Chromosomal abnormalities

Chromosomal abnormalities . Any alteration in the number and / or in the morphology of the chromosomes constitutes a chromosomal alteration. Chromosomal abnormalities are genetic defects that generally occur due to disorders and imbalances in the baby’s chromosomes. Although one of the best known is Down syndrome, there are many kinds of abnormalities. Some of these are less serious but others can even lead to the death of the child before he is born.


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  • 1 Information
  • 2 Overview of Chromosomal Abnormalities
  • 3 Causes of Chromosomal Abnormalities
  • 4 most common chromosomal abnormalities
    • 1 Down syndrome (trisonomy 21)
    • 2 Chronic leukemia
    • 3 Heart defects
    • 4 Marfan syndrome
    • 5 Basal cell nevus syndrome
    • 6 Apert syndrome
  • 5 common sex chromosome abnormalities
  • 6 Sources


About one in 200 babies is born with a chromosomal abnormality. Quite a few of the children with these anomalies (although not all) are characterized by behavioral problems, mental retardation, learning disabilities, etc .; What causes a large number and types of birth defects are errors in the structure or number of chromosomes. Sometimes an infant can be born with fewer or more chromosomes, or one or more broken or altered structures.

Chromosomal Abnormalities Overview

What is a chromosome? The human body is made up of cells. For example, when a person suffers sunburn, skin peels and drops cells of the skin . In the center of each cell there is an area called the nucleus. Human chromosomes are found in the nucleus of the cell.
The chromosome is a structure that is part of the nucleus and that contains genes. Genes determine traits, such as eye color and blood group . How are chromosomes inherited? Normally, each cell in our body has a total of 46 chromosomes, or 23 pairs. We inherit half of the chromosomes (one member of each pair) from the biological mother and the other half (the homologous member of each pair) from the biological father.
Scientists have numbered chromosome pairs 1 through 22, giving pair 23 the name X or Y, depending on the structure. The first 22 pairs of chromosomes are called “autosomes”. The pair 23 chromosomes are known as the “sex chromosomes” because they determine whether the baby will be male or female .
Women have two “X” chromosomes and men have one “X” chromosome and one “Y” chromosome. The graphic representation of the 46 chromosomes, arranged in pairs, is called a karyotype. The normal karyotype for women is written 46, XX, while the normal karyotype for men is written 46, XY. What are trisomies? The term “trisomy” is used to describe the presence of three chromosomes rather than the usual pair of chromosomes. For example, if a child is born with three chromosomes 21 instead of the usual pair, it would be said to have “trisomy 21”. Trisomy 21 is also known as Down syndrome.
Other examples of trisomy include trisomy 18 and trisomy 13. Again, trisomy 18 or trisomy 13 simply means that there are three copies and not the usual pair of chromosome 18 (or chromosome 13) in every cell in the body. What are monosomies? The term “monosomy” is used to describe the absence of a member of a pair of chromosomes. Therefore, there will be a total of 45 chromosomes in each cell of the body, instead of 46. For example, if a baby is born with only one X sex chromosome, instead of the usual pair (either two X sex chromosomes or one sex chromosome X and sex chromosome Y), will be said to have “monosomy X.” Monosomy X is also known as Turner syndrome.

Causes of Chromosomal Abnormalities

Generally, chromosomal abnormalities occur due to an error during the development of a sperm cell or egg. He why these errors is a mystery. But, as far as is known, nothing that either parent does or fails to do before or during their development can cause a chromosomal abnormality in their child.
The reproductive cells (egg and sperm cell) have only 23 individual chromosomes. When these cells come together and pregnancy begins they form a fertilized egg with 46 chromosomes. However sometimes something goes wrong before the pregnancy begins. In the process of cell division, an error occurs that causes a sperm cell or ovum to end up with a number of chromosomes higher or lower than normal.
The moment this cell (with the wrong number of chromosomes) joins with a normal egg or sperm cell, the embryo suffers a chromosomal abnormality. Also, despite a person having the normal number of chromosomes, it may happen that small segments of one or more chromosomes are removed, inverted, duplicated, exchanged with part of another chromosome or alter their normal location.
Embryos with the wrong number of chromosomes often do not survive. In these cases, the pregnant woman has a miscarriage, almost always without knowing it. Up to 70% of spontaneous abortions during the first trimester of pregnancy occur due to chromosomal abnormalities.

Most common chromosomal abnormalities

Prader-Willi syndrome, Ellis-van Creveld syndrome, Rubinstein-Taybi syndrome, Poland sequence, Down syndrome, Nevus basal cell syndrome, Trisomy 13, Marfan syndrome, Achondroplasia, Apert syndrome, among others.

Down syndrome (trisonomy 21)

It is the most common cause of birth malformations in humans, there is 1 case for every 660 births. The risk of this and other trisomies occurring increases with the mother’s age.

  • Have an extra copy of chromosome 21

Children with this syndrome have different degrees of mental retardation, characteristic facial features: smaller than normal and deformed head, flat nose, the inner corner of the eye may present a rounded fold of skin instead of ending in a point. The hands are wide and short, with short fingers, which usually have a single palmar fold.
Congenital heart defects are common in these children and premature death often occurs often as a result of these. Gastrointestinal abnormalities such as duodenal atresia (obstruction of the duodenum) and esophageal atresia (obstruction of the esophagus) are relatively common. Obstruction of the gastrointestinal tract may require surgery shortly after birth.
The prospects for children with Down syndrome are much more encouraging than they used to be. Most have mild to moderate mental retardation. With early intervention and special education, many of these children learn to read and write and participate in various children’s activities like any normal child. In addition, many of them put more effort, interest and concern for studies and sports than most children, and they are as or happier than any child.
Other Trisonomies
There are also trisonomies 13 and 18 that are less common but much more serious than Down syndrome, have many physical birth defects, and severe mental retardation. Unfortunately, most die before the first year of life. One in 5,000 babies is affected by them.

Chronic leukemia

Certain chromosomal and genetic abnormalities may be responsible for the cellular abnormalities that lead to chronic leukemia, immature and abnormal white blood cells are produced, making it difficult to fight infection and clotting.

Heart defects

And of the great vessels produced by disturbances at various stages of fetal development and present at birth, they may not be diagnosed until later.
In this sense, it is suspected that its incidence is 1/120 live births, where sometimes a specific cause can be identified. Chromosomal defects are usually associated with severe congenital heart abnormalities.

Marfan syndrome

Caused by mutations in the fibrillin-1 gene, which is the scaffolding of elastic tissues in the body. Disruption of this produces changes in elastic tissues, specifically in the aorta, skin and eye.
It also causes an exaggerated growth of the long bones of the body, causing a tall stature and long limbs, spider-like fingers (arachnodactyly), a particular group of facial features including a highly arched palate and crowded teeth, thoracic malformations and curvature of the Thorn.

Basal cell nevus syndrome

It is a set of multiple defects that compromise the endocrine glands, the skin, the nervous system, the eyes, and the bones, causing a strange facial appearance and a predisposition to skin cancer. Those affected have separate eyes, prominent eyebrows, a prominent nose and jaw. Skin cancer appears near or at puberty
It is inherited as an autosomal dominant feature.
Bone defects include cysts in the upper and lower jaw, causing abnormal tooth development or spontaneous fractures of the jaw., Rib abnormalities, Scoliosis (curvature of the back), Kyphosis (severe curvature of the back) and Among the disorders of the nervous system are: blindness, seizures
Hydrocephalus (enlarged head), deafness, brain tumors, mental retardation, defects in the iris.

Apert syndrome

It can run in families or occur without a family history. It is characterized by premature closure of the cranial sutures (sutures between the skull bones), so they have a pointed head and unusual face. As the child grows, the bones of the hands and feet gradually fuse, which reduces flexibility and function.
It is transmitted as an autosomal dominant trait. There are some cases of spontaneous presentation in which there is no family history. This condition occurs due to mutations in a gene called fibroblast growth factor receptor 2.
There are some other syndromes with characteristics similar to Apert syndrome and that involve craniosynostosis (premature fusion of the sutures):

  • Saethre-Chotzen syndrome
  • Carpenter syndrome (clover-shaped deformity of the skull)
  • Crouzon disease (craniofacial dysostosis)
  • Pfeiffer syndrome

Common sex chromosome abnormalities

Among the most common abnormalities are those related to the additional presence or lack of sex chromosomes (X and Y). Normally, women have two X chromosomes, and men have one X chromosome and one Y chromosome.
These often affect sexual development, growth abnormalities and infertility, and, in certain cases, learning and behavioral problems. Despite this, most of those affected have a relatively normal life. The best known are:

  • Triple X is an extra X chromosome one in 1,000 to 2,000 women has. These girls do not have the same physical abnormalities, are usually tall, appear to be fertile, and have normal puberty. They are of average intelligence, but learning disabilities are frequent. It is common for it to be detected only if prenatal tests have been done as they look healthy and normal-looking
  • Turner syndrome affects one in 2,500 girls. Girls with this syndrome have only one X chromosome instead of two. Generally, they are sterile and unless they use hormones they will not have the normal puberty changes. Some have other health problems (heart defects), they are short, (but they can grow thanks to hormones), they have normal intelligence, but some have difficulties with spatial concepts and mathematics.
  • Klinefelter syndrome affects about one in 600 to 800 children. Those affected have two or more X chromosomes along with their Y chromosome (they usually have an X and a Y). They have more problems with judgment and impulse control than XYs. They are usually tall with learning disabilities. As adults, they produce less testosterone than normal and are infertile


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