V(D)J recombination on homologous chromosome
There are two alleles (variants of the same genetic region) for the Ig and TCR loci, one on each chromosome. The cell will try first with one allele to obtain a productive arrangement, and then with the other. If the cell fails on both alleles, the cell undergoes apoptosis. If at least one allele has a productive arrangement, the cells move on to selection.

Groups are still uncertain, however, how the cell chooses which allele to initiate recombination on, and ruled out parental imprinting as a culprit last year (1).

To be certain, the regions are the same, though not the same sequence. However, mechanisms during recombination ensure that recombination events among individual cells result in robust junctional diversity and a suitably diverse repertoire of immature antibody or TCR sequences (example).

In activated B cells, however, class switching and somatic hypermutation can result in gross changes to the Ig locus not seen in immature or pro-B cells.