by single genes). Such traits are also described as showingsimple inheritance. However,
many other traits that a plant breeder works with—such as fruit weight, maturity date,
and grain yield—arequantitative(measured on a continuous scale) andpolygenic(con-
trolled by many genes). Such traits are also described as showingcomplex inheritance.
Figure 3.1 illustrates how a quantitative, polygenic trait can still have underlying
Mendelian inheritance. In this illustration, the size of a melon fruit is determined by the
type of alleles that are present at two different genetic loci. This type of mathematical sim-
plification is commonly used to develop or test models that can help explain the numbers of
genes and the types of gene action that are involved in quantitative trait expression.
Although these are mathematical assumptions, models such as this can often approximate
underlying biological phenomena. For example, the “capital” alleles in Figure 3.1 could
represent gene promoters that trigger higher expression of fruit development factors, and
the “small” alleles are less effective versions of these gene promoters.
The distinction between simple and complex inheritance is a common source of con-
fusion. We say that green versus yellow is a simple monogenic trait, because it is often
determined by one of two alternate alleles at a single genetic locus. But there are probably
numerous other genes that might influence the intensity of the green or yellow color, and
there are probably other gene loci that could mutate to block the production of the green
Figure 3.1.A hypothetical melon-breeding scenario that illustrates quantitative inheritance. Alleles
at two loci (A and B) are represented by lower- versus uppercase letters. Assume that the allele rep-
resented by the capital letter increases melon weight relative to the small allele, such that the average
weight of melons (in kg) produced by a variety is determined by 2 times the total number of capital
alleles plus the number of small alleles. Nine different genotypes will be present in the F 2 generation.
If the two loci are not linked on the same chromosome, their expected segregation ratios will be as
depicted in Figure 2.6. However, because some different genotypes result in the same melon size,
only five sizes of melons will be produced, as shown. The expected proportions of each melon
size in a random F 2 population is depicted by the histogram at the bottom of the figure.
50 PLANT BREEDING