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Segregation of Genes: The Plant Breeder's Method of Predicting the Future

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Back to the Wheat Breeding Field

Let’s fast forward to the present and revisit Dr. Baenziger in his wheat breeding field. He is growing F2 plants that originated from crossing true breeding lines that differed in their reaction (resistant vs. susceptible) to a treatment of ALS herbicide. These crosses produced F1 seeds that were planted to grow F1 plants, but Dr. Baenziger did not test the herbicide resistant phenotype of these plants. He had emailed and talked with wheat breeding colleagues who were also working with the inheritance of the ALS herbicide resistance trait and he was quite sure the F1s would show damage from the herbicide.  The F1s might even die if the herbicide application environment was severe.  He needed to have healthy F1s that could produce many F2 offspring. Now he was out in the field with these F2 plants and it was time to think like a geneticist and apply Mendel’s new ideas (which were now old, reliable ideas).

Figure 8. F2 rows from F1 plants after the ALS herbicide is sprayed.  The obvious mixture of phenotypes in the rows, some dead or damaged from the herbicide and some resistant, is often described as ‘segregating’ by a plant breeder.

Since Dr. Baenziger promised to deliver a new line that farmers could be confident in spraying with a weed killing dose of the ALS herbicide, he was very careful as he examined the F2 plants.  His team sprayed with a dose of the herbicide that exceeded the level farmers would apply. He wanted to select ONLY plants in the F2 that were perfect in their resistance response.  Some plants were perfectly resistant.  Others showed severe damage and still others showed damage only when he examined the development of the wheat’s head (the grain producing flowering structure).  When he completed his observation of each F2 and recorded their phenotype, Dr. Baenziger had a data set for analysis.

Figure 9. Careful examination of the F2 plants was critical to make sure that only plants with perfect resistance to herbicide damage were selected by the plant breeder. Notice the damage done to the wheat head by ALS herbicide.

Data Table 3: F2 plant phenotypes after application of ALS herbicide.

Total number of F2s

Number that are

perfectly resistant

Number that are dead or show some damage

Ratio

890 233 657

1:3

The first observation Baenziger made is that the plants that were perfectly resistant were about ¼ of the F2's.  The remaining ¾ had some level of damage. 

Question:  What hypothesis could he have posed based on this result?

a. the resistant plants are RR or Rr
b. the resistant plants are rr
c. these results do not fit with a pattern that would suggest the trait is monogenic.

 


Click on the picture to view a video about Dr. Baenziger's
wheat breeding experiments and the Punnett Square.

Figure 10.  A Punnett square can be used to explain the results of the wheat breeders F2 generation experiment.

Dr. Baenziger now needed to test his hypothesis that the perfect, herbicide damage free plants he selected were the genotype rr.  He and his team carefully harvested the seeds from the individual F2, resistant wheat plants.  He was careful for two reasons.  His genetics hypothesis could be wrong and the breeding behavior of the F2 would reveal the flaw in his prediction.  It is also possible that an occasional F2 plant was just an escape from the herbicide treatment.  Lucky escape from a lethal or damaging dose of the herbicide is an environmental variable and will not be passed on to offspring.

The carefully harvested F3 seed was planted again in family rows and the ALS herbicide was applied to reveal the resistance phenotype.  Dr. Baenziger’s team applied the same careful observation and data collection to these F3s to assemble Data Table 4 below.

Question:  The table of F3 row data (Data Table 4) reveals some rare exceptions to the prediction that the resistant F2s are true breeding.  What is the most likely explanation for this result?

a. Herbicide resistance is actually dominant.
b. The rare, damaged plants are due to random error.
c. The breeder should start over again, because this result shows the breeder made incorrect conclusions.



Data Table 4: F3 plant phenotypes after application of ALS herbicide.

Select F2 number

F3 offspring rows that are

perfectly resistant

F3 offspring rows that show some damage

Notes

197

195 2 Discard rows with some damage

 

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