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The following points highlight the top four methods used in cross-pollinated crops. The methods are: 1. Production of Single Cross Hybrids 2. Production of Double Cross Hybrids 3. Production of Top-Cross Seed 4. Production of Synthetic Cross.
Method # 1. Production of Single Cross Hybrids:
Crossing between two inbreds or varieties is called single cross. Such crosses express maximum amount of heterosis, but if the inbreds are poor then very little amount of seeds are produced. So, it is not widely used for commercial production.
Total number of single crosses by a given number of inbreds (n) can be calculated by the formula n(n-1) and excluding reciprocals by n(n-1)/2. For example, with 4 inbred lines, 12 single crosses and 6 straight crosses can be made. For commercial production of single crosses, a male sterile line is used as female, the male parent is the restorer line, the seed set in the female parent is the hybrid seed, but the seeds of male parent are selfed seed.
Method # 2. Production of Double Cross Hybrids:
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Crossing between two single cross hybrids is called double cross, which also involves four different inbred lines. For example, if A, B, C, D represent 4 inbred lines, one of the possible double crosses will be the crossing between two single crosses, A x B and C x D.
The number of possible double cross hybrids can be calculated by the formula
n(n-1)(n-2)(n-3)/8 , where n is the number of inbred lines. So with 4 inbred
lines, 4x3x2x1/8= 3 double cross hybrids can be produced.
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Two different schemes can be taken for production of double cross hybrids:
Method # 3. Production of Top-Cross Seed:
This scheme is also known as inbred-variety cross, (Inbred x Variety), i.e., inbred line is crossed with an open pollinated variety. Also the cross between a single cross and with another inbred is known as double top cross or 3 ways cross.
This method is used not only for developing a hybrid but to test the combining ability of the inbred. The variety or the single cross product is used as male parent and the inbred line to be tested is used as female parent (emasculated), the seeds from the inbred line are harvested.
Method # 4. Production of Synthetic Cross:
A synthetic variety is produced by crossing in all combinations a number of lines that combine well with each other, this synthetic variety is maintained by open pollination in isolation.
It is produced by open pollination between several inbred lines together in isolated place. Seeds of these inbreeds are mixed in equal proportion and sown in isolated plots, natural cross pollination takes place and the seeds are harvested together.
The term poly cross is used in case of forage crops where crossing is difficult and also it differs from synthetic cross that the inbred lines are not pretested. Another term composite variety is used as synonym for synthetic variety but by definition it is the production of mixture of several phenotypically similar outstanding lines by open pollination.
