Broad sense heritability is defined as:
Or, because Genetic + Environment variation = total phenotypic variation
Broad sense heritability ranges from 0 to1. A high heritability means that most of the variation among individuals is caused by genetic variation among those individuals and not environmental variation. Knowing the heritability can be of value because when the breeder selects for a phenotype (high protein in our case) the genes of that plant will be passed on but not the environment the plant grew in. Breeders prefer to work with traits that have a high heritability so that they can select individuals that will pass the desired trait on to their offspring.
How could Dr. Graef estimate the heritability of seed protein in this F2 family. One method would be to grow the parent lines in the same field. Since these lines are homozygous we can grow identical genotypes in the plots. Any variation in seed protein among these identical plants is caused by environmental differences that influence seed protein. This variation is thus an estimate of the environmental variation among the plants and we can use it to determine the genetic variation.