Showing papers in "Journal of Genetics in 1993"

Why are most mutations recessive

Abstract: It is a long-standing observation that most mutations are recessive. That is, they do not lead to visible phenotypic effects when in heterozygous combination with the wild-type allele. The reason for this has long been debated. Fisher (1930) attributed the observed dominance of the wild type to the action of natural selection at modifier loci. Wright (1929) on the other hand asserted that dominance did not have a selective functionper se, but was a more-or-less automatic offshoot of genetic regulatory mechanisms. The present essay discusses these explanations from a contemporary standpoint and suggests that neither is likely to be valid exclusively. In particular, even when physiology appears to offer a sufficient explanation, evolution of dominance cannot be ruled out.

Genetics and fish breeding

Abstract: Fish are a rich source of nutritious and delicious food to man. With the increasing demand for fish and fish products nearly every country with aquatic resources is making serious efforts to boost fish production. This is not surprising considering the fact that two-thirds of our planet is coverea with water and most of it supports fish life. Therefore the breeding of fish for food, for recreation and aesthetic needs, or in the context of conservation of natural species against environmental harm is of global importance. 'Genetic analysis of.fish species is an old practice. Yet, not many books are available, even for a basic course in fish genetics at the undergraduate level. Therefore, Genetics and Fish Breeding, by the well-known fish geneticist C. E. Purdom, is indeed a timely and much-needed publication. The book is an outcome of Purdom's lifetime work in fish genetics at the famous Lowestoft Fisheries Laboratory in the UK. In this compact book, he has neatly provided a precise and comprehensive account of the wide spectrum of research currently being carried out both for gaining an understanding of the principles of fish genetics and for their ultimate application(s) for higher productivity and disease resistance. The book is divided into 15 chapters. These chapters are written in a lucid and logical manner and can be followed even by beginners. Chapters 1 and 2 deal with the scope of applied fish genetics, and sexuality and reproduction, respectively. Chapters 3 and 4 describe the principles of genetics and Mendelian inheritance of colour and other characteristics in fish. Two chapters (chapters 5 and 6) have been devoted to quantitative genetics, including commercially important characters. In chapter 6 the author has included the use of molecular approaches to ascertain genetic relatedness within and between species. He has very briefly touched upon the use of DNA fingerigrinting (two paragraphs on pages 100 and 101), a technique that is now being used extensively not only for analysis of population structure, but also for behavioural, ecological, conservational and phylogenetic studies in a number of organisms, including fishes. Therefore, in my view, the statement 'In general, however, this wealth of information reflects the uniqueness of individuals and is of little help in population work, though it could be relevant in the study of highly inbred or clonal material' (pages 100, 101) is difficult to justify. I believe that a slightly more detailed coverage of this powerful technique, with special relevance to fish breeding and population studies, would have substantially added to the value of this otherwise thorough book. Chapter 7 gives a brief description of Cytogenetic studies in fishes, including methods for calculation of recombinational distance between genetic loci and the centromere by using the principle of gyriogenesis. Sex determination, hybridization, sexuality and control of sex ratios are included in chapters 8-11. In each of these chapters the author

Duffy blood group determinants and malaria in India

Abstract: Two hundred and fiftyeight Muria Gond subjects from Bastar district in Central India and 97 subjects from Delhi were typed for Duffy blood group determinants, and their blood examined for malaria antibodies as well as for presence of malarial parasites. We found the Duffy-negative phenotype in high prevalence among Muria Gonds, while in Delhi no subject was observed to be Duffy-negative. Frequencies of seropositivity for malaria antibodies (the test did not distinguish betweenP. falciparum andP. vivax) were not significantly different among subgroups of Muria Gond individuals with different Duffy blood group phenotypes. Examination of thin and thick blood films did not reveal infection withP. vivax in Duffy-negative individuals. Our results suggest that Duffy-negative individuals, though resistant to infection withP. vivax, are not resistant to infection withP. falcipanun.

Genetic-evidence for interaction between fita, fitb and rpob gene-products and its implication in transcription control in escherichia-coli

Abstract: ThefitB mutation (Fit, factor involved in transcription) inE. coli was earlier identified as an extragenic suppressor of thefitA76 mutation, which confers a temperaturesensitive transcription defect. Here we show that thefitB mutation by itself confers a temperature-sensitive phenotype depending on the presence or absence of NaCl or glucose, or both, in the medium. ThefitB mutation suppresses the temperature-sensitive phenotype due to thefitA24 mutation also. However, suppression offit A24 byfit B is restricted to rich medium, unlike suppression in thefitA76 fitB combination where it is independent of the medium. The strain harbouringfitA76, fitA24 andfitB mutations shows the extragenically suppressed (as infit A76 fit B) phenotype. Severalrif (rpoB) alleles isolated in afitB genetic background affect growth of thefit B mutant, depending on the medium of growth, temperature, and presence or absence of rifampicin. We propose a model for interaction betweenfitA andfitB gene products and involvement of thefit genes in transcription controlin vivo.

DNA sequence conservation at the gene level in a conserved chromosomal segment in twoPseudomonas species

Abstract: A comparative analysis has been made of the DNA sequences of the isofunctional genes encodingN-acetylglutamate synthase of the arginine biosynthetic pathway of the bacterial speciesPseudomonas aeruginosa andPseudomonas putida Overall homologies of 81% and 84% at the nucleotide and deduced amino acid sequence levels, respectively, were observed This high homology was also reflected in the strikingly similar hydropathy profiles of the encoded proteins; patterns of codon usage, including rare codon usage; and amino acid composition of the proteins This high level of homology at the DNA sequence level is consistent with the location of these genes in the genetically conserved chromosomal region (called auxotrophic-rich region) of the respectivePseudomonas species Despite chromosomal rearrangements identified in this region the conservation observed at the chromosomal level between thesePseudomonas species is also maintained at the level of the DNA sequence, and in the deduced amino acid sequence, of the genes reported here and of six other pairs of genes of the tryptophan biosynthetic pathway, reported by others, which are also located within this chromosomal region

RFLP approach to breeding for quantitative traits in plants—a critique

Abstract: A number of recent papers suggest that use of RFLPs as markers offers a clear advantage in breeding for improvement in quantitative traits (QTs). The concepts underlying establishment of linkage between RFLP markers and QT loci stem from three papers on tomato published during 1987–1991. Essentially, continuously varying QT phenotypes are assigned to RFLP genotype classes, which can be considered to be determined by a single, diallelic gene with codominant alleles. Linkage is inferred through statistical analysis. Similarly, interaction between markers and QT is also tested by an analysis of variance. Here, the statistical methods employed in these three papers to detect linkage are critically evaluated, especially because subsequent investigations take the concepts developed in these papers as proven. In this paper, we examine the three fundamental papersde novo. We scrutinize the methods employed and the inferences drawn to bring to light what we believe are conceptual drawbacks.

Logic of the genetic code: Conservation of long-range interactions among amino acids as a prime factor

Abstract: Any statement on the optimality of the existing code ought to imply that this code is ideal for conserving a certain hierarchy of properties while implying that other codes may have been better suited for conservation of other hierarchies of properties. We have evaluated the capability of mutations in the genetic code to convert one amino acid into another in relation to the consequent changes in physical properties of those amino acids. A rather surprising result emerging from this analysis is that the genetic code conserves long-range interactions among amino acids and not their short-range stereochemical attributes. This observation, based directly on the genetic code itself and the physical properties of the 20 amino acids, lends credibility to the idea that the genetic code has not originated by a frozen accident (the null hypothesis rejected by these studies) nor are stereochemical attributes particularly useful in our understanding of what makes the genetic code ‘tick’. While the argument that replacement of, say, an aspartate by a glutamate is less damaging than replacement by arginine makes sense, in order to subject such statements to rigorous statistical tests it is essential to define what constitutes a random sample for the genetic code. The present investigation describes one possible specification. In addition to obvious statistical considerations of testing hypotheses, this procedure points to the more exciting notion that alternative codes may have existed.

Heritable transformation of tobacco by Agrobacterium-mediated transfer of the Streptomyces-derived herbicide resistance gene bar

Abstract: Thebar gene ofStreptomyces hygroscopicus encodes an enzyme that detoxifies the herbicide Basta. We have transferred theStreptotnyces-derived bar gene to tobacco through theAgrobacterium tumefaciens gene delivery system. Expression ofbar was driven by two different promoters, TR2’ or CaMV 35S, in two DNA constructs. TR2’ is a weak promoter in tobacco. CaMV 35S is, on the other hand, a strong promoter in tobacco, and transformation using the CaMV 35S promoter construct yielded Basta-resistant transgenic plants. Out of the over one hundred transformants obtained, most could be grown to maturity. Four of these were characterized by genetic and molecular methods. Subsequently, one of the four plants was not resistant and did not show presence ofbar DNA. The remaining three plants contained one or more copies ofbar DNA at one or two loci. Segregation data were consistent with this observation: we obtained ratios of either 3:1 (single locus) or 15:1 (two loci) Basta-resistant:Basta-sensitive in the F2 generation. Field-grown plants showed resistance to Basta up to a level of 4000 g of active ingredient per hectare.

Haldane and the analysis of linkage.

Abstract: Haldane described the first clear linkage in mammals and estimated the first linkage in man. He also developed the theory of mapping functions and introduced the terms the morgan and centimorgan, and the approach to linkage, based on the relative likelihoods of trial values of recombination fractions, which is now established. These contributions are discussed against the background of the mendelian controversy and the recent foreground of shotgun linkage.

On an extremum principle in the genetical theory of natural selection

Abstract: Natural selection causes gene frequency changes in a large population leading to genetic evolution over evolutionary time scales. Such gene frequency changes, however, involve an optimizing principle. According to Kimura, such changes, over a short interval of time, occur in a manner such that the increase in population fitness is maximum for a given distance between parent and daughter generation gene frequencies. But according to Ewens, of all gene frequency changes, including those that lead to the same partial increase in mean fitness as the natural selection gene frequency changes, the natural selection values minimize the generalized distance measure between parent and daughter generation gene frequency values. These two optimality principles happen to be mirror images of each other. However, the optimality principles are restricted to the case where the increase in mean fitness is to thefirst order in natural selection gene frequency changes. I show in this paper that, instead of linear approximation to the increase in mean fitness, the treatment can be fairly general, and the exact increase in mean fitness can be considered so as to include the dominance effects of the genes.