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N E Morton

Publications and source records attributed to N E Morton.

At least 37 records · Page 2Linked to original sources
PubMed2001

Combination of linkage evidence in complex inheritance.

The central problem of complex inheritance is to combine evidence from data that typically differ in markers, phenotypes, ascertainment, and other factors, without sacrificing the reliability that lods have given to linkage mapping for major loci. Here we evaluate 5 possible solutions on 200 replicates simulated in Genetic Analysis Workshop 10. Two methods differ from less efficient ones by distinguishing the tails of a normal distribution. Maximum likelihood scores (currently implemented only for the BETA model) and the approach of Self and Liang perform about as well as pooling samples, which is not feasible with heterogeneous data. With moderately heterogeneous data the Self and Liang method appears to be more efficient than maximum likelihood scores. Although improvements are being made in sample design and statistical analysis, the problem of combining linkage evidence from multiple data sets appears to have been solved. Allelic association presents different problems not yet addressed.

Alleles↗
PubMed2001

A tournament of linkage tests in complex inheritance.

The performance of some weakly parametric linkage tests in common use was compared on 200 replicates of oligogenic inheritance from Genetic Analysis Workshop 10. Each random sample for the quantitative trait was dichotomized at different thresholds and also selected through 2 affected sibs, generating 8 combinations of sample and variable. The variance component program SOLAR performed best with a continuous trait, even in selected samples, when the population mean was used. The sib-pair program SIBPAL2 was best in most other cases when the phenotype product, population mean, and empirical estimates of pair correlations were used. The BETA program that introduced phenotype products was slightly more powerful than maximum likelihood scores under the null hypothesis and approached but did not exceed SIBPAL2 under its optimal conditions. Type I errors generally exceeded expectations from a chi(2) test, but were conservative with respect to bounds on lods. All methods can be improved by use of the population mean, empirical correlations, logistic representation for affection status, and correct lods for samples that favour the null hypothesis. It remains uncertain whether all information can be extracted by weakly parametric methods and whether correction for ascertainment bias demands a strongly parametric model. Performance on a standard set of simulated data is indispensable for recognising optimal methods.

Algorithms↗
PubMed2000

A first trial of retrospective collaboration for positional cloning in complex inheritance: assay of the cytokine region on chromosome 5 by the consortium on asthma genetics (COAG).

The central problem of complex inheritance is to map oligogenes for disease susceptibility, integrating linkage and association over samples that differ in several ways. Combination of evidence over multiple samples with 1,037 families supports loci contributing to asthma susceptibility in the cytokine region on 5q [maximum logarithm of odds (lod) = 2.61 near IL-4], but no evidence for atopy. The principal problems with retrospective collaboration on linkage appear to have been solved, providing far more information than a single study. A multipoint lod table evaluated at commonly agreed reference loci is required for both collaboration and metaanalysis, but variations in ascertainment, pedigree structure, phenotype definition, and marker selection are tolerated. These methods are invariant with statistical methods that increase the power of lods and are applicable to all diseases, motivating collaboration rather than competition. In contrast to linkage, positional cloning by allelic association has yet to be extended to multiple samples, a prerequisite for efficient combination with linkage and the greatest current challenge to genetic epidemiology.

Asthma↗
PubMed2000

Allelic association in the FRAX region.

The sex chromosomes enable direct determination of haplotypes, and the analysis of 8 microsatellite markers from the FRAX region on Xq27-q28 contributes 7219 independent haplotypes from our study in Wessex. Allelic association increases with frequency of alleles, and is less for trinucleotide than dinucleotide repeats. The estimate of epsilon, the exponential decline of association with distance in kb, is 0.0023. The swept radius 1/epsilon estimates the distance at which disequilibrium falls to e(-1) approximately .37 of its initial value. The current study estimates the swept radius of association to be 433 kb, which is surprisingly close to estimates for SNPs, and suggests that a marker density of 1/100 kb should be powerful in regions such as FRAX. An explanation for these results is offered, and some speculations made about what will be found when SNPs are subjected to an equally intensive study in multiple regions.

Alleles↗
PubMed2000

Reproductive and menstrual history of females with fragile X expansions.

FRAXA premutations have been associated with premature ovarian failure (POF) or menopause before the age of 40. We have studied women in families ascertained because of a mentally retarded full mutation relative and determined their age of menopause, serum hormone levels in premenopausal individuals and the outcome of any pregnancies. Survival analysis was used as a measure of menopause and demonstrated a significant decrease in age of menopause in premutation carriers compared with their full mutation carrier and normal relatives. Serum FSH was also raised in premutation carriers, although oestradiol, inhibin A and inhibin B were not significantly different. However, we did not find an excess of dizygous twins or pregnancy loss/trisomies, both of which are associated with aging ovaries. Thus premutation carriers as a group have an earlier menopause and raised serum FSH but do not appear to manifest other features of an aging ovary.

Age Factors↗
PubMed2000

Stability and haplotype analysis of the FRAXE region.

FRAXE full mutations are rare and appear to be associated with mild mental retardation. As part of a screening survey of boys with learning difficulties to determine the frequency of full and premutations, we have collected data on the frequency of instability at FRAXE for about 4000 transmissions and the haplotype for over 7000 chromosomes. The distribution of FRAXE repeats was similar to other English populations but differed from two North American Caucasian series. Observed instability at FRAXE was rare but increased with increasing repeat number, and there were no expansions into the full mutation range, except in pedigrees ascertained through a full mutation. Haplotype analysis suggested division into five groups with each group having a characteristic distribution of FRAXE repeats. Fourteen of the 15 full mutations occurred on a single haplotype and this haplotype also had a significant excess of intermediate-sized alleles, suggesting that full mutations originate from large normal alleles. However, a related haplotype also had a significant excess of intermediates but we observed no full mutations on this haplotype, suggesting either loss or gain of stability determinants on it. We suggest that whilst triplet repeat size is a significant predisposing factor for expansion at FRAXE other genetic determinants are also likely to be important.

Alleles↗
PubMed2000

Combined segregation and linkage analysis of inflammatory bowel disease in the IBD1 region using severity to characterise Crohn's disease and ulcerative colitis. On behalf of the GISC.

Inflammatory bowel disease (IBD) is a chronic relapsing disorder affecting the gastro-intestinal tract and is subdivided into two main subtypes: Crohn's disease (CD) and ulcerative colitis (UC). Although the aetiology of IBD is unknown, a strong genetic susceptibility is suggested and different candidate regions have been identified for both CD and UC. The IBD1 region on chromosome 16 has been confirmed to be important for susceptibility to CD, whereas conflicting evidence has been obtained for UC. We performed a combined linkage and segregation analysis in the identified IBD1 region on a sample of 82 extended families with IBD using a parametric method implemented in the computer program COMDS. This approach allows simultaneous evaluation of linkage while estimating the mode of inheritance and to include severity of the trait to characterise the CD and UC phenotypes. Our results are consistent with the presence of a major gene in the IBD1 region close to D16S408 involved in both UC and CD. Furthermore, our data support evidence that a single mutation in the gene leads more frequently to UC, whereas inheritance of two mutant alleles results in the more severe CD. In our study the IBD1 locus was found to have a major role in IBD predisposition in the Italian population.

Adult↗
PubMed2000

The use of long PCR to confirm three common alleles at the CYP2A6 locus and the relationship between genotype and smoking habit.

Long PCR followed by nested PCR has previously been used to determine CYP2A6 160H alleles, but the method proved unreliable. We have optimized this approach in a DNA bank of 1032 subjects (age range 59-74 years) to give reliable results, yielding indirect molecular evidence and very strong statistical evidence of hitherto unrecognized common alleles (designated O) recalcitrant to the long PCR. Coding three alleles (160L, 160H and O) and an approach to association analysis originally developed to deal with null alleles implicit in ABO blood group phenotyping, the contribution of 160H (functionally null) to reduced smoking habit has been clearly measured for the first time, unconfounded by alleles null to the long PCR. The most significant findings (p < 0.01) are that the possession of a 160H allele, compared with not possessing a 160H allele, is associated with a mean age of starting regular smoking 3 years later (95% CI +/- 1.93 years, average start age 20-21 years rather than 17-18 years); and that the average likelihood of quitting smoking at any time is 1.75 fold (95% CI, 1.17-2.61) for those possessing an 160H allele compared with those who have no 160H allele. This suggests that a smoking subject with a genotype predicted to confer 50% of the ability to eliminate nicotine via the CYP2A6 pathway has almost twice the likelihood of quitting smoking.

Aged↗
PubMed2000

Linkage analysis of markers on chromosome 11q13 with asthma and atopy in a United Kingdom population.

Previous studies have suggested that atopy is linked to the beta chain of the high affinity IgE receptor (Fcepsilon R1-beta) on chromosome 11q13. Fcepsilon R1-beta polymorphisms, I181L, V183L, and E237G, are reported to be associated with asthma and atopy. The aim of this study was to investigate linkage to Fcepsilon R1-beta in a UK population and to assess the frequency of the polymorphisms and their association with asthma and atopy. A sample of 131 families was recruited at random with a sample of 109 families ascertained via an asthmatic proband. Each subject completed a written and video-assisted questionnaire and underwent bronchial challenge and skin prick testing. Serum total and specific IgE levels were measured. Quantitative scores were derived for asthma and atopy using principal component analysis. Four microsatellite markers were genotyped, including Fcepsilon R1-beta. The frequency of the I181L and V183L polymorphisms were determined by sequencing, and the E237G polymorphism was determined using the amplification refractory mutation system. We found no evidence for linkage to Fcepsilon R1-beta and only weak evidence for linkage to the less informative marker E237G. We found no examples of the I181L/V183L polymorphism in our population sample. Our study has failed to strengthen the evidence for a candidate gene on chromosome 11q13.

Adult↗
PubMed2000

[Genetic factors involved in asthma and atopy. Studies in British families].

Both asthma and atopy run families indicating a strong genetic component. To investigate possible candidate gene regions, we have recruited 131 families without reference to atopy and asthma (random sample) and 60 extended families with two or more members affected by asthma (multiplex sample). Using both candidate and genome screen approaches, we have been able to provide evidence supporting the presence of candidate genes on chromosome 11q (E237G) and 12q but we have been unable to confirm reports of linkage and association for asthma on 5q. Our experience to date suggests that larger numbers of families are needed to increase the confidence of gene localisation and there is a need to improve the phenotypic description of asthma. Finally, it is essential that claims for linkage or association are confirmed in different populations using the same markers.

Asthma↗
PubMed1999

Genetic epidemiology of single-nucleotide polymorphisms.

On the causal hypothesis, most genetic determinants of disease are single-nucleotide polymorphisms (SNPs) that are likely to be selected as markers for positional cloning. On the proximity hypothesis, most disease determinants will not be included among markers but may be detected through linkage disequilibrium with other SNPs. In that event, allelic association among SNPs is an essential factor in positional cloning. Recent simulation based on monotonic population expansion suggests that useful association does not usually extend beyond 3 kb. This is contradicted by significant disequilibrium at much greater distances, with corresponding reduction in the number of SNPs required for a cost-effective genome scan. A plausible explanation is that cyclical expansions follow population bottlenecks that establish new disequilibria. Data on more than 1,000 locus pairs indicate that most disequilibria trace to the Neolithic, with no apparent difference between haplotypes that are random or selected through a major disease gene. Short duration may be characteristic of alleles contributing to disease susceptibility and haplotypes characteristic of particular ethnic groups. Alleles that are highly polymorphic in all ethnic groups may be older, neutral, or advantageous, in weak disequilibrium with nearby markers, and therefore less useful for positional cloning of disease genes. Significant disequilibrium at large distance makes the number of suitably chosen SNPs required for genome screening as small as 30,000, or 1 per 100 kb, with greater density (including less common SNPs) reserved for candidate regions.

Case-Control Studies↗
PubMed1999

Allelic association between marker loci.

Allelic association has proven useful to refine the location of major genes prior to positional cloning, but it is of uncertain value for genome scans in complex inheritance. We have extended kinship theory to give information content for linkage and allelic association. Application to pairs of closely linked markers as a surrogate for marker x oligogene pairs indicates that association is largely determined by regional founders, with little effect of subsequent demography. Sub-Saharan Africa has the least allelic association, consistent with settlement of other regions by small numbers of founders. Recent speculation about substantial advantages of isolates over large populations, of constant size over expansion, and of F1 hybrids over incrosses is not supported by theory or data. On the contrary, fewer affected cases, less opportunity for replication, and more stochastic variation tend to make isolates less informative for allelic association, as they are for linkage.

Africa South of the Sahara↗
PubMed1999

Genetic aspects of population policy.

Every science begins in folklore and matures as it reacts against dogma and myth. Astronomy developed in the Neolithic, but it did not outgrow astrology until the sixteenth century. Chemistry discarded alchemy at about the same time. On the contrary, the short history of genetics has been concurrent with the pseudo-science of eugenics, which, at times, has been widely accepted and incorporated in population policy and directive genetic counselling, with rare opposition by geneticists. Societal pressures are likely to increase with the power of genetic technology, the fear it generates and the perception that population growth threatens human welfare. Without a pertinent ethical code, geneticists are vulnerable to both temptation and opprobrium. The intrusion of eugenics into genetic counselling has been a recent source of concern to societies and congresses of genetics. This review traces the causes of this concern and the manner of its expression in the absence of an international voice for genetics that could address ethical and other common interests.

Eugenics↗
PubMed1999

Serum concentrations of follicle stimulating hormone may predict premature ovarian failure in FRAXA premutation women.

It is now recognized that female carriers of fragile X premutations are at increased risk of premature ovarian failure. We have studied 51 premenopausal women from fragile X families, to determine whether premutation carriers have variations in the hormonal markers of menopause, compared to full mutations and controls. We found a significant increase in serum follicle stimulating hormone in premutation carriers, suggesting that as a group they will enter menopause before full mutation carriers and unaffected controls. These results have important implications for fertility in these women.

Adolescent↗
PubMed1999

Suggestive evidence for genetic linkage between IgE phenotypes and chromosome 14q markers.

Chromosome 14q was screened for loci modulating immunoglobulin E (IgE) phenotypes in 15 extended and 45 nuclear asthmatic families using a panel of 14 microsatellite markers. We examined the reported linkage between the TCR A/D locus on 14q11.2 and specific (cognate) allergic responses and observed supportive evidence for linkage between a general skin prick test reactivity trait (but not with total serum IgE) and TCRA microsatellite (in the total sample of informative sib-pairs p = 0.039, in selected sample of one or zero affected parent p = 0.017). We also show suggestive evidence for a novel linkage between markers D14S75 and D14S63 on 14q13-23 and log total serum IgE (p = 0.034 and p = 0.0029). The evidence for linkage with marker D14S63 on 14q23 is strengthened by the finding of association of allele 165 to log IgE (p = 0.0029). We conclude that chromosome 14q may contain a locus close to TCR A/D at 14q11.2 linked to skin prick reactivity and a locus at 14q13- 23 linked to total serum IgE.

Adolescent↗
PubMed1998

Allelic association under map error and recombinational heterogeneity: a tale of two sites.

Recombination acts on the genetic map, not on the physical map. On the other hand, the physical map is usually more accurate. Choice of the genetic or physical map for positional cloning by allelic association depends on the goodness of fit of data to each map under an established model. Huntington disease illustrates the usual case in which the greater reliability of physical data outweighs recombinational heterogeneity. Hemochromatosis represents an exceptional case in which unrecognized recombinational heterogeneity retarded positional cloning for a decade. The Malecot model performs well for major genes, but no approach assuming either equilibrium or disequilibrium has been validated for oligogenes contributing to common disease. In this case of greatest interest, the power of allelic association relative to linkage is less clear than for major genes.

Alleles↗