RFLP and Genetic Screening

PCR Analysis of the Cystic Fibrosis Gene

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PCR Analysis of the Cystic Fibrosis Gene

Prenatal screening of the fetus has been used to determine if it is carrying any genetic diseases. The two procedures that provide tissue for analysis are amniocentesis, performed in the beginning of the second trimester, and chorionic villus, which is performed in the first trimester. Until recently these procedures have been used to analyze the whole chromosomes of the fetus to determine if any trisomic conditions exist. Recently, molecular markers have also been employed.

But for some diseases, such as cystic fibrosis, couples prefer to be able to ensure themselves that their child will be normal. This is especially important for two normal parents that have given birth to a child with the disease. This result implies that each are carriers, and therefore the probability that the next child will have the disease is 1/4.

Cystic fibrosis is caused by a 3 bp deletion that leads to a protein which lacks a critical phenylalanine amino acid in the protein. PCR primers have been developed that can distinguish a normal gene from a mutant gene. With these primers a 154 bp product is produced from a normal individual and a 151 bp product is amplified from DNA of an individual with the disease. The following types of patterns can appear after DNA is amplified from individuals with different cystic fibrosis genotypes.

         Normal     Carrier        CF
       Individual  Individual  Individual

154 bp    ___2x       ___

151 bp                ___         ___2x


This screening procedure was successfully used with one couple. Eggs from the female was fertilized in vitro. The embryos were then allowed to develop to the eight cell stage and a single cell was removed and its DNA analyzed by the PCR technique. The results were that two of the embryos were CF, one was a carrier and one was normal. The normal and carrier embryos were implanted into the female. A normal 7 lb., 3 oz. baby was born to the couple. These experiments demonstrate the power of modern techniques such as in vitro fertilization, implantation of embryos and PCR analysis.

Copyright © 1997. Phillip McClean