The success of genetic improvement of any trait depends on the nature of variability present for that trait (Falconer and Mackay, 1996). Therefore, an understanding of the nature and magnitude of variability present in the gene pool for the traits of interest is of greatest importance. Phenotypic variation of any trait is a combination of mainly three components, viz. genetic variation, environmental variation and variation due to the interaction between the genetic and the environmental factors (Falconer and Mackay, 1996; Sleper and Poehlman, 2006; Acquaah, 2009; Brown and Caligari, 2009).
21 1.10.1 Genetic variation
Acquaah (2009) defines genetic or heritable variation as the variation that can be attributed to genes that encode specific traits, and can be transmitted from one generation to the next.
Since genes are expressed in an environment, the degree of expression of a heritable trait is impacted by its environment, some more so than others.
A phenotype (P), defined as the characteristic that is observed, is as a result of a combination of its genetic constitution, called the genotype (G), and the environment (E) and a component attributed to the interaction between the genetic and environmental components (GxE). This is usually expressed as:
Phenotype = Genotype + Environment + G x E (Falconer and Mackay, 1996; Sleper and Poehlman, 2006; Brown and Caligari, 2009). From this equation for phenotypic expression, it follows that any variation seen in the phenotype is due to variation in the factors resulting in the phenotype. The relationship can then be presented as:
VP = VG + VE + VGxE.
Where:
VP = Phenotypic variation VG = Genotypic variation
VE = Variation as a result of the environment
VGxE = variation due to genotype x environment interaction effects
Genotypic variation is generally divided into two components, which are additive and non- additive components (Falconer and Mackay, 1996; Sleper and Poehlman, 2006; Brown and Caligari, 2009). Additive variation is due to the cumulative effect of alleles on all gene loci influencing a trait, and is usually of most value in a crop improvement programme (Falconer and Mackay, 1996). Non-additive variation is divided into dominance variation, caused by the interaction of specific alleles at a gene locus, and epistatic variation, caused by the interaction among gene loci (Falconer and Mackay, 1996). The non-additive variation is normally given little attention since only the additive component of genetic variation is heritable (Falconer and Mackay, 1996; Sleper and Poehlman, 2006; Brown and Caligari, 2009). Genetic or heritable variation in nature originates from gene recombination, modifications in chromosome number, and mutations (Falconer and Mackay, 1996). Rather than wait for them to occur naturally, plant breeders use a variety of techniques and methods
22 to manipulate these three phenomena more and more extensively, as they generate genetic variation for their breeding programmes (Acquaah, 2009).
1.10.2 Environmental variation
Environmental variation is usually associated with environmental conditions prevailing on the site where the crops are grown (Ceccarelli and Grando, 1991; Annicchiarico and Perenzin, 1994). Some of these conditions, such as plant to plant competition and population density can be controlled by use of agronomical practices, where others, such as rainfall, wind are uncontrollable. Environmental variation is normally difficult to control because it is non- heritable. For example, when an individual from a clonal population (identical genotype) are grown in the field, the plants will exhibit differences in the expression of some traits because of non-uniform environments. The field is often heterogeneous with respect to plant growth factors such as nutrients, moisture, light, and temperature (Ceccarelli and Grando, 1991)
1.10.3 Genotype by environment interaction
Genotype x environment interaction (GEI) occurs when different genotypes respond differentially to any changes in the environments (Eberhart and Russell, 1966; Ssemakula and Dixon, 2007). Genotype x environment interaction varies with the genotypes tested and the sites chosen for testing (Buerno, 1986; Lebot, 2009). Especially complexly inherited quantitative traits are influenced by environmental effects. A significant GEI for a quantitative trait such as yield can reduce the usefulness of subsequent analyses, restrict the significance of inferences that would otherwise be valid, and seriously limit the feasibility of selecting superior genotypes (Flores et al., 1998). Differences between genotypic values may increase or decrease from one environment to another which might cause genotypes to rank differently between environments. Genotypes are normally tested over a wide range of diverse environments and agricultural experiments to determine the extent and nature of GEI may involve a large number of genotypes (Egesi et al., 2007; Aina et al., 2009).
Cassava is subject to considerable GEI (Kvitschal et al., 2006; Ssemakula and Dixon, 2007;
Lebot, 2009). Studies with different cassava genotypes tested in contrasting environments have shown that FSRY is subject to strong GEI (Ssemakula and Dixon, 2007; Aina et al., 2009). Tan and Mak (1995) detected that GEI effects were significant for FSRY, commercial storage root number, HI, starch and cyanide content. Although significant, their effects were smaller than the genotype effects, except for commercial storage root number and FSRY.
They found only cyanide content exhibited a linear GXE relationship with the environment.
23 Buerno (1986) reported important genotype x location and genotype x location x year interaction for FSRY when testing a number of genotypes in the humid tropics of Brazil. Huhn (1996) reported that the dry matter content of cassava storage roots had high cultivar-by-year interaction and cultivar-temperature interaction.