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21st March: world Down syndrome day

21st March is the World Down Syndrome Day. This syndrome is a chromosomal combination that has always been part of the human condition. It exists in all regions of the world, and usually it has variable effects on learning styles, physical characteristics or health. It affects 1 in 700 children, making it the most common chromosomal abnormality and the first cause of mental disability. With this article I want to introduce a little more this syndrome.


Its name comes from the English doctor John Langdon Down who described a group of patients with intellectual disabilities and similar physical characteristics, in 1866. These patients had Down syndrome.

However, already existed artworks with people with Down syndrome (Figure 1), but Langdon Down was the first one to group them in a subcategory within individual with cognitive impairment.

quadre oli
Figure 1. “The Adoration of the Christ Child” (1515). This oil painting, made by a follower of Jan Joest van Kalkar, shows two people with Down syndrome (Source: Arte y síndrome de Down)

It is called syndrome because the affected people express a known set of symptoms or signs that they may appear together, although its origin is unknown. Even though physical features are common, each person with Down syndrome is a unique individual and can present the characteristics in different degrees or not.


  • Diminished muscle tone
  • Small ears
  • Slanting eyes
  • Short nose
  • Flat back of head
  • Single crease in the palm of the hand: simian crease: complete fusion between heart line and headline (Figure 2)
  • Tendency to obesity
Figure 2. (1) Common lines, like M, and (2) simian crease, complete fusion between heart line and headline (Source: Incidencia de nacimientos pretérmino y de término con peso bajo al nacer y existencia de línea Sydney)

When they are children present retardation in reaching capabilities as sitting independently, wandering, first words…


In 1959, Jérôme Lejeune, a French doctor, saw that people with Down syndrome had 47 chromosomes in each cell instead of 46. This extra chromosome was 21 (Figure 3). The article  Why I look similar to my parents? reminds us what a chromosome is.

Figure 3. Male karyotype, person with Down syndrome (Source: Mireia Ramos, Cerba Internacional SAE)

So, Down syndrome or trisomy 21, is a result of an extra chromosome. But having and extra copy of chromosome 21 can be given by three phenomena.


It is the major cause and represents 95% of cases. It is produced by an error in the process of cell division. It means that when parent’s cell divides there is an error, and the son inherits two copies of chromosome 21 instead of one.

Then the son has 3 chromosomes 21: 1 comes from one parent and 2 come from the other parent, which are transmitted together.


During the process of cell division of one parent, a chromosome 21 joins with other chromosome, usually a chromosome 14.

Then the son has 3 chromosomes 21: one comes from one parent and two come from the other parent.

It represents 4% of cases, and it is important to identify it to avoid passing the translocation to another child, if the couple wants another child.


It is the least common cause because it represents 1% of cases. After fertilization nondisjunction occurs, but not in all cells. This causes cells with 46 chromosomes and cells with 47, forming a mosaic.

Cells with 47 chromosomes have an extra chromosome 21.


It has been found that the age of the mother is related to have a child with Down syndrome, i.e., the risk of having a baby with Down syndrome is greater among mother age 35 and older.

Trisomy 21 is the most trisomy accepted by nature, so in pregnancy test doctors always study it. If they detect a foetus has Down syndrome, the couple can choose to go ahead or to stop pregnancy.

People with Down syndrome are increasingly integrated into our society. Their IQ is 45-48, when the standard range is around 100, but with a special school integration support is highly beneficial and IQ can go up to 70.

Nowadays, more and more companies offering workplaces for them and this should not surprise us, because after all they only have an extra chromosome (Figure 4).

Figure 4. Keep calm it’s only and extra chromosome (Source: Pinterest)




“I am your father… or not”

Do you remember what Darth Vader told to Luke Skywalker? Well, in Spain, more 25% of cases about paternity tests cannot tell it. As the saying goes: “children of my daughter, my grandchildren will be; but children of my son, I don’t know”. In this article I am going to talk about paternity tests and their evolution.


The objective of paternity tests is prove whether two individuals are biologically parent and child.

There are many reasons to want to do a paternity test (Figure 1), either for personal purposes as legal, such as causes of filiation (to know who is your father), challenging results, unrecognized children…

Figure 1. Doubts about paternity (Source: Infogen)

Time ago women claim these test, but nowadays there are many men who request them to avoid pay maintenance after a divorce or infidelity.


The argument about who is the father of an individual is not recent, we find paternity disputes record on time of Roman Empire.

The methods to solve these disputes have changed. Firstly, the tests were bases on the physical appearance. It is true that behind the resemblances there is inheritance, but the ignorance of the transmission mechanism little objectivity when comparing them were useless.

 At 20s, blood groups allowed exclude paternity. It means that if the child is blood type B, the father may not be A if mother is 0, as shown in Table 1. The blood group of the mother will do the blood group of the child, because we believe that the mother has given birth to the child and is therefore not questioned her motherhood.

taula grups sanguinis
Table 1. Determining blood groups of children according blood group of parents (Source: Mireia Ramos, All You Need is Biology)

Although blood group could exclude paternity, courts did not admit as an evidence time ago. It is what happened to Charlie Chaplin when Joan Barry accused him to be the father of her child. Blood test excluded Chaplin as a father because his blood type was different, but the court did not accept this test. Finally, Chaplin paid the maintenance of Barry’s child. This case promoted passing new laws and forensic tests.

chaplin judici
Figure 2. Trial of Chaplin and Barry (Source: GettyImages)

Nowadays paternity tests are based in a genetic study, with the objective to allow know if there is genetic relationship between father and child by the similarity that must exist between the two samples. So, it allows to confirm or to refuse paternity. But these test not only use to validate that legal father is biological father too.


The identification of individuals based on the study of DNA can be made from any biological sample (blood, buccal mucosa, hair, urine, teeth or even degraded organic material). This is the reason why this analysis is not only useful for the study of paternity and other kinship, but also used in forensic studies, historical research and anthropological studies.

DNA testing is done by analysing genome sequences highly variable, i.e., between individuals of a population may have different forms called alleles. What we analyse to determine paternity are what we call genetic polymorphisms, namely, that the people there are two or more ways (allelic variants) for a gene. These polymorphic variants are inherited and, therefore, the son will present a combination of certain polymorphisms that will be inherited from their parents.

In paternity we analyse repeat polymorphism, i.e., varies the number of times we have repeated the sequence. This sequence has an identifiable physical location and it calls genetic marker.

For each marker a person has two alleles, one comes from the mother and the other comes from the father. This combination of alleles that we receive from our parents is genotype.

To understand this better let’s look at Figure 3. The numbers under each individual display their genotype. This genotype is composed of two alleles, each allele indicates the number of repeats polymorphism.

Figure 3. Difference in the interpretation of a paternity test, where the supposed father and the child coincide (case A), respect a paternity exclusion (case B) (Source: DNAProfile)

Once we have the results for different markers analysed we proceed, finally, to determine paternity or not. If the supposed father is consistent with the child must do a study biostatistician to estimate the probability of paternity, namely, look how many men could randomly matched with the child. But if the supposed father and the child do not coincide we establish an exclusion, and the result is unquestionable.


  • Lorente J.A., Lorente M. El ADN y la identificación en la investigación criminal y en la paternidad biológica. Editorial Comares
  • E. Huguet, Á. Carracedo, M. Gené. Introducción a la investigación biològica de la paternidad. Promociones y publicaciones universitarias 1988
  • Infogen
  • Pruebas de paternidad: información básica
  • DNAProfile