The Field of Quantitative Genetics
Quantitative genetics deals with traits that have complex inheritance – such traits are not controlled by one or two genes but by many genes. Quantitative traits are influenced by environmental factors much more than qualitative traits, and exhibit continuous variation caused by polygenes or multigenes and their interactions among themselves and with environmental factors. Continuous variation does not lend itself to simple analyses because differences among individuals are of quantity (indiscrete classes) rather than quality (discrete classes). Grain yield in crops and meat production in animals are quantitative traits. Such traits cannot be determined/analyzed visually but via statistical methods. In contrast, qualitative traits can be visually determined/analyzed and environmental factors generally have little or no effect on the expression of a gene(s) controlling a qualitative trait.
Because of their complex nature, quantitative traits are much more difficult to analyze than qualitative traits that are controlled by one or two genes. Special statistical procedures and software are needed to analyze quantitative traits. The issues that one is concerned about in quantitative genetics are not only how many and which genes control a trait but also how much of what is observed (phenotype) is attributable to genes and how much to the environment.
Quantitative genetic analyses are also used to answer questions, such as ‘what type of gene action and variances are present for a particular trait’; ‘whether or not there is interaction among genes (epistasis)’; and ‘if gene interaction is present, whether it is of additive-by-additive, additive-by-dominance, or dominance-by-dominance type’. Quantitative genetic methodology allows one to examine the rate of genetic progress under selection. Statistical methods are used to estimate genetic parameters/statistics from phenotypic observations. For such analyses, one relies heavily on statistical or biometrical methods. Thus, Quantitative Genetics is also referred to as Statistical or Biometrical Genetics. Quantitative genetics has contributed greatly to the improvement of crop plants and animals and has helped fine-tune breeding programs.
The second edition of ‘Applied Quantitative Genetics’ authored by Dr. M.S. Kang is currently being prepared. The new edition will cover the various topics included in the first edition in much greater detail.