The principles of QTL analysis (a minimal mathematics approach

Abstract were several such genes segregating in a Mendelian fash-ion in any given population and their eVects were approxi-The 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 F2. Such qualitativeThis 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, whic