Autosomal Chromosome Changes
Chromosomal alterations are quite common and more serious than alterations in a gene, as they involve numerous genes. Each arm of the chromosomes is divided into 4 regions and, within each region, each band is listed in terms of distance to the centromere.
The short arm is called “p” and the long “q”. For example: 1 q 23 indicates: chromosome 1, long arm, second region, third band. The common features of these changes are:
- Maturation and intellectual delay.
- Presence of dysmorphic features and malformations.
- Growth delay.
Classification of chromosomal alterations
Among the chromosomal alterations, we have numerical aberrations and aberrations in the structure of chromosomes.
Chromosomal alterations: numerical aberrations
Of course, the most frequent cause of numerical aberrations in chromosomes is usually non-disjunction. It consists of the failure of homologous chromosomes or chromatids to experience disjunction in the first or second meiotic division or during mitosis.
The consequence of this anomaly is that the members of the chromosome pair do not separate properly. Therefore, in some gametes chromosomes may be left or missing.
When a gamete that does not have a particular chromosome joins a normal gamete, this will lead to a monosomy. Non-disjunction can also occur during mitosis that occurs after zygote formation.
If this happens at the beginning, there will be trisomal cells, while others will be monosomal. These patients are called mosaics. Furthermore, this almost always happens with respect to the X chromosome (mosaics of other chromosomes are usually not viable).
Chromosomal alterations: aberrations in chromosome structure
The most frequent alterations in the structure of chromosomes are: deletion, duplication, inversion and translocation.
- Deletion or suppression. Loss of a fragment of a chromosome so that all the 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 attach to the end of the homologous chromosome. Thus, this change is much less harmful than deletion.
- Inversion. It involves the fragmentation of a chromosome by two breaks, followed by its adhesion to the same chromosome, but in an inverted fashion. In general, this change in gene order does not result in an abnormal phenotype, although it can have consequences for the next generation when the chromosome with the inversion joins with another normal one.
- Translocation. Transfer of part of a chromosome to another non-homologous. Sometimes these translocations are reciprocal.
Structural aberrations usually occur during meiosis in one of the gametes (maternal or paternal).
Autosomal chromosomal alterations
Next, we will briefly expose the main autosomal chromosomal alterations:
Down’s syndrome
- First described by Langdon Down in 1866.
- Most common and known chromosomal abnormality (affects 1 in 700 live births).
- People with Down syndrome have 47 chromosomes, with an extra copy of chromosome 21.
- It can usually be identified at birth or soon after due to the presence of typical characteristics such as the following:
- Hypotonia.
- Mental retardation.
- Brachycephaly (overgrowth of the head due to premature closure of the coronal suture).
- Prominent language.
- Short, wide hands.
- Cardiac abnormalities in 35% of cases.
- Smaller than normal size.
- Although in 95% of cases Down syndrome involves trisomy 21, about 4% have a translocation of the long arm of chromosome 21 (to chromosome 13, 14, 15 or 22). In fact, 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 likely to have affected children than female carriers.
Trisomy 18 (Syndrome E)
- It is a syndrome associated with a trisomy of chromosome 18.
- It is related to multiple birth defects.
- It is much more serious than Down syndrome (most patients die by 6 months of age)
- The most common symptoms include:
- Mental retardation.
- Inability to grow.
- Changes in ears, hands and feet.
Trisomy 13 (Syndrome D)
- It is less frequent than the previous one and the changes are much more severe (patients do not survive beyond the first month of life).
- It includes nervous system defects, mental retardation, cleft lip, cleft palate, skin abnormalities, heart changes, etc.
cat meowing syndrome
- It is due to a partial monosomy of chromosome 5 (deletion of a fragment of the short arm).
- It produces different abnormalities, the most characteristic being that of a child’s cry similar to a cat’s meow. Other changes are mental retardation or microcephaly.
Monosomy 4
- It is produced by the deletion of the short arm of chromosome 4.
- It represents a phenotype characterized by low birth weight, failure to thrive, and abnormal facial appearance.
Chromosomal alterations: conclusions
In conclusion, chromosomal alterations represent, nowadays, an important difficulty for the baby’s evolutionary development. However, there are more and more methods to detect them in time. In addition, there are also better prevention and treatment techniques.
