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Found within the nucleus of our cells, chromosomes are structures that determine our genetic makeup. Chromosomal rearrangement implies a change from the normal size or arrangement of chromosomes. Those with rearranged chromosomes are typically more likely to produce embryos without the correct amount of genetic material. In such individuals, the chances of a successful pregnancy can be low.
In cases where chromosomal rearrangements are suspected, PGT-SR or preimplantation genetic testing for chromosomal structural rearrangements is recommended. Formerly known as PGD, preimplantation genetic diagnosis, PGT-SR involves testing and transferring only normal embryos produced via IVF (In-Vitro fertilization). Thus the chances of a healthy pregnancy become higher.
It is believed that chromosome rearrangements are hereditary. However, it can also happen spontaneously. In fact, many carriers of chromosome rearrangements are completely unaware of their status until the time they try to have children. In such circumstances, PGT-SR comes as a life saviour to help identify healthy embryos that further develop into healthy babies.
At Anderson, we test for three primary types of chromosome rearrangements which include inversions, Robertsonian translocations and reciprocal translocations. Let us look at each of them individually to understand the conditions better.
In this type of chromosome rearrangement, a single chromosome is reversed and reinserted within itself.
This involves only one type of chromosome, and embryos created from these carriers may have missing or duplicated segments of chromosomes.
In this, two chromosomes fuse end to end to form one large chromosome. This translocation occurs only between chromosome numbers 13/14 and 14/21.
With this pairing, the overall chromosome count increases to 46 from 45 and results in conditions such as Trisomy 13, Uniparental Disomy (UPD), and Translocation Down syndrome.
While chromosomes can flip and fuse, they can also swap places. In people with reciprocal translocation, that's exactly what happens. Pieces of genetic material rip off from two different chromosomes and swap places. Embryos created by those with this chromosome rearrangement carry either a balanced or an unbalanced form of translocation or a completely normal set of chromosomes.
Unbalanced translocations often result in missing or extra copies of existing genes which can have serious clinical implications. When either of the parents is a carrier of reciprocal translocation, 80 percent of embryos produced do not contain the correct amount of genetic material.
