Causes of Variation
Based on this explanation we can see why quantitative traits are challenging for breeders to work with. When many gene pairs control a desired trait, it is difficult to generate any given genotype. The impact of environment on variation in quantitative traits creates even more problems. For example, Dr. Graef may hire a hard working undergraduate to individually harvest F2 plants from this cross, carefully collect the F3 seeds on that plant into separate envelopes and then use a NIR machine to measure the protein levels in the seed sample. The result of this work is shown in fig. 2. Dr. Graef can then use this information to select the top lines with the highest seed protein level. Even though seed protein was carefully measured in the F2, the F3 progeny may not express this same high protein phenotype. There are two reasons for this. First, the F2 may be heterozygous at one or more of the seed protein gene pairs (Aa or Bb or Dd etc.) and therefore could produce progeny that are homozygous aa, bb or dd and have lower protein genotypes. Additionally, an F2 plant may be growing in a part of the field that allows it to acquire more nitrogen which results in higher protein in the seeds. Therefore, it would be useful for Dr. Graef to know to what extent the variation in protein was due to environment versus genetic differences among the F2 plants. The relative amount of variation caused by genetic variability is called the broad sense heritability.