*2.3.1.2 Selection for quantitative traits*

These traits are controlled by many genes and are also affected by various environmental factors. There is no sharp distinction among the phenotypes and affected by both additive and non-additive gene action. No trait is 100 per cent heritable, because the environment always affects the phenotype to a certain extent. Therefore, the phenotype of an individual for quantitative traits is not the true indicator of genotype. The phenotypic merit of the individuals for quantitative traits is determined by comparing the individual's own phenotype with that of the average of all the individuals within a group from which it is selected and is called trait ratio [14–16].

> Trait ratio <sup>¼</sup> record of individual for a trait Group mean for the same trait � <sup>100</sup>

The trait ratio depends upon the accuracy of records or available data. The individual's record is of little value unless it shows where the individual ranked relative to others under similar conditions. The environmental part of phenotypic superiority or inferiority will not be transmitted to the offspring or next generation. Therefore, in general, there is a tendency for the average phenotype of the offspring of a phenotypically superior individual will tend to regress toward the average of the population, whereas the average phenotype of the offspring of phenotypically inferior individuals will tend to rise toward the average of the population. Animals own phenotypic value of the character under selection is considered to estimate the probable breeding value (PBV) [5, 13, 14] of that character for that individual.

Probable Breeding Value PBV ð Þ¼ <sup>P</sup> <sup>þ</sup> <sup>h</sup><sup>2</sup> Pi � <sup>P</sup>

Where, P is population mean, h<sup>2</sup> is heritability, Pi is individual phenotypic value.

Comparison is made with the average of other individuals kept under similar environmental conditions of same age and same time; thus, individuals are ranked relative to others under similar conditions. It is also called as mass selection.

In individual selection, the best animals are selected from within a group of animals of the similar age group that has been reared and treated similarly at the same time, i.e., contemporaries. In individual selection, the breeder will be having a single record of each animal's performance (performance test) and hence an estimate of probable breeding value (PBV) [13] for a given trait is calculated as:

$$\text{Probable Breading Value (PBV)} = \overline{\mathbf{P}} + \mathbf{h}^2 \left( \mathbf{P}\_{\text{i}} - \overline{\mathbf{P}\_{\text{c}}} \right)$$

Where, P is population mean, h2 is heritability, Pi is individual phenotypic value, Pc is average of contemporaries.

#### *2.3.1.3 Advantages of individual selection*

