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Biomedical subjects

R Chakraborty

Publications and source records attributed to R Chakraborty.

At least 271 records · Page 15Linked to original sources
PubMed1977

Variation in allele frequencies among caste groups of the Dhangars of Maharashtra, India: an analysis with Wright's Fst statistic.

Variations in allelle frequencies at 11 serological and biochemical loci among 21 endogamous Dhangar castes of Maharashtra, India, have been studied using Wright's FST statistic. Gene differentiation among these castes is only about 4 per cent at these polymorphic loci. The distributional form of FST is explicable by random genetic drift at this early stage of differentiation of these caste groups. The empirical relationship between the average allele frequencies and the ration of within to total gene diversity of the caste groups is also studied.

Alleles↗
PubMed1977

Estimation of time of divergence from phylogenetic studies.

Recent studies with comparative data on base sequences of homologous DNA's or amino acid sequences of homologous proteins indicate that simultaneous estimation of phylogenetic structure and time of divergence is often cumbersome and time consuming. On the other hand, when the topology of an evolutionary tree is known, it is shown in this paper that the least squares theory may be applied to obtain simple estimates of the relative time lengths for each segment of the tree under the assumption of uniform random substitutions in each segment. The method is illustrated with amino acid sequence data on various globin molecules and cytochrome c. The evolutionary significance of some of the estimates is also discussed.

Amino Acid Sequence↗
PubMed1977

Statistical studies on protein polymorphism in natural populations. I. Distribution of single locus heterozygosity.

Surveying the literature, the frequency distribution of single-locus heterozygosity among protein loci was examined in 95 vertebrate and 34 invertebrate species with the aim of testing the validity of the mutation-drift hypothesis. This distribution did not differ significantly from that expected under the mutation-drift hypothesis for any of the species examined when tested by the Kolmogorov-Smirnov goodness-of-fit statistic. The agreement between the observed interlocus variance of heterozygosity and its theoretical expectation was also satisfactory. There was an indication that variation in the mutation rate among loci inflates the interlocus variance of heterozygosity. The variance of heterozygosity for a homologous locus among different species was also studied. This variance generally agreed with the theoretical value very well, though in some groups of Drosophila species there was a significant discrepancy. The observed relationship between average heterozygosity and the proportion of polymorphic loci was in good agreement with the theoretical relationship. It was concluded that, with respect to the pattern of distribution of heterozygosity, the majority of data on protein polymorphisms are consistent with the mutation-drift hypothesis. After examining alternative possible explanations involving selection, it was concluded that the present data cannot be explained adequately without considering a large effect of random genetic drift, whether there is selection or not.

Animals↗
PubMed1976

Electrophoretically silent alleles in a finite population.

The expected number of silent alleles in an electromorph is computed for various values of population size (N), mutation rate (u), and sample size (s) under the assumption of no selection. The proportion of alleles undetectable by electrophoresis is higher when Nu is large than when this is small. It is shown that an electromorph of high population frequency has more silent alleles than an electromorph of low frequency if the sample size is the same.

Alleles↗
PubMed1976

Empirical relationship between the number of nucleotide substitutions and interspecific identity of amino acid sequences in some proteins.

There are three different methods of estimating the number of nucleotide substitutions between a pair of species from amino acid sequence data, i.e. the Poisson correction method, random evolutionary hit method, and counting the actual but minimum number of nucleotide substitutions. In this paper the relationships among the estimates obtained by these methods are studied empirically. The results obtained indicate that there is a high correlation among these estimates and in practice any of the three methods may be used for constructing evolutionary trees or relating nucleotide substitutions to evolutionary time. The effects of varying rates of nucleotide substition among different sites on the Poisson correction and random evolutionary hit methods are also studied mathematically. It is shown that these two methods are quite insensitive to the variation of the rate of nucleotide substitution.

Amino Acid Sequence↗
PubMed1976

The genetics of shovel shape in maxillary central incisors in man.

From dental casts of 94 parent-offspring and 127 full-sib pairs, sampled from two Chilean populations, shovelling indices are computed to measure the degree of shovelling of maxillary central incisors quantitatively. Genetic correlations are computed to determine the role of genetic factors in explaining the variation in this trait. Assuming only hereditary factors to be responsible for the transmission of shovel shape, 68% of total variability is ascribed to the additive effect of genes.

Anthropology, Physical↗
PubMed1976

Infinite allele model with varying mutation rate.

Available data suggest that the variation in mutation rate among protein loci follows the gamma distribution. Thus, taking into account this variation, formulae are developed for the distribution of allele frequencies, mean and variance of heterozygosity, expected number of alleles, proportion of polymorphic loci, and genetic distance. These formulae should be more appropriate for the analysis of gene frequency data for protein loci than equivalent formulae with constant mutation rate.

Alleles↗
PubMed1976

Hidden genetic variability within electromorphs in finite populations.

The amount of hidden genetic variability within electromorphs in finite populations is studied by using the infinite site model and stepwise mutation model simultaneously. A formula is developed for the bivariate probability generating function for the number of codon differences and the number of electromorph state differences between two randomly chosen cistrons. Using this formula, the distribution as well as the mean and variance of the number of codon differences between two identical or nonidentical electromorphs are studied. The distribution of the number of codon differences between two randomly chosen identical electromorphs is similar to the geometric distribution but more leptokurtic. Studies are also made on the number of codon differences between two electromorphs chosen at random one from each of two populations which have been separated for an arbitrary number of generations. It is shown that the amount of hidden genetic variability is very large if the product of effective population size and mutation rate is large.

Codon↗
PubMed1975

Power of assaying inbreeding through sampling of phenotypes and mating types.

Power of detecting inbreeding coefficient through a phenotype or mating type bioassay is evaluated as a function of sample size and level of inbreeding. Efficiency of a mating bioassay over a phenotypic one is then defined as the ratio of the two power functions for a given sample size while surveying the same population (i.e., with the same F). Empirical analysis of this efficiency function indicates that efficiency depends largely on n and F apart from the gene frequencies at the locus.

Alleles↗
PubMed1975

Estimation of race admixture--a new method.

The contribution of a parental population in the gene pool of a hybrid population which arose by hybridization with one or more other populations is estimated here at the population level from the probability of gene identity. The dynamics of accumulation of such admixture is studied incorporating the fluctuations due to finite size of the hybrid population. The method is illustrated with data on admixture in Cherokee Indians.

Alleles↗
PubMed1975

On some theoretical studies on gene differentiation in natural populations.

Different mathematical approaches to study the extent of genetic variation of natural populations are reviewed. The modern understanding of the gene structure permits new interpretations of existing concepts like fixation or inbreeding. A more recent measure of genic divergence, which at molecular level is designed to measure net codon differences is also seen to be related with gene diversity in a substructed population. It is argued that such variations are produced and preserved possibly by simultaneous action of migration, mutation, selection, and random genetic drift. At the present moment it is very difficult to isolate out the effect of each factor because of varying degrees of variation at the different gene sites and between different sets of populations.

Biological Evolution↗