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The genome is the set of genetic instructions that are found in a cell and allow biological life. In the case of humans, these instructions are stored in 23 pairs of chromosomes (23 maternal and 23 paternal), being the 23rd chromosome different regarding the gender of the individual (XX for women and XY for men).

The photo of those 23 chromosomes constitutes the human karyotype. Any change that alters the amount of genetic information will be reflected in karyotype changes and will trigger syndromes. Among these possible changes on the karyotype, we find trisomies. Trisomy is a genetic disorder in which a person has three copies of a chromosome instead of two.

Human karyotype without genetic alteration
Figure 1. Human karyotype without genetic alterations.

Most of the syndromes generated by a trisomy are the result of meiosis errors. Meiosis is a biological process in which sperm and egg cells are formed. Human beings are diploid individuals (two copies – 2n), which means that our cells are also diploid (2n). The only haploid cells (one copy - n) are the gametes (eggs and sperm) and thanks to meiosis, the cells are able to go from 2n to n.

For this reason, when meiosis is not carried out satisfactorily, trisomies are generated. We can say that they are the result of the union of a "normal" gamete (n) and a "defective" gamete (partially 2n) originating an individual with three copies on one of its chromosomes.  

Down syndrome is the best-known trisomy, but there are many others depending on which chromosome does not separate correctly in the process of meiosis. Let's take a look at some of them.

Down syndrome or trisomy 21

The Down's Syndrome is due to the existence of a third chromosome 21. This trisomy usually occurs due to a misseparation of chromosome 21 in the Ovum (female gamete). However, it is not the only reason that can trigger trisomy 21, but it is the most common.

Human karyotype with trisomy 21
Figure 2. Human karyotype with trisomy 21.

Down syndrome is usually accompanied by intellectual disability, muscular hypotonia, shortness in height, flat face, vision and hearing loss, cardiac complications... Its worldwide prevalence is 10 cases per 10,000 births.

Edwards syndrome or trisomy 18

Edwards syndrome is due to the existence of a third chromosome 18. In this case, meiosis can fail both in the generation of the egg cell (female gamete) and sperm cell (male gamete).

Human karyotype with trisomy 18
Figure 3. Human karyotype with trisomy 18

Those affected by this syndrome usually die before birth or shortly after birth, with an incidence of 1 in 7,000 births per year. The symptoms they present are usually hypotonia, low consciousness and reactivity to the environment, growth retardation, small and elongated head, heart defects and other malformed organs...

Patau syndrome or trisomy 13

Patau syndrome or trisomy 13 can be due to a failure in meiosis in any of the parental gametes. It is estimated to have an incidence of between 1/8,000 and 1/15,000 births in one year, although 95% of those diagnosed die before birth, and those that are born have a reduced life expectancy, usually dying from cardiorespiratory problems.

Human karyotype with trisomy 13
Figure 4. Human karyotype with trisomy 13

The symptoms of Patau syndrome include severe psychomotor retardation, a decrease in intelligence quotient, facial alterations, brain malformations, cardiac malformations...

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