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Corn Breeding: Introduction to Concepts in Quantitative Genetics

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The Concept of Heritability

The goal of the plant breeder is to improve phenotypic values by identifying and selecting superior genotypes. Because environment also affects phenotype, there is not a perfect correspondence between phenotypic and genotypic values. To predict the outcome of selection in a collection of genotypes, a breeder must know the level of correspondence between phenotypic and genotypic values. Heritability (H) is a value that expresses the degree of this correspondence.

Specifically, H is the percentage of the phenotypic variance that is attributable to differences among individuals in genotypic value. This can be seen in the equation below.

When there is no genetic variance, then the numerator in this equation is 0 and H = 0. When there is no environmental variance, then all the phenotypic variance is genetic variance (i.e., ) and H = 100. Breeders can estimate heritability by determining values of both the phenotypic and genetic variances. An example of calculating the phenotypic variance of ear length already has been presented. Values of genetic variance also can be obtained, but the procedures for doing this are beyond the level of this lesson.

Although the heritability of a trait depends on how it is measured, in what environment(s) it is measured, and which plants are measured, different traits of corn tend to have different values of heritability. Qualitative traits, such as flower color, often have a value of heritability close to 100. Heritability values of quantitative traits are typically less but can vary greatly. For example, the number of days to flowering or the percentage of protein in the kernel, often have heritabilities in excess of 50%. In contrast, grain yield of individual plants often has a heritability that is much less than 50%. This means that differences in the yields among individual plants of an open-pollinated variety are determined typically much more by environmental factors than by genetic factors, even though they are grown in the same field at the same time.

The higher the heritability of a trait, the easier it is to modify that trait by selection. This intuitively makes sense. If heritability is very high, then phenotypic value is a good estimator of genotypic value because σE2 is small compared to σG2 (hence, environmental differences between plants are small). Therefore, selecting the plants of an open-pollinated variety with the best phenotypic values to be parents of the next generation should result in substantial improvement in the variety. The opposite will be true when heritability is low. In fact, if H = 0, then even though the plants that are phenotypically superior are chosen as parents, no improvement from this selection is expected. In the lesson on mass selection in this series, the relationship between gain from selection and heritability will be discussed further.

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