Training Manual Application of Genetics and Biotechnology in ...

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CHAPTER 1 Quantitative Genetics in Aquaculture G. Gopikrishna, M.S.Shekhar and S. Kannappan Genetics is that branch of biology, which deals with heredity and variation. Qualitative Genetics deals with traits that are easily classified into distinct phenotypic categories and are under the genetic control of only one or a very few genes with very little or no environmental modifications to obscure the gene effects. In contrast to this, quantitative genetics deals with traits that exhibit continuous variation and the phenotypic measurements form a spectrum. Quantitative traits are governed by many genes, each contributing a small amount to the phenotype, such that their individual effects cannot be detected by Mendelian methods. Genes of this nature are called polygenes. The phenotypic variability expressed in most of the quantitative traits has a relatively large environment component and a correspondingly small genetic component. The task before the quantitative geneticist is to determine the magnitude of the genetic and environmental components of the total phenotypic variability of each quantitative trait in a population. This can be achieved by application of special statistical procedures. Dr Ronald A. Fisher (1890-1962) a British Statistician and geneticist was instrumental in providing the foundation for modem quantitative genetics by hsing the newly emerged Mendelian theory with a biometical approach to the study of heredity. 1.1 Statistics in Quantitative Genetics The data on any quantitative trait can be presented graphically as a frequency distribution. The horizontal axis or abscissa, measures values of he trait that are encountered in a sample from the population. The abscissa is usually subdivided into regular intervals. The vertical axis or ordinate measures the frequency of the observations in each interval.
1.2 Normal Distribution The study of a quantitative trait in a large population reveals that very few individuals possess the extreme phenotype and that progressively more individuals are found nearer the average value for that population. This type of a symmetrical distribution is characteristically bell-shaped and is called the normal distribution or Gaussian distribution. - The average phenotypic value for a normally distributed trait is expressed as X or arithmetic mean. It is not possible to measure all the individuals in a population, hence, measurements are taken on a sample from that population in order to estimate the population value (parameter). If the sample is truly representative of the larger population of which it is a part, then would be an accurate estimate of the mean of the entire population (p). Usually, English alphabets are used to represent statistics i.e. measurements derived from the sample, whereas Greek letters are used to represent population parameters i.e. attributes of the population from which the sample was drawn. Larger the sample size, the more accurately the statistic estimates the parameter. 1.4 Measurement of variability Let us suppose that we are comparing two populations for the same quantitative trait. Initially, we would compute the respective means. Although the means do represent the two populations, they do not tell us anything about the variability of the populations. A useful measure of the amount of variability in a population is the standard deviation denoted by (a).A sample drawn from this population will have a standard deviation (S). It is computed by subtracting the sample mean from each individual measurement, squaring the deviations and summing them up for all the individuals in the sample. The resulting sum is divided by (n-I) where 'n' is the sample size.
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Page 24 and 25: CHAPTER 6 MITOCHONDFUAL DSA EXTRACT
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Page 28 and 29: Role of nutrition in captive broods
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