Trisomy: Definition, History, Description, Production, Terminology and Trisomic Analysis

It is a type of polysomy in which there are three instances of a particular chromosome, rather than the normal two.

A trisomy is a type of aneuploidy (an abnormal number of chromosomes).

Other cases of trisomy are also known in various species of different plants and animals.

Trisomy is not an inherited genetic disorder indicated by family history. In fact, it is always a completely new abnormality caused by improper ripening. The risk of trisomy increases with the age of the mother.

Chromosome number abnormalities are caused by random errors in cell division caused by rapid development and intense cell division in the early stages of fertilization.

In some cases, an extra copy of a chromosome is created, which means that there are three instances of a particular chromosome instead of the normal two. This phenomenon is called trisomy, an abnormality that has an extremely serious impact on the later development of the unborn baby.


Trisomy of a chromosome often leads to the induction of a specific phenotype for the chromosome in question.

In a series of classical studies carried out between 1923 and 1931, Blakeslee demonstrated specific phenotypic effects associated with the trisomy of each type of chromosome in Datura stramonium, Jimson’s herb.

The number of haploid chromosomes in Jimson’s weed diploid weed is 12 and Blakeslee observed 12 different characteristic phenotypes expressed as changes in the overall size and shape of the seed capsules and the structure of the hooks carried on their surface.

Furthermore, Blakeslee observed that the phenotypic changes induced by trisomy were significantly greater than those caused by the induction of triploidy or tetraploidy and concluded that the genetic imbalance of trisomy was the main cause of the phenotypic changes.

Description and causes

Most organisms that reproduce sexually have pairs of chromosomes in each cell, with one chromosome inherited from each parent.

In these organisms, a process called meiosis creates cells called gametes (eggs or sperm) that have only one set of chromosomes. The number of chromosomes is different for different species.

Humans have 46 chromosomes (that is, 23 pairs of chromosomes). Human gametes only have 23 chromosomes.

If the chromosome pairs do not separate properly during cell division, the egg or sperm may end up with a second copy of one of the chromosomes.

If such a gamete results in fertilization and an embryo, the resulting embryo may also have a complete copy of the extra chromosome.

Trisomy production

It can arise spontaneously due to the production of n + 1 type of gametes due to the rare nondisjunction of a bivalent.

However, more frequently, trisomy occurs artificially either by selfing triploids (produced by crossing diploids and autotetraploids) or by crossing these Iriploids as females with diploids as males (3x x 2x).

In either case, trisomy is obtained in large numbers and can be identified through the phenotypic effects of individual chromosomes.


The number of chromosomes in the cell where trisomy occurs is represented as, for example, 2n + 1 if one chromosome shows trisomy, 2n + 1 + 1 if two show trisomy, etc.

“Complete trisomy” : Also called “primary trisomy,” it means that an entire extra chromosome has been copied.

In this type of trisomy, the extra chromosome is normal and completely homologous to a pair of homologues in the chromosomal complement. Each chromosome has a separate effect on the phenotype and therefore trisomy can be identified for different chromosomes.

“Partial trisomy” means that there is an extra copy of a part of a chromosome.

“Secondary trisomy” : the additional chromosome has quadrupled arms (the arms are identical, it is an “isochromosome”).

“Tertiary trisomy” : This is a rare occurrence, the extra chromosome is made up of arm copies of two other chromosomes.

After fertilization with a normal gamete, this extra chromosome gives rise to the tertiary trisomy of the embryo.

A trisomic has an extra chromosome that is homologous to one of the complement chromosomes. Therefore, it forms a trivalent. This trivalent can have a variety of forms in primary and secondary trisomy. In a tertiary trisomic a characteristic pentavalent is observed.

Trisomies are sometimes characterized as “autosomal trisomies” (trisomies of the non-sex chromosomes) and “chromosomal sex trisomies.” Autosomal trisomies are described by referring to the specific chromosome that has an extra copy.

Thus, for example, the presence of an extra chromosome 21, found in Down syndrome, is called trisomy 21.

Human trisomy

Trisomies can occur with any chromosome, but they often result in miscarriage, rather than live birth.

For example, trisomy 16 is the most common trisomy in human pregnancies, occurring in more than 1% of pregnancies; only pregnancies in which a few normal cells are produced in addition to trisomic cells, or mosaic trisomy 16, survive.

This condition, however, generally results in miscarriage in the first trimester. The most common types of autosomal trisomy that survive to birth in humans are:

Down syndrome – trisomy 21

Down syndrome is caused by trisomy 21, a disorder that results from the presence of an extra chromosome 21.

Only two-thirds of Down syndrome pregnancies will end in normal delivery. About 30% of pregnancies will end in miscarriage.

This disorder has a serious impact not only on the growth and general well-being of the baby, but also on the shape of his body. It is characterized by different facial features and different levels of psychological and mental dysfunction.

The most common complications include immune and circulatory disorders or gastrointestinal disorders. Children with trisomy 21 require special medical attention depending on the degree of their disability.

In some cases, the symptoms of Down syndrome can be mild and the patient can enjoy a relatively long life.

Edwards syndrome – trisomy 18

Edwards syndrome occurs as a result of an extra chromosome 18.

The consequences of this chromosomal abnormality are serious: the baby is born with a low birth weight, an abnormally shaped head, a small jaw, a small mouth, often with a cleft lip or palate.

In addition to breathing and feeding problems, the baby is also prone to heart disease. The prognosis is very unfavorable.

Edwards syndrome pregnancies are accompanied by a high risk of miscarriage, and most live-born children do not live beyond one year.

Patau syndrome – trisomy 13

Trisomy on chromosome 13 is called Patau syndrome. Trisomy 13 is a serious genetic disorder that can affect all organs, including the brain, heart, and kidneys. These children are sometimes born with a cleft palate or deformed limbs.

People with this congenital disorder have a very small chance of survival. Patau syndrome pregnancies are characterized by a high risk of miscarriage or stillbirths.

Trisomy 9

Trisomy 9 is a rare and often fatal chromosomal abnormality that occurs in about 2.5 percent of miscarriages, pregnancy losses that occur before the 20th week of pregnancy.

Similar to trisomy 21, trisomy 9 occurs when there are three copies of chromosome 9 present in a baby’s cells, as opposed to two copies.

Unlike trisomy 21, however, trisomy 9 is rarer, leads to more severe manifestations, and has a much lower survival rate.

Complete trisomy 9 is almost always fatal, with the vast majority dying in the first trimester. While most live-born babies have mosaic trisomy 9, many die in infancy from health problems caused by the disorder.

That said, there are some who survive beyond the first year of life.

Partial trisomy 9 may not affect the life expectancy of the baby, but affected babies can have a variety of common health and developmental problems.

The signs and symptoms present in trisomy 9 are variable. Common findings on ultrasound include fetal heart defects and malformations of the brain and spinal cord.

Trisomy 8 (Warkany syndrome 2)

Trisomy 8, also known as Warkany 2 syndrome, is a human chromosomal disorder caused by having three copies of chromosome 8. It can appear with or without mosaicism. Complete trisomy 8 causes serious effects on the development of the fetus and can be a cause of miscarriage.

Complete trisomy 8 is usually an early life-threatening condition, while trisomy 8 mosaicism is less severe, and individuals with a low proportion of affected cells may exhibit a comparatively mild range of physical abnormalities and developmental delay.

People with trisomy 8 mosaicism are more likely to survive into childhood and adulthood, and they exhibit a distinctive and recognizable pattern of developmental abnormalities.

Common findings include delayed psychomotor development, moderate to severe mental retardation, variable growth patterns that can result in abnormally short or tall stature, a blank face, and many musculoskeletal, visceral, and ocular abnormalities, as well as other abnormalities.

A deep plantar sulcus is considered pathognomonic for this condition, especially when seen in combination with other associated features.

The type and severity of symptoms depend on the location and ratio of trisomy 8 cells compared to normal cells.

Trisomy 8 mosaicism affects large areas of chromosome 8 that contain many genes and therefore can be associated with a variety of symptoms.

Mosaic trisomy 8 has been reported in rare cases of Rothmund-Thomson syndrome, a genetic disorder associated with RECQL4 DNA helicase on chromosome 8q24.3.

The syndrome is “characterized by skin atrophy, telangiectasia, hyper- and hypopigmentation, congenital skeletal abnormalities, short stature, premature aging, and increased risk of malignancy.”

Some individuals trisomic for chromosome 8 were deficient in coagulation factor VII production due to a factor 7 regulation gene (F7R) mapped to 8p23.3-p23.1.

Trisomy and other rearrangements of chromosome 8 have also been found in trich-rhino-phalangeal syndrome.

Small regions of chromosome 8 trisomy and monosomy are also created by recombinant chromosome 8 syndrome (San Luis Valley syndrome), causing abnormalities associated with tetralogy of Fallot, which results from recombination between a typical chromosome 8 and one carrying a parental paracentric inversion.

Trisomy is also found in some cases of chronic myeloid leukemia, possibly as a result of karyotypic instability caused by the bcr: abl fusion gene.

Of these, Trisomy 21 and Trisomy 18 are the most common. In rare cases, a fetus with trisomy 13 can survive, leading to Patau syndrome. Autosomal trisomy can be associated with birth defects, intellectual disability, and shortened life expectancy.

Trisomy of the sex chromosomes can also occur and include:

XXX (Triple X syndrome)

Triple X syndrome is a genetic condition found only in women. About 1 in 1,000 girls have it.

Girls with triple X syndrome, also known as XXX syndrome, trisomy X, and 47, XXX, may be taller than other girls. Other symptoms can include problems with spoken language and spoken word processing, coordination problems, and weaker muscles.

Most girls with Triple X syndrome can grow up healthy, have normal sexual development and fertility, and lead productive lives.

Depending on what symptoms a girl has and how severe they are, doctors may recommend various treatments.

Girls who have triple X syndrome are born with it. It is called triple X because they have an extra X chromosome in most or all of their cells.

Generally, a person has 46 chromosomes in each cell, divided into 23 pairs, which includes two sex chromosomes. Half of the chromosomes are inherited from the father and the other half from the mother.

Chromosomes contain genes that determine the characteristics of an individual, such as eye color and height. Girls typically have two X (or XX) chromosomes, but girls with triple X syndrome have one extra X (XXX) chromosome.

Triple X syndrome is not caused by anything the parents did or did not do. The disorder is a random error in cell division. This error can occur before conception in the reproductive cells of the mother or father, or early in the development of the embryo.

When the extra chromosome is due to improper cell division in the embryo, a girl may have a mosaic of triple X syndrome. This means that some of the cells have an extra X chromosome, but not all do. Girls with this type of triple X syndrome generally have fewer symptoms.

Girls with triple X syndrome can develop anxiety, depression, and attention deficit hyperactivity disorder. These problems can be alleviated as they age and reach adulthood. Otherwise, treatment can help control them.

Less often, girls may have abnormal development of the ovaries and / or uterus, delayed onset of puberty, and problems with fertility.

Rarely, a girl may develop heart and kidney problems, frequent urinary tract infections, stomach pain, constipation, flat feet, and an abnormally shaped chest wall and rib cage (called pectus excavatum).

XXY (síndrome de Klinefelter)

Klinefelter syndrome (KS) also known as 47, XXY or XXY, is the set of symptoms that result from two or more X chromosomes in men. The main characteristics are sterility and small testicles.

Often the symptoms can be subtle and many people do not realize they are affected. Sometimes symptoms are more prominent and can include weaker muscles, taller height, poor coordination, less body hair, breast growth, and less interest in sex.

Often only at puberty are these symptoms noticed. Intelligence is generally normal; however, reading difficulties and speech problems are more common. Symptoms are usually more severe if there are three or more X chromosomes (XXXY or 49, XXXXY syndrome).

Klinefelter syndrome usually occurs randomly. An older mother may have a slightly higher risk of a child with Klinefelter syndrome. The condition is not usually inherited from the parents.

The underlying mechanisms involve at least one additional X chromosome in addition to a Y chromosome such that the total number of chromosomes is 47 or more instead of the usual 46. Klinefelter syndrome is diagnosed by a genetic test known as a karyotype.

While there is no known cure, a number of treatments can help. Physical therapy, speech and language therapy, counseling, and adjustments to teaching methods can be helpful.

Testosterone replacement can be used in those who have significantly lower levels. Enlarged breasts can be removed by surgery.

About half of affected men have a chance to father children with the help of assisted reproductive technology, but this is expensive and not without risks.

Men appear to have a higher risk of breast cancer than typical, but still lower than that of women. People with this condition have a near normal life expectancy.

Klinefelter syndrome is one of the most common chromosomal disorders, occurring in one to two per 1,000 live male births. It is named after Harry Klinefelter, who identified the condition in the 1940s.

In 1956, the identification of the extra X chromosome was first noticed. Mice can also have XXY syndrome, making them a useful research model.


XYY syndrome is a genetic condition in which a man has an extra Y chromosome. Symptoms are generally few. They can include being taller than average, acne, and an increased risk of learning disabilities. The person is generally normal, including normal fertility.

Generally, the condition is not inherited from a person’s parents, but occurs as a result of a random event during sperm cell development. Diagnosis is by chromosomal analysis. There are 47 chromosomes, instead of the usual 46, giving a 47, XYY karyotype.

Treatment may include speech therapy or extra help with homework. The results are generally good. Prevention is not possible.

The condition occurs in about 1 in every 1,000 male births. Many people with the condition are unaware that they have it. The condition was first described in 1961.

47, XYY is not inherited, but generally occurs as a random event during the formation of sperm cells. An incident in chromosome separation during anaphase II (from meiosis II) called nondisjunction can result in sperm cells with an extra copy of the Y chromosome.

If one of these atypical sperm cells contributes to a child’s genetic makeup, the child will have an extra Y chromosome in every cell in the body.

In some cases, the addition of an extra Y chromosome results from nondisjunction during cell division during post-zygotic mitosis in early embryonic development. This can produce 46, XY / 47, XYY tiles.

Compared to autosomal chromosome trisomy, sex chromosome trisomy usually has less serious consequences. People may show few or no symptoms and have a normal life expectancy.

Trisomic analysis

Trisomics are also used to locate genes on specific chromosomes. If a particular gene is located on the chromosome involved in the trisomy, segregation in the progeny of this trisomy will not follow a Mendelian pattern, but the ratio will deviate from the normal 3: 1 F2 and 1: 1 cross test ratios.

The expected relationships in trisomy can be resolved. Relationships based on chromosome segregation are provided as well as those based on chromatid segregation.

Chromosomal segregation will be maintained, when the gene is located very close to the centromere and does not allow the crossing between the gene and the centromere, so both sister chromatids will be similar. In chromatid segregation, the gene is located away from the centromere and allows crossover between the gene and the centromere.