Methods of genetic selection
As pointed out at the beginning of the last section, the development of some of the earlier strains of common carp and trout has not always been as a result of planned selection. Carp farming in different regions has led to the establishment of strains which appear to have adapted to the general climatic conditions under which they are grown, and which grow faster than the wild strains. In rainbow trout, the usual practice has been to pick the best-looking fish from a stock to be the parents of the next generation. These common-sense approaches cannot be depended upon in a cultivation programme to achieve genetic improvement.
Most economically important characteristics of cultivated organisms are measurable and their variation within a population usually takes the form of a ‘normal’ distribution (Purdom, 1972). Such a distribution of measurements occurs because the magnitude of a characteristic is determined by a large, often variable, number of factors, some of which are environmental and some genetic. The separation of environment and heredity has been one of the main aims of studies on population genetics. The reliable models of the inheritance of the measurable characteristics can be used for predicting and controlling the gains from genetic selection within cultivated species.
The variation of a character between individuals can be expressed as ‘variance’, the mean square deviation of individual values from the mean. This is called the phenotypic variance (VP) of a sample or population and is the sum of two components, the environmental variance (VE) and the genotypic variance (VG). Hence VP = VE + VG. The proportion of phenotypic variance that is genetic (VG/VP) is approximately equal to the value of ‘heritability’ which measures the proportion of additive genetic inheritance in the phenotypic variance. It is a measure of the degree to which multiple genes control resemblance between offspring and their parents in the face of a particular set of modifying environmental factors. Heritability can be used to predict selection gains in the formula R = h2S, where R is the response, measured as the change in the mean from one generation to the next, and S is the ‘selection pressure’, or the difference between the mean of the selected parents and the mean of the population from which they were chosen.
As indicated, the ratio of VG to VP is only an approximate measure of h2. More reliablevalues can be obtained through parent/off-spring correlations or by the use of the above formula in a selection. Though laborious and time-consuming, it is essential to have an indication of the magnitude of h2 before starting an extensive selection programme.
The primary reason for desiring estimates of heritability is to enable prediction of results expected from a given level of selection. The effectiveness of selection depends upon:
a) heritability of the attribute (h2);
b) degree of variation in the attribute (sP); and
c) intensity of selection applied in (1).
According to Falconer (1981), the antici-pated response to selection (R) can be stated algebraically as
R = iσPh2
R= mean of offspring from selected parents minus mean of all adults before selection mean of group selected minus
σP = phenotypic standard deviation of the attribute
h2 = heritability estimate for a particular attribute