The birth and reproduction of Carrier(Heterozygous) organisms, (i.e) heterozygous individuals that have a dominant allele masking the effects of the recessive 'lethal gene.' However even though the gene is masked, it is still there. Therefore, if the heterozygous individual is to reproduce with another heterozygous (carrier) organism there is a higher chance that their offspring will have the active form of the 'lethal gene' causing the death of the offspring.