Trisomy

In genetics, a trisomy is the existence of an extra chromosome in a diploid organism: instead of a chromosomes counterpart is a triplet (2+1 chromosomes).

The effects of trisomies go parallel to those of monosomies. The addition of an extra chromosome results in somewhat more viable individuals than in the case of loss of a chromosome. The variation of sex chromosomes of the trisomy type has a less serious effect on the phenotype than autosomal variations. In the human species, the addition of an Extra X O Y chromosome and a woman or a male gives rise to viable individuals who have various syndromes (such as Klinefelter syndrome, triple X syndrome or Xyy syndrome). The addition of a big self -process to the diploid endowment has serious effects and is normally lethal during its development.

both in vegetables and animals, trisomies can be detected by cytological observation of the meiotic division. Since there are three copies of one of the chromosomes, the mating settings are normally irregular. In a region given along the chromosome, only two of the three counterparts can establish synapses, although different regions of the trio may be paired. When three copies of a chromosome are in synapse, the configuration is called trivalent. In some cases, instead of a trivalent one can find a bivalent and a univalent (an unproven chromosome) in the first meiotic division. Thus, meiosis produces gametes with a chromosomal (n + 1), which can perpetuate the trisomic situation.

Trisomies in humans

The most frequent autosomal trisomies in humans that survive birth are:

• Trisomnia 22
• Trisomy 21 (Down syndrome)
• Trisomy 18 (Edwards syndrome)
• Trisomy 13 (Patau syndrome)
• Trisomy 9
• Trisomy 8 (Warkany syndrome 2)

Of these, trisomies 21 and 18 are the most common.

The trisomies that affect sex chromosomes are the following:

• XXX (triple X syndrome)
• XXY (Klinefelter syndrome)
• XYY (XYY syndrome)

Compared to trisomies that affect autosomas, trisomies in sex chromosomes generally have less consequences for the health of individuals, they may not even suffer symptoms and have normal life expectancy.

Down syndrome

The only autosomal trisomy of the human species that survives a significant number of individuals beyond the year after birth was discovered by John Langdon Haydon Down in 1866. The syndrome is a consequence of the trisomy of chromosome 21, belonging to the G group of the chromosomal groups of the human karyotype, and is called Down Syndrome (or "Trisomy of 21").

People with this condition have low height, rounded head, high and flattened front, and dried and cracking tongue. They present epicanto , leather fold in the internal corner of the eyes. The palms of the hands show a single transverse fold, and the soles of the feet have a fold from the heel to the first interdigital space (between the first two fingers). In many cases they suffer from congenital heart disease and tend to develop leukemia. The intelligence ratio varies from 20 to 60 but under an intervention and early stimulation program, these individuals can achieve significant cognitive development.

The global incidence of Down's syndrome approaches 1 in 700 births, but the risk varies according to the mother's age. The incidence in 25 -year -old mothers is 1 every 1250 live births; In 30 -year mothers it is 1 per 1000; In 35 -year -old mothers is 1 by 400; in 40 -year -old mothers is 1 percent; In 45 -year -old mothers is 1 by 30. To detect chromosomal abnormality during the prenatal period, amniocentesis and biopsy of chorionic villi can be used. Some maternal blood alterations can suggest the gestation of a child with Down syndrome: low levels of alpha-feet and abnormal levels of unconclosed striol and human chorionic gonadotropin.

The chromosomal anomaly that causes most cases of Down Syndrome (95%) is free trisomy 21, which has three copies of this chromosome. Therefore, patients have 47 chromosomes instead of 46 (normal human genome figure) in all their cells. This anomaly is a consequence of the failure in the separation of chromosomes during cell division. In a less frequent type of Down syndrome, produced by translocation, part of the genetic material of one of chromosomes 21 is adhered to another chromosome (usually 14). In another type of less frequent Down syndrome, individuals have chromosomal alterations only in some cells of their organism, not all; In this case it is said that they present mosaicism.

Patau Syndrome

Klaus Patau and his collaborators (1960) observed a child with serious developmental malformations and a 47 chromosomes' karyotype. The extra chromosome had an average size, one of the acrocentric of group D, which is now called chromosome 13. The trisomy of 13 , which is called Patau Syndrome (47, +13) . Children have a characteristic lip-palatine fissure and polydactyly. Autopsies have revealed congenital malformations in many organic systems, indicative of abnormal development that occurs at five or six weeks of gestation. The average survival of these children is about three months. The average age of parents of affected children is greater than that of parents of normal children, but is not as high as the average maternal age in cases of Down syndrome. This case only occurs in 1 in 19,000 births.

Edwards syndrome

John H. Edwards and his collaborators published in 1960 the case of a trisomic child for a chromosome from group E, which is now known is chromosome 18. This trisomy (47, +18) is currently called Edwards syndrome In recognition of your discoverer.

The phenotype of these children illustrates that the presence of an extra self -somage produces congenital malformations and a lower life expectancy. These children are smaller than normal. Their skulls are elongated according to the antero-posterior axis and their ears are deformed and located lower than normal, they present the wide neck, congenital dislocation of the hip and depressed chin. The average survival is less than four months. Death is normally produced by pneumonia or heart failure. The maternal middle ages is high, babies with Edwards syndrome are preponderantly women.

Viability of aneuploidies in the human species

The reduced viability of individuals diagnosed with monosomies and trisomies suggests that many other aneuploids could appear, but the affected fetuses do not reach term in most cases. The gametes that lack a chromosome are so abnormal functionally that either they do not participate in fertilization or monosomic embryos die so early that their recovery is very rare. Certain studies revealed some striking statistical data

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. In the human species it was estimated that between 15% and 20% of all conceptions end with abortions due to some form of chromosomal anomaly. For example, it is estimated that 1% of pregnancies occur trisomies in 16, not becoming mostly viable the individuals that carry it. 6% of all pregnancies begin with an abnormal number of chromosomes. Many of these end before birth. It is estimated now that between 10% and 30% of all fertilized ovules have some error in the number of chromosomes

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The highest percentage of chromosomal anomalies are aneuploidia. An aneuploidy with one of the highest incidents between abortions is situation 45, X, which gives rise to girls with Turner's syndrome if the fetus survives. Between 70% and 80% of abortions and living births with Turner's syndrome carry maternal X chromosome. The meiotic error that gives rise to this syndrome occurs in spermatogenesis.

These observations support the hypothesis that normal embryonic development requires an exact endowment of chromosomes, which maintain a delicate balance of genetic information expression. The prenatal mortality of most aneuploidies is a barrier against the introduction in human populations of a series of chromosomal anomalies.