In these conditions, the genetic alteration is of such magnitude that it can be visualized in the chromosomes, either by alterations in their number or by their structure.

Syndromes of alterations due to changes in the number of autosomes.

Patau syndrome.

It is a disease produced by a trisomy of chromosome 13, that is, somatic cells have 44 autosomes, two heterochromosomes and one additional chromosome 13 (44 + 2 sex chromosomes + an additional chromosome 13).

Babies born with this syndrome have characteristics such as:

• Mental deficiency.
• Deafness.
• Polydactyly (more than five fingers on one of the hands or feet).
• Cleft lip.
• Cardiac abnormalities.

In general, it occurs with a frequency of 1 in every 5000 newborns. About 88% of babies with this syndrome die during the first month of life, and a 5 survive until the sixth month.

Karyotype of Patau syndrome
Source

Edwards syndrome.

This chromosomal aberration is also called trisomy of chromosome 18 (44 + 2 sex chromosomes + an additional chromosome 18). The individuals that suffer it are infants who manage to survive less than four months since they die of pneumonia or heart attack. These children show the following characteristics:

• Deformed ears.
• Hypertension.
• Cardiac abnormalities.
• Alterations in the nervous system.
• Mental retardation and elongated skull in the anterior and posterior part.

Karyotype of Edwards syndrome
Source

Down's Syndrome.

It is one of the most known aneuploidies in humans, so named because it was originally studied in 1866 by John L. Down (1828-1896). This condition occurs when somatic cells, instead of having a pair of chromosomes 21, possess three chromosomes 21. It is also known as trisomy of chromosome 21 (44 + 2 sex chromosomes + an additional sex chromosome). This syndrome is the result of the union of a gamete with two chromosomes 21 and another gamete with a chromosome 21.

In general, the greater probability that a person suffers from Down syndrome is due to the formation of ovules with two chromosomes 21. It has been shown that as a woman ages, she has a greater probability of having ova with two chromosomes 21, and therefore, there is also a greater possibility of having children with Down syndrome.

The phenotypic characteristics of individuals with this trisomy are:

• Mental retardation in different degrees.
• Short stature.
• Slanted eyes.
• Rounded face.
• Nose flattening.
• Small ears.
• Short neck.
• Low muscle tone.
• Alterations of the cardiovascular system.
• Sterility.
• Wide and shorthands.

Today people who have a trisomy of chromosome 21 are over 50 years of age and are actively integrated into social life.

Source

Syndromes of alterations due to changes in the number of heterochromosomes.

There are several alterations in the number of heterochromosomes, ranging from the duplication of X or Y chromosomes to the presence of four X chromosomes, both in women (XXXX) and in men (XXXXY). The main chromosomal aberrations are:

Turner syndrome.

It only occurs in women. It is a consequence of the alteration of the number of heterochromosomes X, which is caused by the union of a gamete carrying an X chromosome with another gamete lacking this chromosome.

Gametes lacking the X chromosome can be ovules or sperm formed during gametogenesis.

The somatic cells of these individuals possess 45 chromosomes: 44 autosomes (the same as normal individuals) and a single X heterochromosome, so it is represented as X0 individuals.

This monosomy produces a woman with the following characteristics:

• Cardiovascular disorders.
• Moderate mental retardation.
• Child traits.
• Abnormal gonads.
• Absence or atrophy of the uterus, fallopian tubes, and ovaries, so it is sterile.
• When it reaches adulthood, it is of small stature and its secondary sexual characteristics, such as the development of the breasts, are atrophied.

Source

Klinefelter syndrome.

It only occurs in male individuals. It results from variations in the number of heterochromosomes. Karyotypes vary between the ends 47, XXY (an additional X chromosome) and 49, XXXXY (three additional X chromosomes). These chromosomal aberrations occur when the gametes that give rise to the new being that has an additional X chromosome.

Trisomy XXY is the most frequent since it is found in 80% of cases. These men present the following characteristics:

• They are usually tall.
• Sterile, with little-developed gonads.
• They have larger pectorals and smaller testicles than the average man.

Karyotype of Klinefelter syndrome
Source

References:

• Human Genes and Genomes: Science, Health, Society. By Leon E. Rosenberg and Diane Drobnis Rosenberg.
• https://www.biology.iupui.edu/biocourses/N100/2k4csomaldisordersnotes.html
• https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/chromosome-abnormality
• https://www.ndss.org/about-down-syndrome/down-syndrome/
• https://ghr.nlm.nih.gov/
• https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1634840/

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