This is also called recurrent crossing. Characters which are desirable are selectively transferred from one individual to the other in this method. For example, a trait such as disease resistance frost resistance etc.
Present in a variety where the yield is not good are sought to be transferred on to a variety with a high yield but is easily susceptible for the disease. Back cross is done as follows.
Let us assume plant A is a high yielding variety but susceptible to the disease and plant is hardy but with less yield. In order to bring together high yield as well as hardiness, the two plants are crossed and an F, progeny is obtained.
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F, individuals with both the desired traits (high yield and resistance) are selected and crossed with the original parent A. The F, products of this back cross is again crossed back to A. This will be done a number of times until the characters of B are selected.
Finally these selected plants are selfed to make the traits of B homozygous. In this way, the desired traits of B are introduced into a plant.
The main objects of back cross are:
(a) Reduction of unnecessary variations in further generations.
(b) Increasing the proportion of the desired, gene in the progenies.
(c) Increasing the fertility by back crossing.
Multiple crossing also known as compound crossing involves crossing several varieties simultaneously to combine the desired traits.
The resultant progeny from the above cross is used as the starting point for selection. Selection from this stock is made either by pedigree method or bulk method. Plants with desirable combination of traits only are selected.
This technique has been developed by Dr. Norman Borlaug, who is often called the Father of green revolution. In this technique developed for wheat, a high yielding variety is crossed with a number of varieties.
Progenies (each one) are back crossed with the original variety and selections are made to obtain the combination of both the traits. These are crossed among themselves and the progeny selected showing greatest resistance to the rust disease.
By using multilineal crosses, Borlaug obtained day neutral varieties of wheat which could be cultivated in all areas of the world. He was also successful in transferring the dwarf genes of the Mexican wheat (which prevents lodging) to Japanese strains to evolve a very successful variety.