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Backcross Breeding

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Backcross Breeding 1 - Basic Gene Inheritance

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Inheritance of Transgenes

Which is better, one copy or two?

For ECB protection, it does not matter if a plant has one or two Bt gene copies. One copy produces an estimated toxicity level 400X the minimum lethal dose to ECB, more than enough to do an effective job of protecting corn plants.

A plant homozygous with the Bt gene is selfed or mated to another homozygous Bt plant resulting in the following progeny.

The ratio of possible progeny of a homozygous cross.

However, some traits do require two copies of a gene to be effective. An example would be herbicide resistance in ’Clearfield’ or ’Immi’ corn. Plants with one IR resistant allele per cell were not as resistant to ALS herbicides as homozygous IR plants. Therefore, two copies were needed. Those hybrids have since been replaced by IT corn, which is adequately effective with only one allele copy. The alternative resistance allele, IT, confers field resistance in heterozygous hybrids and has become a more popular choice for plant breeders.

In some situations, plant breeders need to know if a plant is homozygous for an important gene. How do plant breeders determine if a plant has one gene copy or two? For example, if the gene is recessive, all progeny that express the trait must have two gene copies. On the other hand, if the gene is dominant, even plants with only one copy will express the trait. Plant breeders can determine if a plant has one or two dominant gene copies by selfing it (mating it to itself), and planting the resulting seeds in a row. If the parent had two Bt gene copies, all of the offspring will have ECB resistance and two gene copies as well. If the parent had only one gene copy, 3/4 of the offspring will have ECB resistance, and 1/4 will not (see Punnett square above). Therefore, homozygous plants will breed true and produce progeny like themselves.

A plant that breeds true for all important traits will produce an inbred line of progeny when self pollinated.

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This project was supported in part by the National Research Initiative Competitive Grants CAP project 2011-68002-30029 from the USDA National Institute of Food and Agriculture, adminstered by the University of California-Davis and by the National Science Foundation (NSF), Division of Undergraduate Education, National SMETE Digital Library Program, Award #0938034, administered by the University of Nebraska. Any opinions, findings, conclusions or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the USDA or NSF.