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B C Lamb

Publications and source records attributed to B C Lamb.

12 recordsLinked to original sources
PubMed1992

New equations and a method for finding nine parameter values for two alleles at one locus to study gene conversion using Ascobolus immersus.

A quantitative treatment is given for meiotic gene conversion with its parameters and equations for their interactions to determine allele segregation class frequencies from heterozygotes. The possible pairing of both pairs of nonsister chromatids in a bivalent at exactly the same point is included. Using sets of data from Ascobolus immersus, it is shown that values for all nine parameters for hybrid DNA models of recombination can be obtained using an iterative computer program. The accuracy of the values is estimated and the double-strand gap repair model is considered. The parameter values obtained invalidate most of the simplifications used in previous quantitative analyses of gene conversion data. They showed total bias in strand preference in asymmetric hybrid DNA formation and some bias in which type of chromatid is the invading one. There were slight differences in repair frequency between the two types of mispair and very large differences in the direction of repair. Conversion control factors had major effects on hybrid DNA formation and repair of mispairs.

Alleles↗
PubMed1989

The effects of gene conversion control factors on conversion-induced changes in allele frequencies in populations and on linkage disequilibrium.

Conversion control factors (ccfs) are widespread. They control conversion properties at their target loci, affecting the conversion frequency and the amount and even the direction of gene conversion disparity. Three major types of ccf can be recognised. Experimental studies of the effects of ccfs have been combined with theoretical studies and modelling to examine the effects of ccfs on the evolutionary population genetics of alleles at the target locus. The ccf alleles present can greatly affect the rate and the direction of conversion-induced changes in target locus allele frequencies. Gene conversion can both cause and remedy linkage disequilibrium, with causation being related to polymorphism for ccfs. Disparity in conversion direction does not by itself necessarily cause linkage disequilibrium.

Alleles↗
PubMed1986

Gene conversion disparity: factors influencing its direction and extent, with tests of assumptions and predictions in its evolutionary effects.

The evolutionarily important characteristics of gene conversion disparity extent and direction are surveyed in fungi. Temperature and background genotype can have small or large effects, sometimes even changing the direction of disparity. Disparity results from Sordaria and Ascobolus were very similar, with between-strain, between-data set and between-locus differences being larger than those between species or genera. In general, different loci in an organism show similar disparity properties when comparable types of mutation are considered, but may not do so in pooled results containing different proportions of different mutation types. Frameshifts typically have strong disparities, usually with negative signs for single base additions and positive signs for single base deletions. Base substitutions tend to have moderate disparities, favoring wild type more often than mutant in most data sets. Large deletions usually have significant disparity, either positive or negative. For comparable molecular types of mutation, spontaneous and induced mutations had roughly similar disparity properties.--Experimental tests and theoretical considerations generally failed to support a number of assumptions and predictions made in previous treatments of gene conversion in evolution. In general, a mutation's conversion properties depend much more on its molecular type in relation to wild type than on any evolved conversion advantages or disadvantages.

Alleles↗
PubMed1986

A general method for identifying correct solutions in the quantitative analysis of gene conversion data.

Past attempts to obtain values for meiotic parameters relating to hybrid DNA formation and the correction of mismatched bases in hybrid DNA have not given unique solutions unless various simplifying assumptions were made. A method is given for identifying correct sets of solutions after calculating the frequency of hybrid DNA formation at a heterozygous site and using the fact that closely linked sites within a locus have very similar hybrid DNA formation frequencies. The method is illustrated with simulated data and Sordaria fimicola data; it can also show up incorrect assumptions in analysis. A method is suggested for assessing the importance of double-strand gaps in producing conversions.

Chromatids↗
PubMed1985

The effects of mispair and nonpair correction in hybrid DNA on base ratios (G + C content) and total amounts of DNA.

Base ratios and total DNA amounts can vary substantially between and within higher taxa and genera, and even within species. Gene conversion is one of several mechanisms that could cause such changes. For base substitutions, disparity in conversion direction is accompanied by an equivalent disparity in base ratio at the heterozygous site. Disparity in the direction of gene conversion at meiosis is common and can be extreme. For transitions (which give purine [R]/pyrimidine [Y] mispairs) and for transversions giving unlike R/R and Y/Y mispairs in hybrid DNA, this disparity could give slow but systematic changes in G + C percentage. For transversions giving like R/R and Y/Y mispairs, it could change AT/TA and CG/GC ratios. From the extent of correction direction disparity, one can deduce properties of repair enzymes, such as the ability (1) to excise preferentially the purine from one mispair and the pyrimidine from the other for two different R/Y mispairs from a single heterozygous site and (2) to excise one base preferentially from unlike R/R or Y/Y mispairs. Frame-shifts usually show strong disparity in conversion direction, with preferential cutting of the nonlooped or the looped-out strand of the nonpair in heterozygous h-DNA. The opposite directions of disparity for frame-shifts and their intragenic suppressors as Ascobolus suggest that repair enzymes have a strong, systematic bias as to which strand is cut. The conversion spectra of mutations induced with different mutagens suggest that the nonlooped strand is preferentially cut, so that base additions generally convert to mutant and deletions generally convert to wild-type forms. Especially in nonfunctional or noncoding DNA, this could cause a general increase in DNA amounts. Conversion disparity, selection, mutation, and other processes interact, affecting rates of change in base ratios and total DNA.

Animals↗
PubMed1977

The use of gene conversion to study synaptinemal complex structure and molecular details of chromatid pairing in meiosis.

Gene conversion can be used to study: the topography and pairing relationships of the four chromatids of a bivalent at the time of crossing over and hybrid DNA formation, the lengths of intimately paired segments and the frequency of intimate pairing at particular sites. Conversion ratios of different types, corresponding-site interference, co-conversion, and the range and distribution of conversion frequencies are discussed in relation to DNA and chromatid pairing, and synaptinemal complex organisation. Conversion data from Ascobolus immersus and other fungi are compared with electron microscope data from various organisms and with models of the synaptinemal complex.

Alleles↗
PubMed1976

Randomness tests on the sequence of ascal segregation classes in Neurospora crassa.

Various statistical tests for randomness were made on the order of ascal classes in groups of asci from wild-type X asco crosses. There was no significant nonrandom clustering of asci of the same segregation class, nor a regular twinning of similar asci. Any apparent observed clustering of similar ascal classes is probably an artefact or due to chance. 2 x n X2 tests showed that frequencies of individual ascus classes from different perithecia were generally homogeneous, as were second division segregation frequencies. The tests described here for randomness in sequences of occurrences could be of general use in other areas of genetics.

Cell Division↗
PubMed1975

Cryptic mutations: their predicted biochemical basis, frequencies and effects on gene conversion.

Cryptic mutations are undetected base changes in genetic DNA (or hereditary RNA). Some kinds of base change are normally undetected; others may or may not be detected, depending on experimental conditions, procedures and genotypes. Cryptic mutations could affect gene conversion results because when heterozygous they cause mismatched base pairs in hybrid DNA in the same way as known mutations, but the experimenter is unaware of them. Cryptic heterozygosity will usually be much more frequent in heterothallic than homothallic organisms. The effects of cryptic mutation heterozygosity on recombination and conversion of known mutations are predicted with reference to co-conversion, map expansion and polarity. Relevant evidence is considered.

Base Sequence↗