The algorithms that drive the ldb location database are described. The program captures data on genetic and physical maps and combines information from different sources into a summary map. To assure portability it was developed in Fortran on a SUN SPARCStation under Unix. The algorithms, which combine rule-based seriation with a minimum deviance bootstrap, allow investigators and chromosome committees to produce a composite location in Mb that integrates partial maps. The program and manual are now available from the authors.

Integration of gene maps: chromosome 1

A composite map of 177 loci has been constructed in two steps. The first combined pairwise logarithm-of-odds scores on 127 loci into a comprehensive genetic map. Then this map was projected onto the physical map through cytogenetic assignments, and the small amount of physical data was interpolated for an additional 50 loci each of which had been assigned to an interval of less than 10 megabases. The resulting composite map is on the physical scale with a resolution of 1.5 megabases. In the future these methods may be used to incorporate locations from linkage, contigs, radiation hybrids, restriction fragments, and somatic cell maps. Dense, reliable, and well-documented maps are essential for long-range sequencing and to localize and clone disease genes.

Construction of a map of chromosome 16 by using radiation hybrids

A human-hamster cell hybrid carrying a single copy of chromosome 16 as the only human genetic material was irradiated with a single dose of gamma-rays (7000 rads; 1 rad = 0.01 Gy) and then fused with a thymidine kinase-deficient hamster cell line (RJKM) to generate radiation hybrids retaining unselected fragments of this human chromosome. In two experiments, 223 hybrids were isolated in hypoxanthine/aminopterine/thymidine (HAT) medium and screened with 38 DNA probes, corresponding to anonymous DNA or gene sequences localized on chromosome 16. The most likely order and location of the 38 DNA sequences were established by multiple pairwise analysis and scaled to estimate physical distance in megabases. The order and the distances thus obtained are mostly consistent with available data on genetic and physical mapping of these markers, illustrating the usefulness of radiation hybrids for mapping.

Radiation hybrid mapping

A theory is developed to predict marker retention and conditional retention or loss in radiation hybrids. Applied to multiple pairwise analysis of a human chromosome 21 data set, this theory fits much better than proposed alternatives and gives a physical map consistent with other evidence and robust with respect to errors to typing. Radiation hybrids have great promise to provide order and physical location at two levels of resolution, spanning the techniques of linkage and restriction fragments and not limited to polymorphic loci.

Error filtration, interference, and the human linkage map

Typing error is a major problem in constructing human linkage maps, leading to incorrect orders and inflating map lengths. An error filter is incorporated into multiple pairwise analysis that corrects for inflation of map lengths and improves recovery of the correct order. Multipoint mapping is more sensitive to error, but when its output is adjusted for both error and interference, map lengths are no longer inflated in proportion to the number of loci and are close to those obtained by multiple pairwise analysis.

A genetic linkage map of 27 markers on human chromosome 21

We have constructed a genetic linkage map of the long arm of human chromosome 21 comprising 27 DNA markers. This map is an updated version of that reported earlier by group (1989, Genomics 4: 579-591), which contained 17 DNA markers. The current markers consist of 10 genes and 17 anonymous sequences. Traditional methods (restriction fragment length polymorphisms) were used to map 25 of these markers, whereas 2 markers were studied by polymerase chain reaction amplification of (GT)n dinucleotide repeats. Linkage analysis was performed on 40 CEPH families using the computer program packages LINKAGE, CRI-MAP, and MAPMAKER. Recombination rates were significantly different between the sexes, with the male map being 132 cM and the female map being 161 cM, assuming Kosambi interference and a variable ratio of sex difference in recombination. Approximately one-half of the crossovers in either sex occur distally, in terminal band 21q22.3, which also contains 16 of the markers studied. The average distance between adjacent markers was 6 cM.

Standard maps of chromosome 10

To achieve consensus more exact definitions of genetical maps are required, of which standard, comprehensive and skeletal might be some. A standard genetic map gives distance from pter in centimorgans (cM), uses the international nomenclature for assigned loci, is sex-specific, and allows as well as possible for interference and typing errors. A standard physical map gives distance from pter in megabases (Mb). A standard map is called comprehensive if it aims to include all syntenic loci, and skeletal if it is limited to loci whose order is well supported. Loci with established order are called skeletal, and are used to define regional assignments of other loci. These principles are illustrated using the CEPH data for chromosome 10. Map lengths by multiple 2-point analysis under supported interference are in good agreement with other evidence, but multipoint mapping gives a substantial overestimate. There are currently 21 loci in the skeletal genetic map and 40 loci in the comprehensive genetic map. From these data, cytogenetic assignments, and partial genetic maps the physical location has been estimated for 85 loci. MEN2A is in a region close to the centromere in which male recombination per megabase is much reduced. Order of DNA markers in this densely mapped region has not been determined, and therefore the exact location of MEN2A is uncertain, although it is likely to lie between D10S34 and D10S30 and close to D10S11.

Counting algorithms for linkage

The Lander-Green algorithm is algebraically different from the EM algorithm that maximizes likelihood. In our experience the numerical difference is very small. Since computations are no easier or convergence faster for the Lander-Green algorithm, there is no reason to prefer it to EM. As suggested by Green (personal communication), layering is advantageous. Occasional updating of the information weights might also be considered to accelerate mapping.

The CEPH consortium primary linkage map of human chromosome 10

The first CEPH consortium map, that of chromosome 10, is presented. This primary linkage map contains 28 continuously linked loci defined by genotypes generated from CEPH family DNAs with 37 probe and enzyme combinations. Cytogenetic localization of some of the genetic markers indicates that the consortium map extends, at least, from 10p13 to 10q26. The order of loci on the consortium map agrees with the physical localization data. The female map spans 309 cM (206 cM if an approximation of interference is included in the mapping function used to construct the map), and the mean genetic distance of intervals is 11 cM (7 cM). Also presented are maps of chromosome 10 from each of five CEPH collaborating laboratories, based on genotypes for all relevant markers in the CEPH database. The CEPH consortium map of chromosome 10 should be useful for localization of any gene of interest falling within the span covered. The genotypes in the chromosome 10 consortium map database are now available to the scientific community.

A centromere map of the X chromosome from trisomies of maternal origin

A centromere map is derived from XXX and XXY trisomies of maternal origin. Preliminary data suggest reduced recombination in the tetrads giving rise to mei I nondisjunction, but an excess of recombination in the pericentric region. As in Drosophila, multichiasmate tetrads may be more at risk of nondisjunction than nullochiasmate tetrads.