Table of Contents
Chromosome changes are quite common and more serious than gene changes because they affect numerous genes. Each arm of the chromosomes is divided into 4 regions, and each band is listed in each region with respect to the distance to the centromer.
The short arm is called “p” and the long one is called “q”. For example: 1 q 23 means: chromosome 1, long arm, second region, third band. The common characteristics of these changes are:
Mature and intellectual delay.
Presence of dimorphic features and malformations.
Classification of chromosomal changes
Within the chromosome changes we have: the numerical aberrations and the aberrations in the structure of the chromosomes.
Chromosome changes: numerical aberrations
The most common cause of numerical aberrations in chromosomes is usually non-disjunction. This is because paired chromosomes or chromatids experience no disjunction in the first meiotic division, the second, or during mitosis.
The consequence of this anomaly is that members of the chromosome pair do not separate properly. Therefore, chromosomes may remain or be missing in some gametes.
If a gamet with an additional chromosome binds to a normal gamete during fertilization, the resulting zygote has three units of that particular chromosome. This phenomenon is called trisomy.
When a gamet that lacks a particular chromosome connects to a normal gamete, it leads to a monosomy. Non-disjunction can also occur in mitoses that occur after the zygote is formed.
When this happens in the beginning, there are trisoma cells while others are monosomic. These patients are called mosaics. In addition, almost always in relation to the X chromosome (mosaics of other chromosomes are usually incapable).
Chromosome changes: aberrations in the structure of the chromosome structure
The most common changes in the structure of the chromosomes are deletion, duplication, inversion and translocation.
Deletion or deletion Loss of a fragment of a chromosome so that all genetic information stored in the missing fragment is missing from the affected chromosome.
Duplication Presence of an additional chromosome fragment. Sometimes a deleted fragment of a chromosome can adhere to the end of the homologous chromosome. This change is much less harmful than deletion.
Investment It involves fragmentation of a chromosome by two breaks, followed by its adhesion to the same chromosome, but inverted. In general, this change in the order of the genes does not result in an abnormal phenotype. Although this can have consequences for the next generation if the chromosome with inversion reaches a different normal.
Relocation Transfer of part of a chromosome to another non-homologous one. Sometimes these translocations are mutual.
The structural aberrations generally occur during meiosis of one of the gametes (maternal or paternal).
Autosomal chromosomal changes
Next, we briefly reveal the major autosomal chromosome changes:
First described by Langdon Down in 1866.
Chromosomal abnormalities more common and known (affects 1 in 700 live births).
People with Down syndrome have 47 chromosomes, with chromosome 21 being an additional one.
It can usually be identified at the time of birth or shortly afterwards when typical features such as the following occur:
Brachycephaly (excessive growth of the head due to premature occlusion of the coronary suture).
Short, broad hands
Heart abnormalities in 35% of cases.
Size lower than normal.
Although Down syndrome includes trisomy for chromosome 21 in 95% of cases, about 4% have long arm translocation of chromosome 21 (to chromosomes 13, 14, 15, or 22). These patients therefore have 46 chromosomes, although they are phenotypically indistinguishable from trisomy 21.
The average maternal age is 34 years.
Male carriers are less affected than female carriers.
Trisomy 18 (syndrome E)
It is a syndrome related to trisomy for chromosome 18.
It refers to several congenital defects.
It is much more serious than Down syndrome (most patients die at 6 months of age).
The most common symptoms are:
Inability to grow.
Changes in ears, hands and feet.
Trisomy 13 (Syndrome D)
It is less common than the previous one and the changes are much more serious (patients do not survive the first month of life).
Contains defects in the nervous system, mental retardation, lip edges, cleft palate, skin abnormalities, changes in the heart, etc.
It is due to a partial monosomy of chromosome 5 (deletion of a short arm fragment).
It produces various abnormalities, with the child’s cry being the most similar, similar to the meowing of a cat. Other changes include mental retardation or microcephaly.
It arises from the deletion of the short arm of chromosome 4.
It is a phenotype that is characterized by low birth weight, growth disorders and unusual facial features.
Chromosome changes: conclusions
In summary, chromosomal changes present an important difficulty for the evolutionary development of the baby today. However, there are more and more methods to recognize them in time. There are also better prevention and treatment techniques.