Bohn, G. W.
Most plant breeders work with numerous plant and fruit characters. The work would be difficult, and time- and space-consuming if each character were controlled in inheritance by a single genetic factor. Actually, many economic characters (such as plant size, disease resistance and yield; fruit size, shape and appearance; and flesh firmness, texture and flavor) are controlled individually by several genes. The several genotypes of any one character difference sort out in a normal or skewed distribution curve with little indication of distinct phenotypes in the F2 population. The breeder selecting in the F2 population from a wide cross, such as that often required in disease-resistance work, has the difficult task of selecting in material segregating in a dozen or more distribution curves (one for each character) superimposed upon one another. With limitations on time, space and funds he may endeavor to select in comparatively small populations. That is especially true of naturally cross-pollinated crops, such as the muskmelon, that must be hand-pollinated before selection information is available; it is true, also, of self-pollinated crops such as the tomato that require considerable field space. The folly of such a procedure and the need for tremendous F2 populations for progress with a strict inbreeding program can be illustrated with segregation studies in tomato crosses with several simply inherited, visible markers.
A small F2 population (Rick's 48-110) from the cross ms\9 A a c d l r y was grown at La Jolla last season. We identified only 36 of possible phenotypes among 82 plants obtained from 100 seeds, plants occurred (linkage in repulsion); but, then, only 1 of the 82 plants was ry (independent). Plants with combinations of other 2-character recessives (all independent) ranged from 2 to 9. Interestingly, the distribution of plants homozygous for O, 1, 2, 3, 4, 5, 6 and 7 markers was 14, 20, 32, 12, 4, 0, 0, 0; nons of the 82 plants was homozygous for more than 4 markers.
One can visualize, in such plant material, the chances of obtaining in the F2 from a wide cross a plant superlative for any one economic character. The observations on these easily observed characters indicate that a breeder would have little chance (theoretically 1 in 1024) of obtaining a high yielding plant among 100 or 200 F2 plants from a cross in which high yield was dependent on 5 recessive genes. Combining high yield with similarly inherited disease resistance (1 in 1,048,576) and quality (1 in 1,073,741,824) would require numbers outside the range available to most vegetable breeders.