Chromosome analysis is often performed on parents who have a history of miscarriages or a child with a chromosome abnormality. Sometimes, one partner is found to be a carrier of a chromosome translocation. A chromosome translocation is the result of pieces of two chromosomes that have broken off and switched places; it is “balanced” because there is no extra or missing chromosome material.
All of the chromosome material is still present in an individual with a balanced translocation. A carrier, however, has a significant increased risk of repeated miscarriages or a child with a serious chromosome problem. The goal of preimplantation genetic diagnosis (PGD) for chromosome translocation carriers is to reduce the number of pregnancy losses and reduce the number of affected offspring.
PGD Procedure for Chromosome Translocation
Couples electing preimplantation genetic diagnosis undergo an in vitro fertilization (IVF) cycle to create embryos. Genetic analysis is then performed on cells from each embryo prior to transfer into the woman’s uterus. The embryo is biopsied on the third day of its development when the embryo has approximately 6-8 cells. One or two cells are taken from the embryo. The embryo is incubated until testing is complete.
The biopsied cells are analyzed using a technique called fluorescence in situ hybridization (FISH). FISH analysis uses DNA probes that attach to the chromosomes being tested in order to count the chromosomes. Each probe is labeled with a different fluorescent dye (color). The fluorescent probes are applied to the biopsied cell and will attach to specific chromosomes. Under a fluorescent microscope the number of chromosomes of each type (color) can be determined.
The FISH analysis involves two rounds of testing for each cell. For couples undergoing IVF and PGD for translocations, the embryos are first tested for unbalanced translocations. If possible, a second test is completed for chromosomes 13, 18, 21, X, and Y which are those chromosomes most likely to result in a liveborn child with a chromosome abnormality.
Using this procedure can identify appropriate embryos for transfer. Embryos that are found to have an unbalanced chromosome abnormality or aneuploidy are not transferred. The gender of embryos is not revealed.