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Most breeding programs follow a similar procedure, with major variations to the procedure (described below) occurring during the selection process in later generations of inbreeding. The process starts with the need to introduce new traits through manual cross pollination. Most breeders in North America make crosses in the greenhouse during the winter season while breeders in the tropics make crosses in the field. Beans are manually cross pollinated in the bud stage before the flowers open (Figure 2.4). Once a flower is fully open, it has already been self‐pollinated. The flower structure consists of a central female style that ends in a stigma, which is coiled and surrounded by a filament that consists of 10 stamens, which suspend the anthers that contain the pollen. Detailed drawings of dissected bean flowers are illustrated in Freytag and Debouck (2002). In the past, breeders would open the immature bud and physically remove a portion of the filament with all 10 anthers. Since the stigma is receptive before the pollen is released from the anthers, the breeder then transfers pollen from male parent to that receptive stigma. Pollen is transferred by breaking off the pollinated stigma (carries excess pollen) from the male donor and hooking that excised stigma with the receptive stigma of the female parent. The flower bud is closed and tagged to indicate the pollen donor. This entire process severely damages the flower, resulting in a low success rate (<50%) with few hybrid seeds produced per pod. Forage legume flowers such as clovers and alfalfa have a natural tripping mechanism to facilitate outcrossing by insects. Beans have a similar mechanism, which breeders manipulate to trip flower buds by depressing downward on the base of two petals to force the stigma out of the filament and away from the immature anthers. At this point, pollen using excised stigma from male parent can be transferred to the tripped stigma with minimum damage resulting in a significantly higher success rate (>90%), with more hybrid seeds in the pod. Since developing hybrid pods are less competitive in development to normally self‐ pollinated pods, many of the latter are removed from the female parent by breeders to favor the production of F₁ hybrid seed. Breeders mostly work with populations developed from a single cross of two parents, but in some instances, crosses between F₁ hybrids are used to produce a four‐way cross as a means of introducing more traits into the progeny, from four parents rather than two.

Fig. 2.4. Open bean flower is already self‐pollinated. The immature bud in the background is used to make a cross pollination with pollen from another open flower. (For color detail, please see color plate section.)

Source: Original image by author, J.D. Kelly.

Once the tagged pods mature, the hybrid seed is harvested as F₁ generation seed. The inbreeding process is initiated at this stage with self‐pollination occurring about 10 times to the F₁₀ generation before the final new inbred variety is released to farmers. Inbreeding procedures vary as different breeding methods are used to fix traits that are inherited differently. Some highly heritable traits such as seed color are under single gene control and are referred to as qualitative traits. Other traits such are yield are controlled by many genes that are influenced by environmental conditions and are under more complex quantitative genetic control. The methods used to fix these different traits vary and will be discussed in more detail in later sections under specific traits. Bean breeders rarely practice selection in the F₁ generation, as there is no segregation in typical biparental crosses. In those instances where four‐way crosses are made, selection may be initiated for single dominant gene traits segregating in that cross, assuming breeders choose to deploy a system of gamete selection (Singh 1994). Aside from this one variation in methodology, breeders allow the F₁ generation plants to self‐pollinate to produce the next‐generation F₂ seed. Breeders will check the F₁ plants to ensure that they result from a cross and are not the result of an accidental self‐pollination. Seed production at this step is conducted in greenhouses or other favorable environments to maximize quantities of disease‐free seed. Most programs will bulk F₁ hybrid seed coming from different crosses (pods) between the same parents once they confirm they are a genuine cross. The F₂ is the generation where most breeders will initiate field selection as segregation is first observed in this generation.