FIG. 18.13 Relationship between number of genes and their effect on complex traits. Many Most of the genes affecting the trait have relatively small effects. For a typical complex trait, only a few genes have a relatively large effect. Few Small Large Magnitude of gene effect Number of genes affecting the trait
Genetic Variation
Genetic variation refers to the variation in the genome sequences between individual organisms of a species. Individual differences or population differences can both be referred to as genetic variations. It is primarily caused by mutation, but other factors such as genetic drift and sexual reproduction also play a major role.
Quantitative Genetics
Quantitative genetics is the part of genetics that deals with the continuous trait, where the expression of various genes influences the phenotypes. Thus genes are expressed together to produce a trait with continuous variability. This is unlike the classical traits or qualitative traits, where each trait is controlled by the expression of a single or very few genes to produce a discontinuous variation.
When genes for complex traits have effects that are distributed as shown in Fig. 18.13,which are easier to identify: those that are numerous with small effects, or those that are few with large effects?
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