Practice question

Question

Two independent transgenic plants, one with single copy of gene A and another with single copy of gene B were expressed under the same seed-specific promoter. In transgenic plants, seed formation on self-pollination was similar to that of untransformed plants. When homozygous plants with gene A (male parent) were crossed with homozygous plants with gene B (female parent), viable seed formation did not occur although, pollen production was normal. What could be the possible reason for this observation?