Down syndrome trisomy and sex chromosomes

Let’s know about Down syndrome trisomy and sex chromosomes. A trisomy It is the chromosomal condition of individuals who carry three chromosomes instead of the normal pair. In diploids, the common envelope consists of two chromosomes, each of which is complementary to the chromosome that defines the species.

Down syndrome trisomy and sex chromosomes
Down syndrome trisomy and sex chromosomes

A change in number in only one of the chromosomes is called aneuploidy. It would be a true effect, or euphemism, if it involved changes in the total number of chromosomes. Humans have 46 chromosomes in each somatic cell. If they have a single trisomy, they will have 47 chromosomes.

Trisomies are statistically consistent and represent great changes for the carrier organism. They can also occur in humans, and are associated with complex diseases or syndromes.

Trisomies in humans are known by all. The most commonly present, frequent and known to all is trisomy of chromosome 21, the most common cause of the so-called Down syndrome.

There are other trisomies in humans that represent a great physiological cost to the carrier. Among them we have trisomy of the X chromosome, which represents a great challenge for the afflicted woman.

Any living eukaryotic organism can present a trisomy. In general, any change in the number of chromosomes (aneuploidy) in plants is much more harmful than an increase in the number of chromosomes of the species. In other animals, as a general rule, aneuploidies are also the cause of many conditions.

Trisomy of chromosome 21 (Down syndrome: 47, 13:)

The complete (trisomy) of chromosome 21 in humans determines the presence of 47 chromosomes in diploid somatic cells. Twenty-two pairs provide 44 chromosomes, while the trio of chromosome 21 provides three more – one of which is extraterrestrial. That is, it is a chromosome that is “depleted”.

Description and a little history

Trisomy of chromosome 21 is the most common disease in humans. Similarly, this trisomy is also the most frequent cause of Down syndrome. However, in other somatic triads more frequent than in chromosome 21, they are usually more lethal in the embryonic stages.

That is, fetuses with trisomy 21 may arrive at birth, while other trisomic fetuses may not. In addition, children with trisomy 21 on chromosome 21 have a much higher postnatal survival due to the low gene frequency of this chromosome. In other words, there are some genes that will increase in copy number because chromosome 21 is the smallest autosome.

Down syndrome was first described by the English physician John Langdon Down in the period 1862 to 1866. However, the association of the disease with chromosome 21 was established a hundred years later. French researchers Marth Gautier, Raymond Turpin and Jerome Lejeune participated in these studies.

genetic causes of disease

The triad of chromosome 21 is caused by the union of two gametes, one of which is a carrier of more than one, total or partial, of chromosome 21. There are three ways in which this can happen.

In the first one, one of the parents, non-disjunction of chromosome 21 during meiosis gives rise to gametes with two chromosome 21s instead of one. Non-dissociation means “separation or lack of separation”. This is the gamete that can give rise to true trisomy by adding another gamete with a copy of chromosome 21.

There is another less frequent cause of this trisomy called a Robertsonian translation. In this, the long arm of chromosome 21 is changed to another chromosome (usually 14). The union of one of these gametes with another normal will give rise to an embryo with a normal karyotype.

However, there will be extra copies of the hereditary material of chromosome 21, which is sufficient to cause disease. The syndrome can also be caused by other chromosomal aberrations or mosaicism.

In a mosaic, the individual presents cells with normal karyotype, alternating with cells with dissimilar karyotype (trisomic for chromosome 21).

manifestation of disease

The underlying cause of Down syndrome is the increased expression of certain enzymes due to the existence of three copies of the gene on chromosome 21 instead of two.

This increased expression leads to changes in the normal physiology of the individual. Some of the enzymes so affected include superoxide dismutase and beta-synthase cysts. Many others are related to the synthesis of DNA, primary metabolism and cognitive abilities of the individual.

The disease manifests itself at different levels. Congenital heart defects are some of the most important that determine the life span of people affected by trisomy.

Other conditions that afflict sick individuals include abnormalities of the gastrointestinal tract, hematological, endocrine, otorhinolaryngological and musculoskeletal disorders, as well as visual abnormalities.

Neurological disorders are also significant, and include mild to moderate learning difficulties. Most adult individuals with Down syndrome develop Alzheimer’s disease.

Diagnostics and studies in other living systems

Prenatal diagnosis of Down’s can be done in several ways. This includes ultrasound, as well as sampling of chorionic hairs and amniocentesis. Both can be used for chromosome counting, but this represents some risk.

Other more modern assays include chromosome analysis by FISH, other immunological techniques based on DNA amplification by PCR, and genetic polymorphism testing.

The study of chromosome 21 trisomy in rodent systems has allowed us to analyze the syndrome without experimenting with humans. In this way, genotype/phenotype relationships are analyzed safely and reliably.

In the same way, it has become possible to proceed to test strategies and therapeutic agents that can later be used in humans. The most successful rodent model for these studies has turned out to be the mouse.

Trisomy of sex chromosomes in humans

In general, assisted sex chromosomes have fewer medical consequences than autosomes in humans. The female of the human species is XX, and the male is XY.

The most common sexual triads in humans are XXX, XXY and XYY. Obviously, there cannot be person YY, let alone YYY. XXX individuals are morphologically female, while XXY and XYY are male.

Triple X Syndrome (47, 2016)

In humans XXX individuals are females with an extra X chromosome. The phenotype associated with the condition varies with age, but in general adult individuals have a normal phenotype.

Statistically, one out of every thousand women are XXX. A phenotypic trait common in XXX females is premature growth and development, and an unusually long lower extremity.

At other levels, XXX women often have hearing or language development disorders. At the end of adolescence, they usually overcome problems associated with their social adaptation, and improve their quality of life. However, psychosis occurs more frequently in women than in age 50.

In females, one of the X chromosomes is inactivated during the normal development of the individual. It is believed that XXX women disable two of them. However, it is believed that most of the results resulting from trisomy are due to genetic imbalance.

This means that such inactivation is not efficient or sufficient to avoid differences in the expression of some (or all) genes. It is one of the most studied aspects of the disease from a molecular point of view.

In the case of other trisomies, prenatal identification of triple X trisomy is still based on karyotype studies.

Kleinfelter syndrome (47, XXY)

It is said that these individuals are males of the species with an extra X chromosome. The signs of aneuploidy vary with the age of the individual, and are usually only a condition diagnosed when they are adults.

This means that it does not affect sexual dysfunction because of the great effects caused by trisomies in autosomal chromosomes.

XXY adult males produce little or no sperm, testicles and small penis and have decreased libido. They are longer than average, but have less facial and body hair.

They may have enlarged breasts (gynecomastia), decreased muscle mass, and weak bones. The administration of testosterone is usually helpful in treating certain endocrinological aspects related to the condition.

XYY Syndrome (47, XYY)

This syndrome is experienced by males of the human species (XY) who have an extra Y chromosome. The consequences of the presence of an extra Y chromosome are not as dramatic as those seen in other trisomies.

XYY individuals are phenotypically male, regularly tall, and with slightly elongated limbs. They produce normal amounts of testosterone and do not present special behavior or learning problems as thought in the past.

Many XYY individuals are unaware of their chromosomal status. They are phenotypically normal, and moreover, fertile.

Tridosha in other organisms

The effect of aneuploidies on plants has been analyzed, and compared to the effect of euploidy change. In general, changes in the numbers in one or fewer chromosomes are more harmful to the normal functioning of the individual than changes in the full set of chromosomes.

As in the cases described, an imbalance in expression seems to be responsible for the detrimental effects of the differences.