Backcross Breeding
It is important for the plant breeder to understand the inheritance of transgenes, as explained in the last section, so that they can efficiently proceed with their breeding methods.
How is backcross breeding done?
The backcross breeding method has been used by plant breeders for decades to incorporate specific traits into elite lines. This method works by crossing the transgenic inbred line with an elite inbred line of choice. The offspring of this cross, called the F1 generation, will have 50% of the transgenic line’s alleles and 50% of the elite line’s alleles.
The F1 plants are crossed back to the elite inbred again. These offspring, called the BC1 generation for backcross 1, will have 75% of the elite alleles and 25% of the transgenic line’s alleles. Not all of the BC1 offspring will be transgenic, so the plant breeder must have a way to determine which plants have the transgene and which do not.
This may be done by selecting for a herbicide resistance marker gene, the protein encoded by the transgene or the transgene itself. The breeder could select by spraying all of the plants with the herbicide. Identifying the transgenic plants will be discussed in the next section.
In the following step, the breeder crosses the selected transgenic offspring back to the elite inbred again. The offspring of this cross is the BC2 generation and will have 87.5% elite alleles. Every backcrossing generation increases the percentage of elite alleles by half of the total percentage of transgenic line alleles of the previous generation. After BC5, the average percentage of elite alleles will be almost 99%. The other 1% are alleles from the transgenic line and will include the selected ’transgene’.
After the transgenic elite inbred has reached an adequate percentage of elite alleles, the breeder self crosses it to obtain offspring with 2 copies of the transgene. An inbred homozygous for the transgene is desirable when making a cross with another inbred line because it allows one copy of the transgene to be passed on to all offspring.*This animation has no audio.*
Each generation of backcrossing increases the percentage of elite alleles in the transgenic line until its yield potential is close to that of the original elite line. This may take up to 5 or 6 generations. Image by P. Hain
One in four offspring of the self-cross will have 2 copies of the transgene and 2 in four will have 1 copy. Since one copy of the Bt gene is enough to give adequate ECB resistance, the breeder cannot determine which plants are homozygous by phenotype. S/he must self-cross these progeny and plant the seeds from each plant in individual rows. If a heterozygous plant is selfed, 3/4 of the progeny will have the Bt trait, but 1/4 will not. If a homozygous plant is selfed, all progeny in the row will have the Bt trait. The breeders will select and use the progeny from these rows.