Heritability of Quantitative Characters in Farm Animals

NY distinction in principle between qualitative and quantitative A characters disappeared long ago, although in the early days of Mendelisni it was often conjectured that they might be inherited according to fundamentally different laws.If it is still convenient to call some characters qualitative and others quantitative, it is only to denote that the former naturally have a discontinuous and tlie latter a continuous distribution, or that the former are not easily measured on a familiar metrical scale.Colors are an example.Differences between colors can be measured in ternis of length of light waves, hue, brilliance, etc., but most of us find it difficult to compare those measurements with our own visual impressions.Most quantitative characters are affected by inany pairs of genes and also importantly by environmental variations.It is rarely possible to identify tlie pertinent genes in a Mendelian way or to map the chroiiiosome position of any of them.Forlunately this inability to identify and describe the genes individually is almost no handicap to the breeder o f economic plants or animals.What he would actually do if he knew the details about all tlie genes which affect a quantitative character in that population differs little from what he will do if lie nierely knows how heritable it is and whether much or little of tlie hereditary variance comes froin dominance or overdominance, and from episla tic interactions between the genes.Delinition. --Heritability is the fraction of the observed or phenolypic variance which was caused by differences between the genes or the genotypes of the individuals.It is used in both a narrow and a broad sense.In the broad sense it refers to tlie luiictioning of the whole genotype as a unit and is used in contrast with environmental.But the genotype is not transmitted as a unit.Instead its constituent genes segregate and come together in new combinations.The genes niay interact with each other in non-adjditive ways, so that in certain combinations they have effects quite different froin their average effects in 4 b g 2 + a,*' J + Biz + 6,Z + ajz _ _ _ _ _ _ ~_ ~ -__ and in the broad sense it is: 16015223,