The principles of QTL analysis (a minimal mathematics approach)

were several such genes segregating in a Mendelian fashion in any given population and their eVects were approxiThe combination of molecular marker and trait data to mately additive ( Kearsey and Pooni, 1996). explore the individual genes concerned with quantitat- It is diYcult to define a quantitative trait precisely. The ive traits, QTL analysis, has become an important tool best that can be said is that such a trait appears to show to allow biologists to dissect the genetics of complex a continuous range of variation in a population, which is characters. However, the mathematical and statistical more or less normally distributed. There are no obvious techniques involved have deterred many from under- discontinuities in the distribution as might be expected of standing what the methods achieve and appreciating a classical, single gene trait, such as the 15251 distribution their strengths and weaknesses. of genotypes and phenotypes in an F 2 . Such qualitative This paper is designed to give a non-mathematical genes have a large eVect on the phenotype compared to explanation of the principles underlying these ana- the environment and, dominance apart, genotypes have lyses, to discuss their potential and to provide an intro- recognizably diVerent phenotypes. Very often one of the duction to the techniques used in the subsequent alleles is non-functional or very dysfunctional, which papers in this series of articles based on the SEB results in the clear phenotype. symposium. However, allelic diVerences may occur in structural or regulatory genes which alter the genes’ action slightly and