Karyotype and chromosomal abnormalities: everything you need to know

Humans have the same karyotype consisting of 46 chromosomes: 22 pairs of autosomes (the same for men and women) and one sex chromosome pair. A person’s karyotype remains the same throughout his or her life.

While the egg contains 22 chromosomes and an X sex chromosome, the sperm consists of 22 chromosomes and an X or Y sex chromosome.

Each chromosome contains genes. When the egg and sperm cells are fused, their genetic material is combined to form a future human being with 46 chromosomes.

The sex of the unborn child depends on the type of sex chromosome that the sperm will bring. If fertilization of the egg occurs with a sperm that carries an X chromosome will result in a girl. The combination of the X and Y chromosomes will give a boy.

Chromosomes differ in size and internal structure, which makes it possible to distinguish one chromosome from another during the study.

65% of early pregnancy losses are due to a chromosomal pathology of the embryo (miscarriage, ectopic pregnancy.)

What are the chromosomal abnormalities?

Chromosomal abnormalities represent a change in the number or structure of chromosomes. There are several types of abnormalities:

• Trisomy, the presence of a supernumerary chromosome (trisomy 21 or Down syndrome).
• Monosomy, the absence of a chromosome: the embryo karyotype, 45 X0, is characterized by the absence of a sex chromosome (Turner Syndrome).
• Polyploidy is an accident of fertilization when a set of chromosomes doubles or triples: instead of 46 XX, the embryo can have 69 XXX. This is the case when an egg is fertilized by two sperm at the same time, or when dysfunction occurs at the very beginning of embryo division).
• Translocations, the exchange of genetic material between two non-homologous chromosomes. There can be balanced translocations (the person may not be aware of them) and unbalanced translocations (when the organism is not viable).
• Microdeletions, the loss of chromosome segments. Microdeletions cause specific genetic syndromes (in this case Lejeune’s syndrome, which is the deletion of part of chromosome 5).

When should a karyotype be carried out?

• In case of repeated miscarriages (2 or more spontaneous terminations of pregnancy up to the 22nd week).
• In case of unsuccessful attempts to transfer embryos without PGD excluding uterine/endometrial pathologies.
• In the case of primary amenorrhoea (absence of menstruation during life).
• If the anamnesis of the woman/man or their relatives includes cases of congenital chromosomal diseases.
• If the sperm concentration in the ejaculate is less than 10 million/ml.
• In cases of azoospermia (absence of sperm in the ejaculate).

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