They result in the death of an organism. The recessive lethal alleles involve an identical pair of alleles exhibiting recessiveness. Most of them occur in the homozygous state. Hence, they are fatal. These alleles not only code for the recessive traits but also code for the dominant traits. Diseased phenotype sometimes gets expressed in the heterozygote individuals. An example includes heterozygote individuals in Achondroplasia, a disorder related to dwarfism. If one lethal allele occurs, the individual still tolerates it. Hence, the heterozygotes having one lethal allele still manage to survive. However, if the lethal allele occurs in a double dose, it becomes lethal. The condition is known as a homozygous condition. Lethality gets often associated with the death of the individual or the organism. In homozygous Achondroplasia individuals, death occurs before birth or in the perinatal stage. Consider another example of recessive lethal alleles in cystic fibrosis. It is an autosomal recessive disorder affecting 1in 2500 people. The mutations in the CFTR gene reduce the intracellular sodium chloride levels. The CFTR gene mutation thereby induces the secretion of thick mucus. The cystic fibrosis carriers do not show the mutant phenotype even though they have one recessive lethal allele. However, if the two heterozygote carriers marry and plan to have children, 25% of them experience lethality. Hence, such individuals die due to the presence of two recessive lethal alleles obtained from each of the carrier parents.