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PubMed1974

Genetic load in natural populations: is it compatible with the hypothesis that many polymorphisms are maintained by natural selection?

Recent studies of genetically controlled enzyme variation lead to an estimation that at least 30 to 60% of the structural genes are polymorphic in natural populations of many vertebrate and invertebrate species. Some authors have argued that a substantial proportion of these polymorphisms cannot be maintained by natural selection because this would result in an unbearable genetic load. If many polymorphisms are maintained by heterotic natural selection, individuals with much greater than average proportion of homozygous loci should have very low fitness. We have measured in Drosophila melanogaster the fitness of flies homozygous for a complete chromosome relative to normal wild flies. A total of 37 chromosomes from a natural population have been tested using 92 experimental populations. The mean fitness of homozygous flies is 0.12 for second chromosomes, and 0.13 for third chromosomes. These estimates are compatible with the hypothesis that many (more than one thousand) loci are maintained by heterotic selection in natural populations of D. melanogaster.

Animals↗
PubMed1996

Additive effects, but no synergistic interaction of stressful life-events and genetic loading in affective disorders.

Life-event research as well as neurobiological findings point to the relevance of adverse stress for the pathogenesis of affective disorders. The well established genetic root might be related to the sensitivity to stress. In concordance, recent studies showed a synergistic interaction between genetic loading and life-events concerning the precipitation of depression, i.e. there might exist a genetic sensitization to the adverse effects of stressors. The present investigation, using information extracted from 877 case records, did not reveal a synergistic interaction concerning the age at onset and the mean frequency and duration of episodes.

Age of Onset↗
PubMed1995

A model for the evolution of the genome: the effect of stochasticity on genetic loads.

In this paper a general model is given for the evolution of the genome incorporating stochastic factors. The model is applied to the substitutional genetic load problem. All of the major hard selection load formulae in the literature are extended and, where necessary, corrected (for stochasticity). Turning to rank selection, formulae for stochastic factors are also corrected and harmful mutations included. A simple formula for the selection coefficient as a function of the nonneutral substitution rate and the mutation profile is obtained. Further, it is noted that the formulae derived also apply (for different parameter values) to the mutation load, thus unifying the two loads under a single theory. A general formula for the mutation load under hard selection is given, extending previous results. Finally, the author derives a formula showing how many harmful mutations can be effectively eliminated by rank selection and discusses its relevance to the question of the possible buildup of harmful mutations in the human gene pool due to long-term exposure to low-level radiation.

Animals↗
PubMed1996

Genetic load in chromosomes 3 in a population of Drosophila melanogaster in Lagos, Nigeria.

Among 194 chromosomes 3 tested, 48.2% were lethal, 13.4% semilethal, 14.4% subvital, and 29.4% non-lethal. The genetic load in the population is high since only about 30% of the chromosome did not reduce viability, and the frequency of drastic (lethals and semilethals), 56%, is among the highest reported for the chromosome. The frequency of lethals is also high. The rate of allelism is low, only 0.5% among the 28 lethals tested. There were, however, 5 probable cases of pseudoallelism in which some non-allelic lethal chromosomes were either semilethal or subvital in the heterozygous condition.

Animals↗
PubMed1990

Genetic load in northern populations of Drosophila subobscura.

O-chromosomes were sampled from two natural populations in Finland, one population in Sweden and one population from Spain. Viability was assessed using the Va/Ba balanced lethal strain. The genetic load for lethals and semilethals was 0.165/0.065 in Helsinki and 0.151/0.101 in Tvärminne, Finland; 0.309/0.118 in Gävle, Sweden and 0.351/0.112 in Barcelona, Spain. With the exception of the Gävle population, the results confirm the general picture that marginal populations are characterized by light loads in comparison with the central ones. Homozygotes for normal or almost normal chromosomes from the Gävle population were less viable than corresponding homozygotes from other marginal populations. Random combinations of wild chromosomes from the Spanish and Finnish populations did not show increased viability over random Spanish heterozygotes.

Alleles↗
PubMedDate not supplied

Control and prevention of the genetic load of haemoglobinopathies in India.

Haemoglobinopathies and thalassaemia are inherited disorders which affect a large number of individuals in India. With a population of 950 million and a birth rate of 28 per thousand, it has been estimated that there would be about 42 million carriers and about 12,000 infants born each year will be inheriting a major haemoglobin disorder in India. In view of this heavy genetic load, frequent blood transfusions, high cost of treatment and management, physical trauma, and psychological and mental harassment to the patients and their families, it has been realized that the preventive genetic approach is the most suitable for the Indian setting. After carrier detection, prenatal diagnosis and genetic counselling are important options for couples at high risk for haemoglobinopathies. A prerequisite for a successful prevention and control programme is health education, public awareness and sensitization, and screening of the population for identification of heterozygotes or carriers in the community.

Female↗
PubMed1993

Genetic loadings in schizophrenia: a dermatoglyphic study.

Finger and palmar dermatoglyphics of 120 male and 120 female schizophrenics with and without a family history of schizophrenia in first-degree relatives were studied in the northwestern part of India. Patients were selected according to specific diagnostic criteria. Significant dermatoglyphic differences were observed for fingerprint patterns, total finger ridge counts and 'atd' angle between the schizophrenics with and those without a positive family history of schizophrenia, suggesting a strong "genetic loading" (i.e., hereditary factors) in familial cases of schizophrenia. Dermatoglyphic features of isolated schizophrenics also significantly differed from those of controls, thus indicating the involvement of genetic factors in the etiology of schizophrenia.

Dermatoglyphics↗
PubMedDate not supplied

[Human genetics studies in areas of high natural radiation. VII. Genetic load].

Two methods to estimate the inbreeding load (Morton, Crow and Muller17 1956; Freire-Maia and Freire-Maia6 1965) are reviewed. Both are employed in the analysis of our data. Besides the total population, a sample constituted of individuals with no alien ancestral is also analysed. No clean effect of natural radioactivity, as measured by genetic load models, has been found (this is especially valid for abortions, pre-natal mortality, anomalies, and abnormalities in general). The results on stillbirths and post-natal and total mortalities are discussed, and it is concluded that most probably the differences found are due to uncontrolled concomitant variables (if not to chance alone). Further analysis are under way.

Brazil↗
PubMed2001

High genetic load in the Pacific oyster Crassostrea gigas.

The causes of inbreeding depression and the converse phenomenon of heterosis or hybrid vigor remain poorly understood despite their scientific and agricultural importance. In bivalve molluscs, related phenomena, marker-associated heterosis and distortion of marker segregation ratios, have been widely reported over the past 25 years. A large load of deleterious recessive mutations could explain both phenomena, according to the dominance hypothesis of heterosis. Using inbred lines derived from a natural population of Pacific oysters and classical crossbreeding experiments, we compare the segregation ratios of microsatellite DNA markers at 6 hr and 2-3 months postfertilization in F(2) or F(3) hybrid families. We find evidence for strong and widespread selection against identical-by-descent marker homozygotes. The marker segregation data, when fit to models of selection against linked deleterious recessive mutations and extrapolated to the whole genome, suggest that the wild founders of inbred lines carried a minimum of 8-14 highly deleterious recessive mutations. This evidence for a high genetic load strongly supports the dominance theory of heterosis and inbreeding depression and establishes the oyster as an animal model for understanding the genetic and physiological causes of these economically important phenomena.

Alleles↗
PubMed1975

Population genetic studies in bees (APIDAE, Hymenoptera). I. Genetic load.

Three populations of Apis mellifera each predominantly of a different subspecies (mellifera, ligustica and adansonii) and 7 species of stingless bees (Meliponinae, Apidae) were manipulated for applying the MORTON, CROW & MULLER's methodology in order to estimate the lethal equivalents (B) of each population. A total of 249 queens were used, 27 being meliponids and 222 Apis mellifera. The populations of Apis have a B that does not differ significantly when they are compared to each other (1.29, 1.36, 1.32) and the balanced average equals 1.33. When the x-alleles are not considered, this balanced value is 0.262. The figures for B in the seven species of stingless bees ranged between 0.104 and 0.159 with balanced average of 0.132. The main reason for this smaller load in meliponids may rest in their effective population numbers, which are smaller than those of Apis. The average mortality for diploid females (0.141) and for haploid males (0.163) allows the estimation of the total elimination (sigma E = 0.073). Since for haplo-diploid systems the total mutation rate is sigma mu = 2 sigma E divided by 3 the figure 0.048 is obtained. Since about 15% of the genes in Apis mellifera are sex limited, this value of sigma mu should be added of 0.0072 (that is 0.15 X 0.048) and then, the total mutation rate becomes 0.055. Using a quite different method, the one by MORTON, CROW & MULLER, the figure 0.076 was obtained. If a mutation rate of 10(-5) is assumed, the number of genes in Apis mellifera that can make a contribution to the genetic load would vary between 5,500 and 7,600.

Animals↗