Estimates Of Variance For Each Trait Biology Essay


The estimates of variance for each trait were exploited from analysis of variance for the computation of genotypic variance (Vg), environmental variance (Ve), and phenotypic variance (Vp). These components of variation were further used for the calculation of broad sense heritability (h2 B.S) estimates and genetic advance (G.A). The estimates of genetic variance, phenotypic variance, heritability estimates, heritability %age, and genetic advance have been shown in Table 4.2.1.

It is evident from the table that plant height had maximum (129.92) genotypic variance. Plant height had been found to express highest values for components of variance as well as genetic advance and broad sense heritability. It is also clear from the table that there existed great difference between the values of genetic variance and environmental variance for the characters plant height and peduncle length. The higher values of genetic variance for these traits revealed that they were less influenced by environment. The higher values of genotypic variance may affect the heritability in positive manner. The parameter spike density expressed very low values for Vg, Vp, and Ve. Largest values of environmental variance (94.597) were observed for number of grains per spike which meant that this character was much influenced by environmental conditions. Phenotypic variance of grains per spike was found high as compared to environmental variance so selection on phenotypic basis for this trait could not be reliable.

4.3. Heritability and genetic advance

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Selection has been considered a basic tool for the development of new cultivars of field crops. The efficacy of the selection of yield related components depends on the genetic variation and percentage of heritability (Mahpara,2008). Heritability estimates pointed that certain morphological traits influencing grain yield in wheat, were more heritable than yield because yield is a polygenic trait and is greatly influenced by the environment. Ketata et al. (1976) reported moderately high heritability estimates for grains weight and plant height, moderate estimates for number of tillers, and low for spikelets per spike, grains per spike, and grain yield per plant when studying winter wheat crosses.

Heritability and genetic advance are important selection parameters. High heritability estimates along with high genetic advance are usually more helpful in the forecasting of the performance of genotypes and the selection based on these factors could play a vital role in the improvement of grain yield (Iqbal and Khan, 2003). However, it is not necessary that a character expressing high heritability will also exhibit high genetic advance (Singh and Narayanan 1993).

It is obvious from the Table 4.2.1 that the trait plant height has occupied top position for heritability (0.7541). Aycicek and Yildrim (2006) found low heritability estimates for plant height (0.0402). In contrast, Mehta et al. (1997) found middle value and Mossad et al. (1990) and Bhutta et al. (2006) reported high heritability for plant height. Environmental conditions affect the studies conducted for the evaluation of biometrical analysis to a great extent. Although, a breeder can significantly control the environmental variations but cannot eliminate them completely. So a great difference may be seen among the findings of different breeders. According to Rambaugh et al. (1984) heritability studies must be conducted in favorable environments as genetic factors like genetic variance, broad sense heritability and genetic advance are decreased under unfavorable environments.

Followed by plant height, peduncle length (0.6688), grain yield per plant (0.6112), and fertile tillers per plant (0.5296) displayed high heritability magnitudes. Number of tillers per plant is a yield component. Selection of the parameter of large number of tiller production is important for developing high yielding varieties. Plants grown under normal conditions produce more number of tillers than drought conditions (Mahpara, 2008). Many researchers studied this trait in wheat and obtained different results about its heritability. High degree of heritability was observed by Collaku and Harrison (2005) and Ahmed et al. (2007). While low to medium heritability was noted by Singh et al. (2004). Medium to high heritability estimates were noted by Khan et al. (2003). Zaheer and Ahmad (1991) achieved low values for grain yield and Ahmad (1990) and Riaz and Chowdhry (2003) were able to find highest estimates for the same parameter. The trait 100-grain weight had moderate heritability estimates (0.4117). Chaturvedi and Gupta (1995) found intermediate while Mehta et al. (1997) observed high heritability for this character.

Spikelets per spike and spike density showed low values (0.3040) and (0.3333) respectively. Number of spikelets per spike is also important for increasing grain yield but low values of heritability for this trait have been reported in the studies conducted by Singh et al. (2004). High values of heritability were observed by Dechev (1995) and Saleem et al. (2003) for spikelets per spike. Lowest heritability value was observed for spike length (0.2738). Improvement in existing yield status can be achieved by selecting spike length with high heritability (Khan et al., 2003; Ahmed et al., 2007). While low heritability in spike length was observed by Singh et al. (2004). Laghari et al. (2010) determined highest % age of heritability (79.35 %) for the character spike length. The character grains per spike exhibited low to moderate heritability (0.3519). High heritability for number of grains per spike was observed by Riaz and Chowdhry (2003) and Singh et al. (1999). Moderate to high heritability was determined (Chaturvedi and Gupta, 1995 and Ahmed et al., 2007), low values for this trait were observed by Pawas et al. (1989) and Al-Marakby et al. (1994). Fida et al. (2001) also obtained the low heritability (0.35) for grains per spike while Aycicek and Yildrim (2006) got highest figure (0.2218) for the same trait.

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Genetic advance is the advanced measure of the tendency of transfer of characters under one cycle of selection (Poehlman and sleeper, 1995). The higher values of genetic advance were observed for plant height (20.0608), peduncle length (11.7730), grains per spike (12.6137), and grain yield per plant (6.6052). High genetic advance and high heritability for plant height and peduncle length exhibited that heritability is due to additive gene effects so selection based on these factors would be reliable. Lower values of genetic advance in spike length (0.9415), spike density (0.1669), spikelets per spike (1.9275), and 100-grain weight (0.6035) indicated the presence of non additive genetic effects. High heritability values for those characters would be perhaps due to the favourable influence of environment instead of their genetic make up hence selection of those traits would not be fruitful.