<prompt> In a study of inheritance of the cystic fibrosis gene CFTR, the genetic mutations in carriers and affected individuals are documented. Based on these findings, investigators determine that there is no simple screening test to detect all carriers of mutations of the CFTR gene. Which of the following is most likely to be the greatest limitation to development of a screening test for CFTR mutations?Both copies of the gene must be abnormal for detectionFluorescence in situ hybridization is labor-intensive and expensiveFrequency of mutations among ethnic groups limits sensitivityLess than 1 individual in 10,000 is a heterozygote  <response>: When a genetic disease e.g., cystic fibrosis is caused by many different mutations, with different frequencies among populations, there is no simple screening test that can detect all the mutations. Although 70 of patients with cystic fibrosis have a 3-base pair deletion that can be readily detected by PCR the AF508 mutation, the remaining 30 have disease caused by several hundred allelic forms of CFTR. To detect all would require sequencing of the CFTR genes. This prohibits mass screening. The other listed options do not apply. 