Nowadays, we know that all the genetic information is enclosed in the inner of our cells, specifically in the cell nucleus. The genetic information could be compared to a book, whose half of the chapters derive from the mother and the other half from the father. This book contains sentences- the chromosomes, which consist of words- the genes.
Human beings have 23 pairs of chromosomes in each somatic cell, which in total add up to 46 chromosomes. Half of these 46 chromosomes are of maternal origin and the other half are of paternal origin. Some chromosomes have a large number of genes, while other carry only a small number. Chromosomes consist of what we all have heard as “DNA”, which refers to the chemical structure of Deoxyribonucleic Acid. Chromosomes of the pairs 1-22 are called autosomes and are common for both sexes. The last pair consists of chromosomes X and Y, which are called sex chromosomes and are differently distributed between males and females: a male human has one Y and one X chromosome (symbolized XY), while a female human has two X chromosomes (symbolized XX).
The chromosomes that form a pair (homologous chromosomes) are identical in terms of having the same kind of genes at the same location. However, genes of the same location in the two homologous chromosomes may differ on the nature of their genetic information. These genes are called allelic (there is a gene that defines a characteristic, the allelic gene A results in phenotype A and the allelic gene B results in phenotype B. Both genes are in the same location on the homologous chromosomes and control the same characteristic, but in a different way).
Every human inherits one chromosome of each homologous pair from the mother and one from the father. Especially, regarding the sex chromosomes, all humans inherit one X chromosome from their mother, who has two X chromosomes (XX). The second sex chromosome is inherited by the father and could be either X or Y, as he has an X and an Y chromosome (XY). As a result, sex is defined by the Y chromosome and all females have as sex chromosomes a pair of XX, while all males have a pair of XY.
Chromosomal abnormalities are cases of irregularities in the structure or the number of the chromosomes. Therefore, there are structural or numerical chromosomal abnormalities. These abnormal situations are present when someone has one or more chromosomes with detriment. Situations in which there is a numerical chromosome abnormality are called aneuploidies and could be the result of surplus chromosomes (trisomy or polysomy) or on the opposite, due to lack of some of them (monosomy). The most characteristic case of trisomy- existence of one surplus chromosome- is Down syndrome. People with Down syndrome have one extra 21 chromosome, instead of just a pair of chromosomes 21, and for this reason the syndrome is also called trisomy 21. Regarding monosomies, the most characteristic case is Turner syndrome or X0. People showing Turner syndrome are women, who lack one X chromosome.
The embryos that result from an IVF cycle can be tested regarding their chromosomal status (aneuploid or euploid) after Pre-implantation Genetic Screening (PGS or as it is called nowadays PGT-A). The aim of PGS is to screen the embryos and replace only the healthy ones- avoiding failed embryotransfers and their emotional and financial cost. Additionally, pregnancies with embryos affected by a syndrome, can be avoided, as well.