What is pre-implantation genetic diagnosis (PGD) and what is pre-implantation genetic screening (PGS)?

It would be useful to draw a distinction between pre-implantation genetic screening (PGS) and pre-implantation genetic diagnosis (PGD).

The term Pre-implantation Genetic Diagnosis refers to the distinction between genetically normal embryos, which are carriers of a disease and affected embryos , whose parents had a genetic disease. This could be:

1. A monogenic disease, namely a disease caused by a malfunctioning gene. The most common examples are thalassemia and cystic fibrosis.
2. A polygenic disease, which is attributed to a functional disorder of a group of genes.
3. A chromosome disorder (numerical-functional), namely a disease caused by the lack or surplus of a specific chromosome or a fault in its structure. Common numerical chromosome abnormalities include trisomies ( the most widely known is trisomy 21 or Down syndrome) and monosomies (e.g. X-chromosome monosomy or Turner syndrome). Structural chromosome abnormalities include deletions, duplications, translocations and inversions of portions of chromosomes.