If 'n' is no. of Inbred line then total no. of double cross is

If 'n' is no. of the Inbred line then total no. of the double cross is $$\frac{{{\text{n}}\left( {{\text{n}} - 1} \right)\left( {{\text{n}} - 2} \right)\left( {{\text{n}} - 3} \right)}}{8}$$
The double cross was the basic technique used in developing modern hybrids and has been used by commercial firms since. Jones's invention was to use four inbred lines instead of two in crossing.
In the poly cross, selected inbreds are allowed to intermate by open pollination with a random sample of pollen from a number of selected lines.