An overview of chromosomal aberrations
Chromosome are the structures with definite organization. They carry genes which remain in them in liner order. In ordinary conditions, though the chromosomes remain unchangeable, but under certain natural and artificial conditions some structural changes may occur in them. This structural changes are collectively called chromosomal aberrations.
The standard number of the chromosomes of a normal human being are 46. So what will happen if it was 47 with an additional chromosome or let’s suppose something extremely abnormal is that possible for a cell to have 92 pairs of chromosomes? The short answer is yes.
In humans, chromosomal abnormalities presently known to be associated with pathologic conditions. These aberrations involve alteration of the number or the structure of the chromosome.
1-cri-du chat syndrome (cat cry) deletion in the short arm of the chromosome 5: losses of a portion of the short arm of one of the two, fifth chromosome lead to condition named the cri du chat syndrome. The child born with it has a small head, mental retardation and characteristics cry like the Cat. Cri-du-chat syndrome occurs in an estimated 1 in 20,000 to 50,000 newborns. This condition is found in people of all ethnic backgrounds. The segment that is missing from the chromosome number 5 may be less than 1/100 of total genetic material in the karyotype.
Down syndrome occurs when an individual has a full or partial extra copy of chromosome 21. This additional genetic material alters the course of development and causes the characteristics associated with Down syndrome. A few of the common physical traits of Down syndrome are low muscle tone, small stature, an upward slant to the eyes, and a single deep crease across the center of the palm – although each person with Down syndrome is a unique individual and may possess these characteristics to different degrees, or not at all.
How common is Down syndrome?
According to the Centre for Arab Genomic Studies, Down syndrome is one of the most common chromosomal anomalies affecting approximately 1 in every 800 birth.1
Are there different types of Down syndrome?
1- TRISOMY 21 (NONDISJUNCTION)
Down syndrome is usually caused by an error in cell division called “nondisjunction.” Nondisjunction results in an embryo with three copies of chromosome 21 instead of the usual two. Prior to or at conception, a pair of 21st chromosomes in either the sperm or the egg fails to separate. As the embryo develops, the extra chromosome is replicated in every cell of the body. This type of Down syndrome, which accounts for 95% of cases, is called trisomy 21.
In translocation, which accounts for about 4% of cases of Down syndrome, the total number of chromosomes in the cells remains 46; however, an additional full or partial copy of chromosome 21 attaches to another chromosome, usually chromosome 14. The presence of the extra full or partial chromosome 21 causes the characteristics of Down syndrome.
Mosaicism (or mosaic Down syndrome) is diagnosed when there is a mixture of two types of cells, some containing the usual 46 chromosomes and some containing 47. Those cells with 47 chromosomes contain an extra chromosome 21.
Mosaicism is the least common form of Down syndrome and accounts for only about 1% of all cases of Down syndrome. Research has indicated that individuals with mosaic Down syndrome may have fewer characteristics of Down syndrome than those with other types of Down syndrome. However, broad generalizations are not possible due to the wide range of abilities people with Down syndrome possess.2
Patau’s syndrome is a serious rare genetic disorder caused by having an additional copy of chromosome 13 in some or all of the body’s cells. It is also called trisomy 13.
This chromosomal condition associated with severe intellectual disability and physical abnormalities in many parts of the body. Individuals with trisomy 13 often have heart defects, brain or spinal cord abnormalities, very small or poorly developed eyes (microphthalmia), extra fingers or toes, an opening in the lip (a cleft lip) with or without an opening in the roof of the mouth (a cleft palate), and weak muscle tone (hypotonia). Due to the presence of several life-threatening medical problems, many infants with trisomy 13 die within their first days or weeks of life. Only 5-10% of children with this condition live past their first year. Trisomy 13 occurs in about 1 in 16,000 newborns. Although women of any age can have a child with trisomy 13, the chance of having a child with this condition increases as a woman gets older.3-4
Trisomy 18, also called Edwards syndrome, is a chromosomal condition associated with abnormalities in many parts of the body. Individuals with trisomy 18 often have slow growth before birth (intrauterine growth retardation) and a low birth weight. Affected individuals may have heart defects and abnormalities of other organs that develop before birth. Other features of trisomy 18 include a small, abnormally shaped head; a small jaw and mouth; and clenched fists with overlapping fingers. Due to the presence of several life-threatening medical problems, many individuals with trisomy 18 die before birth or within their first month. 5-10% of children with this condition live past their first year, and these children often have severe intellectual disability.
Trisomy 18 occurs in about 1 in 5,000 live-born infants; it is more common in pregnancy, but many affected fetuses do not survive to term. Although women of all ages can have a child with trisomy 18, the chance of having a child with this condition increases as a woman gets older.5
Turner syndrome (xo) syndrome
Normal females have two X chromosomes, when they possess only one X chromosome this syndrome is developed. Turner syndrome is a chromosomal condition that affects development in females. The most common feature of Turner syndrome is short stature, which becomes evident by about age 5. An early loss of ovarian function (ovarian hypofunction or premature ovarian failure) is also very common. The ovaries develop normally at first, but egg cells (oocytes) usually die prematurely and most ovarian tissue degenerates before birth. Many affected girls do not undergo puberty unless they receive hormone therapy, and most are unable to conceive (infertile). A small percentage of females with Turner syndrome retain normal ovarian function through young adulthood.6
Some people with features of Klinefelter syndrome have an extra X chromosome in only some of their cells; other cells have one X and one Y chromosome. In these individuals, the condition is described as mosaic Klinefelter syndrome (46,XY/47,XXY). Boys and men with mosaic Klinefelter syndrome may have milder signs and symptoms than those with the extra X chromosome in all of their cells, depending on what proportion of cells have the additional chromosome.
Several conditions resulting from the presence of more than one extra sex chromosome in each cell are sometimes described as variants of Klinefelter syndrome. These conditions include 48,XXXY syndrome, 48,XXYY syndrome, and 49,XXXXY syndrome. Like Klinefelter syndrome, these conditions affect male sexual development and can be associated with learning disabilities and problems with speech and language development. However, the features of these disorders tend to be more severe than those of Klinefelter syndrome and affect more parts of the body. As doctors and researchers have learned more about the differences between these sex chromosome disorders, they have started to consider them as separate conditions.7
Triple X syndrome
Triple X syndrome results from an extra copy of the X chromosome in each of a female’s cells. As a result of the extra X chromosome, each cell has a total of 47 chromosomes (47, XXX) instead of the usual 46. An extra copy of the X chromosome is associated with tall stature, learning problems, and other features in some girls and women.8
Williams syndrome is a developmental disorder that affects many parts of the body. This condition is characterized by mild to moderate intellectual disability or learning problems, unique personality characteristics, distinctive facial features, and heart and blood vessel (cardiovascular) problems.
It is caused by the deletion of genetic material from a specific region of chromosome 7. The deleted region includes 26 to 28 genes, and researchers believe that a loss of several of these genes probably contributes to the characteristic features of this disorder.9
22q11.2 deletion syndrome
Most people with 22q11.2 deletion syndrome are missing a sequence of about 3 million DNA building blocks (base pairs) on one copy of chromosome 22 in each cell. This region contains 30 to 40 genes, many of which have not been well characterized. A small percentage of affected individuals have shorter deletions in the same region. This condition is described as a contiguous gene deletion syndrome because it results from the loss of many genes that are close together.10
Tetraploidy is an extremely rare chromosomal anomaly, polyploidy, when an affected individual has four copies of each chromosome, instead of two, resulting in total of 92 chromosomes in each cell. The phenotype is severe with multiple congenital anomalies, including central nervous system, ocular, cardiac, renal, and/or genital malformations and limb defects. Most patients show severe intrauterine growth retardation, hypotonia, failure to thrive and developmental delay. It is usually associated with miscarriage.11
- Center for Arab Genomic Studies Cags.org.ae. (2019). [online] Available at: http://www.cags.org.ae/Down%20Syndrome-English.pdf
2. NDSS. (2019). What is Down Syndrome? | National Down Syndrome Society. [online] Available at: https://www.ndss.org/about-down-syndrome/down-syndrome/
3. nhs.uk. (2019). Patau’s syndrome. [online] Available at: https://www.nhs.uk/conditions/pataus-syndrome/
4.Trisomy 13. [online] Genetics Home Reference. Available at: https://ghr.nlm.nih.gov/condition/trisomy-13#genes
5. Genetics Home Reference. Available at: https://ghr.nlm.nih.gov/condition/trisomy-18#statistics
6. Genetics Home Reference. Available at: https://ghr.nlm.nih.gov/condition/turner-syndrome
7.Genetics Home Reference. Available at: https://ghr.nlm.nih.gov/condition/klinefelter-syndrome#genes
8. Genetics Home Reference. Available at: https://ghr.nlm.nih.gov/condition/triple-x-syndrome
9. Genetics Home Reference. Available at: https://ghr.nlm.nih.gov/condition/williams-syndrome
10. Genetics Home Reference. Available at: https://ghr.nlm.nih.gov/condition/22q112-deletion-syndrome#genes