What is Pure Line theory?
Johannsen developed concept of pureline while working with princess bean (Phaseolus vulgaris nana.). Princess bean exhibits self-pollination.
Procedure
- Started experiments in 1901. Established 19 purelines by selection of individual beans from the mixed seed lot of 8 kg. (named those as A, B, …, T)
- Followed selection among these purelines too and selected the smallest and largest beans (seeds). Planted those selected seeds. (1902-1907). And noted results.
Observations
| Weight class of mother bean | Properties of offspring | ||
|---|---|---|---|
| Avg. weight | Number of beans | SD | |
| 250-300 | 445 | 40 | 84.0 |
| 300-350 | 498 | 53 | 64.8 |
| 350-400 | 453 | 164 | 69.9 |
| 400-450 | 447 | 155 | 65.5 |
| 450-500 | 434 | 103 | 74.9 |
| 500-550 | 468 | 102 | 79.1 |
| 550-600 | 458 | 95 | 79.9 |
| Whole line | 454.4 | 712 | 74.0 |
Result
There were seeds of various sizes within each line, but the variability within the line was much lesser than in original lot. Largest and smallest seed from a line produced progeny with the same mean weight. This is because the seeds from each of the mother beans (1901) are the result of two generations of selfing from a single grandmother bean. Grandmother bean itself may be the result of many generations of selfing. Variation if present, it is environmental. Johannsen introduced the terms genotype and phenotype to explain this distinction.
Experiment showed that a land variety in a self pollinated crop is composed of a mixture of pure lines. Diversity, or variation, is between lines and not within lines.
Johannsen’s Conclusions
- Continuous inbreeding (selfing) leads to homozygosity.
- Variation within a pureline results from environment only.
- Selection within pureline is not effective because all the plants in a pure line have same genotype.
- Selection in the original population is effective because the plants have genetic variation.