In carrier pigeons there is a rare inherited condition that causes the death of the chicks before hatching. in order for the disease to be passed from generation to generation there must be parent birds that

The parent birds must carry the condition within their genotype. A carrier's genotype would be "Aa", with the recessive coding for the fatal condition so that it can be passed on to offspring, but the dominant allele assures that the parent does not express the condition.