Additional PGD Techniques
Polymerase chain reaction (PCR)
If a suspected hereditary problem is at the gene rather than chromosomal level, PCR is commonly used. PCR makes millions of copies of a DNA code segment, which allows scientists to assess whether this segment is normal or mutant. First, a primer or copy is made to a specific conserved region of the target chromosome. For gender selection, the Y chromosome is targeted. The primer is a perfect match for its target region, like a lock and key. If the primer finds its target region, PCR then vastly amplifies the region. If the amplified DNA is detected, the target region (and the target chromosome) is present. If the amplified DNA is not detected, the target chromosome is not present.
The drawback: 25% of cases, the segment fails to amplify even when it’s present (called allele dropout). This severely limits PCR’s effectiveness in gender selection. PCR is used primarily to detect mutations that cause single-gene disorders such as cystic fibrosis.
Single nucleotide polymorphismanalys (SNP) technique
SNP allows rapid examination of all 23 pairs of chromosomes and single-gene disorders with a very high degree of accuracy. Developed using data from the Human Genome Project and from computer microchip technology, this technique amplifies numerous small sequences on each chromosome with PCR and analyzes them to determine the presence of each and the number of copies.
It's also used to identify mutations that may cause single-gene disorders.
Comparative genomic hybridization (CGH)
CGH holds great promise. Originally used to detect abnormalities in cancer cells, CGH has been used to analyze embryonic cells. CGH can very accurately determine the number of copies of the 23 chromosomes. The downside: CGH takes about 1 week to complete, thus the embryos must be frozen while the analysis is performed. Because frozen embryos have lower success rates than fresh embryos, CGH’s usefulness for PGD and sex selection is low.