sentences:  In the case of homozygous achondroplasia, death almost invariably occurs before birth or in the perinatal period.Not all heterozygotes for recessive lethal alleles will show a mutant phenotype, as is the case for cystic fibrosis carriers.If two cystic fibrosis carriers have children, they have a 25 percent chance of producing offspring having two copies of the lethal allele, eventually resulting in the death of the child.