PGD for abnormalities in chromosome number

Recently, the ASRM and SART have advocated the use of the term PGS or preimplantation genetic screening instead of PGD screening to be used in cases when the genetic parents are known or presumed to be chromosomally normal and their embryos are screened for aneuploidy .

Chromosomes in normal embryos

Sperm and eggs (oocytes) should normally each contain 23 chromosomes. This is called the haploid state. The embryos created by human beings during the union of the sperm and the egg are supposed to have 46 total chromosomes. This is called the diploid state. The chromosomes are arranged in 23 pairs. They are number 1 through 22. The 23rd pair are not given a number, these are the sex chromosomes.

Embryos with abnormal chromosome numbers

Occasionally, an embryo can be created that does not have 23 pairs of chromosomes. There are a few different types of abnormalities that can occur.

Aneuploidy

Aneuploidy is when an abnormality occurs only on a single chromosome pair. For example, a common type of aneuploidy is the presence of three copies of chromosome #21, or trisomy 21. The embryos have a total of 47 chromosomes instead of the correct 46.

 

This particular trisomy can result in a live born child. A child with this abnormality is said to have Down’s Syndrome. If there is only one copy of a chromosome pair present in the embryo it is called a monosomy. The most commmon example of monosomy is an embryo which has only one X chromosome. The total number of chromosomes present is 45 and you can see it written like this: 45,XO (the “O” represents the missing chromosome). A 45,XO embryo will often miscarry but can also result in a live born child. A child born with this abnormality is always a female and is said to have Turner’s Syndrome.

Polyploidy

Less commonly, an embryo can contain an entire extras set of chromosomes. Instead of 46 chromosomes (2 copies of all 23 chromosomes), the embryo many contain 69 chromosomes (3 copies of all 23 chromosomes) This is called triploidy. There can also be tetraploidy for four sets and so on. The picture to the right shows triploidy since there are three copies of every chromosome.

 

Preimplantation genetic diagnosis (PGD) can be used to determine how many copies of a particular chromosome are present in an embryo. There are several different laboratory techniques that can be used to accomplish this such as NGS (Next Generation Sequencing), aCGH (CGH microarray) or older techniques like FISH (Flourescent In Situ Hybridization.