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Embryologist's tips – 3: Embryo genetics

As we already know, embryo quality can be rated combining three indicators – morphology, kinetics and genetics. So, let's repeat: the ability of an embryo to be implanted by 25% depends on its morphology, 25% on kinetics and 50% on genetics. Finally, we'll talk about embryo genetics today.
Our molecular geneticists has prepared an explanation, what is PGT-A (PGS according to the old classification).
PGT-A (PGS) is a method for testing embryos, which allows to detect changes in the number of chromosomes before implantation. With the help of this test, it is possible to select genetically healthy embryos to increase the chance of successful implantation and reduce the risk of spontaneous abortion and fetal chromosomal abnormalities.
Changes in the number of chromosomes can occur spontaneously during fertilization or embryo development. It could be both changes of one chromosome - monosomy (one chromosome instead of two), trisomy (three chromosomes instead of two), and polyploidy (69 chromosomes instead of 46).
Figure 1. Aneuploidy examples obtained using NGS (Next generation sequencing).
(Zheng, H., Jin, H., Liu, L. et al. Mol Cytogenet (2015) 8: 38.)

It is possible to detect chromosomal changes using PGT-A also in cases when one or both parents are carriers of balanced translocations and there is a possibility that a gamete with an incorrect chromosome set (with structural chromosomal changes) took part in the process of fertilization.
Figure 2. Unbalanced translocations examples obtained using NGS ((Next generation sequencing).
(Zheng, H., Jin, H., Liu, L. et al. Mol Cytogenet (2015) 8: 38.)

A group of molecular geneticists and geneticists assess the results, carefully analyze and check detected changes and give their interpretation.
The only limitations for the analysis are 69XXX detection, which is not distinguishable from normal 46XX; it is impossible to detect small chromosomal deletions not exceeding 20 MB; it is impossible to determine the risk of uniparental disomy.

 


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