A healthy individual is a carrier of a lethal allele but is unaffected by it. What is the probable genotype of this individual?

To carry a lethal allele but not be affected by it, said allele would most likely be a recessive allele, masked by a healthy dominant allele. The dominant allele would take precedence and thus protect the individual from the lethal one. Therefore, if the gene was labeled with the letter a, the gene would be Aa, the A being the dominant (healthy) allele and the a being the recessive (lethal) gene.