Genetic consequences
 Repeated backcrossing to the recurrent parent accelerates homozygosity equals to selfing and improving the recovery of recurrent parent type progenies.
 Heterozygous F1 will have genes in ratio of 50:50 from nonrecurrent and recurrent parents.
 In subsequent backcross generations, this ratio will tilt towards recurrent parent with the proportion of genes from donor parent being reduced by one –half and genes from recurrent parent increasing by the same rate.
 Contents of the donor parent in any backcross are given by the formula (1/2)n where n is number of crosses and backcrosses attempted with recurrent parent so in BC4 the genes from donor parent would be (1/2)5 = 1/72 or 1/32 x 100 = 3.125% indicating that contribution of recurrent parent by 96.875%
The proportion of homozygosity in any generation is given by the formula.
[(2m1/2m)]n
 m = number of generation
 So after 3 generation of backcrossing homozygosity will be 31/3 = 81/8 = 7/8 or 7/8 x 100=175/2 = 87.5%.
For n number of genes, the proportion of homozygous genotypes would be
(21/2) n
 So if we consider 4 genes then after 3 generation of backcrosses, the proportion of homozygotes would be = (231/23)4 = (7/8)4 = 76.5%
Objective in backcrossing is to recover recurrent phenotype along with target gene.
 Briggs and Allard (1953) suggested a complete the gene transfer through six backcrosses. Some important considerations in backcrossing program are:minimum of 53 plants from backcrossed seeds, 96 F2 plants and 68 F3 rows are required to
 If selection is not rigid for recurrent parent, the number of backcrosses required may increase to ten.
 Smaller populations necessitate more number of backcrosses.
 If the parental lines agronomically resemble then even fewer backcrosses are sufficient.
 If the target gene is linked to undesirable gene then more number of backcrosses need to be attempted.
 Normally six backcrosses that follow strict selection for recurrent genotype are adequate for effective gene transfer

Last modified: Monday, 2 April 2012, 6:45 PM