Phenotype versus Genotype (FENOTIF Versus GENOTIF)

An important term used throughout this book is phenotype, which simply means “what something looks like.” We often speak about the phenotype of a specific trait, in which case it takes on units of measurement. For example, the phenotype of a quantitative trait such as seed weight in wheat might range between 30 and 80 mg. The term phenotype is also used to distinguish what a plant looks like from its genotype (what genes are present) or genotypic value (what we would expect the phenotype to be if we could predict it exactly from the genotype). A fundamental concept in plant breeding is that genotypic value is something that we try to measure and predict. If we could identify or control all the unpredictable effects of error and environment, then the phenotype of a plant (P) would be equal to its genotype (G) plus the effects caused by error and environment (E). Virtually all of the fancy equations that you will see in plant breeding books are derivations of this basic formula.

P= G + E

or more precisely: P=  ∑G + E

where S indicates that genetic effects may be summed over multiple genes, as they are

in Figure 3.1.

The equations above refer to the genotypic or phenotypic values of a single plant or observation. However, breeders work with populations of many plants, and they often summarize a set of observations by calculating the variance, which is simply a mathematical formalization of variability, and genetic variability is the key to creating varieties through artificial selection. The basic breeding equation can also be written to describe a population of plants in terms of phenotypic variance (VP), genetic variance (VG) and environmental variance (VE), such that

VP = V_G + V_E

It is imperative for any breeder to understand the relative proportion of genetic variance that contributes to phenotypic variance for a given trait. This concept is formalized using the term heritability (H), which, in its simplest form, is measured as

H= V_G/V_P

Since VP is always greater than or equal to VG, the heritability of a trait can range from 0 to 1. If H is equal to one, then all variance is caused by genetic effects, and the breeder will be very successful at selecting better plants. Such is the case for the imaginary melon trait illustrated in Figure 3.1. However, if H is zero, then VG must also be zero, and there is no possibility of selecting plants that are genetically superior because all variation is environmental. Most traits that breeders work with show intermediate levels of heritability, between zero and one.

 

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PLANT BIOTECHNOLOGY AND GENETICS: Principles, Techniques, and Applications