PGD most often refers to “single gene” diseases. Genes sit on chromosomes and are not identifiable through CCS/PGS. Genetic diseases, such as cystic fibrosis, thalassemia, Huntington’s disease, factor V Leiden can be identified if:
- There is a family history of such
- An ethnic predisposition for which screening was done (e.g. thalassemia in Asians) or
- A patient has independently opted to undergo whole genome sequencing and incidentally found to be a carrier of a genetic disease.
PGD is also done for known reciprocal translocations of chromosomes diagnosed by chromosome studies (karyotype). PGD can also avoid X-linked genetic diseases either by transferring a female XX) embryo in cases when a gene cannot be identified (e.g. severe autism) or identifying the single gene (e.g. fragile X gene) on the X chromosome.
In testing for a single gene disorder, the disorder is identified and a mutational analysis is completed of the gene for the individual patient, then a patient-unique probe is built to identify presence or absence of the abnormal gene in the embryo.
The steps for PGD are then identical to IVF and CCS (embryo biopsy, vitrification, and genetic testing) with similar risks. However, CCS (comprehensive chromosomal screening) can be done with PGD to increase the chances of both a chromosomally normal embryo and an embryo which is also unaffected by the abnormal gene identified being transferred with the highest pregnancy rates.