PGD or pre-implantation genetic diagnosis is an assisted reproductive technique used to make sure that the embryo has not developed any genetic disorder or chromosomal alteration.
Chromosomes are located within the nucleus of the cells. They are made of DNA and contain the genes (each gene is a short piece of DNA). Human beings with no genetic alteration have 46 chromosomes, 23 from the mother and 23 from the father. Thus, alterations can be caused by genes from the mother, the father, or both.
Alterations in genes and chromosomes
There are two types of genetic disorders:
- Autosomal recessive: it is necessary that the gene alteration is found both in the maternal copy and the paternal copy to be affected by an autosomal recessive disorder.
- Autosomal dominant: only one mutated copy of the gene will be enough for a person to be affected by an autosomal dominant disorder.
Both types of genetic disorders can be detected with PGD.
There are two types of chromosomal alterations:
- Numerical alterations: in this case, the individual does not have 46 chromosomes but either is missing a chromosome from a pair or has more than two chromosomes of a pair. It usually causes death, except in the case of the Down’s syndrome –with three copies of chromosome 21–, the Klinefelter syndrome –there are three sex chromosomes: two X chromosomes and one Y chromosome–, and the Turner syndrome –there is only a single sex chromosome, an X chromosome–.
- Structural alterations: it occurs when the chromosome’s structure is altered. It is the same genetic information but wrongly distributed.
Clinics which perform PGD have a list of monogenic disorders -in which the disease is caused by an alteration or a mutation in the gen. Thus, they can be analyzed in order to see if some of them are on the embryo or not. The amount of chromosomes that the embryo has is also evaluated.
This way, if the parents present or are carriers of a genetic illness that can be detected through DNA catheters, it can be identified in the laboratory and then the pre-implantation genetic diagnosis can be used to detect it in the embryos. Those embryos that present the genetic alteration will be discarded, and only the embryos free of this mutation will be used for the embryo transference.
Thanks to this diagnosis technique, thousands of children free of genetic illnesses have been born throughout the world.