Principles of Plant Breeding (2+1)

Pure Line Theory

• A pure line is the progeny of a single self fertilized homozygous plant.
• The concept of pureline was proposed by Johannsen on the basis of his studies with beans (Phaseolus vulgaris) variety called Princess. He obtained the seeds from the market and observed that the lot consisted of a mixture of larger as well as smaller size seeds. Thus there was variation in seed size. Johannsen selected seeds of different sizes and grown them individually. Progenies of larger seeds produced larger seeds and progenies from smaller seeds produced small seeds only.
• This clearly showed that there is variation in seed size in the commercial lot and it has a genetic basis. He studied nineteen lines altogether. He concluded that the market lot of the beans is a mixture of pure lines. He also concluded whatever variation observed with in a line is due to environment only.
• Confirmatory evidence was obtained in three ways. In line 13 which is having 450 mg seed wt he divided the seeds on weight basis. He divided the line into seeds having 200, 300, 400 and 500 mg weights and studied the progenies.
• Ultimately he got lines having weight ranging from 458 to 475. Thus the variation observed is purely due to environment. The second evidence was that selection with in a pure line is ineffective. From a pure line having 840 mg selection was made for large as well as small seeds.
• After six generations of selection the line for large seed as well as for small seed gave progenies having 680-690 mg. did not change further. Thus it was proved that selection within a pure line is ineffective. In third evidence when parent - offspring regression was worked in line thirteen found be to zero indicating that variation observed is non heritable and it is due to environment only.