A chromosomal disorder is a missing, extra, or unequal share of chromosomal DNA. It can be an abnormality in the number of chromosomes (Increase or decrease in the number of chromosomes)or a structural aberration in one or more chromosomes. In this article, we are going to discuss different types of chromosomal abnormalities and their genetic causes.
Types of chromosomal abnormalities or disorders/aberrations:
There are primarily two types of chromosomal disorders or abnormalities
- Numerical abnormalities or disorders
- Structural abnormalities or disorders
Numerical chromosomal abnormalities (one of the Major types of chromosomal disorders or abnormality) or disorder are mainly the types of chromosome defect that occurs when there are different numbers of chromosomes present in the cells from actual numbers of chromosomes. E.g. in numerical chromosomal abnormality, there could be 47 or 45 chromosomes instead of 46.
In numerical chromosomal abnormality, we mostly use two terms i.e. polyploidy and aneuploidy
- Firstly, It is a numerical abnormality in which the number of chromosomes is an exact multiple of haploid number but larger than the diploid number.
- Secondly, Polyploidy mainly arises from the fertilization of an egg by two sperms or failure in one of the divisions of a sperm or an egg so that diploid gamete is produced.
- In polyploidy, the survival of a fetus is rare.
- Firstly, Aneuploidy is also the type of numerical chromosome abnormality that occurs when the number of chromosomes is not an exact multiple of haploid number, and this results in the failure of paired chromosomes at first meiosis or sister chromatids at second meiosis to divide at anaphase.
- Therefore, two cells are produced, one with an extra copy and one with a missing number of chromosomes.
Example of numerical chromosomal disorder:
Trisomy 21 is the first human chromosomal abnormality that shows an extra copy of chromosome 21 and causes Down syndrome.
- Firstly, In Down’s syndrome genes of all the 3 copies of chromosome 21 are normal.
- Secondly, In this syndrome, not all individuals show the same physical characteristics but their phenotypes can vary.
- Thirdly, These individuals have an abnormally small chin, poor muscle tone, skin folds on the inner corner of eyes, a flat nasal bridge, an enlarged tongue and have a small neck.
- The height, weight head circumferences are smaller in children with Down’s syndrome and they have some degree of mental retardation.
Patau syndrome shows an extra copy (3 copies) of chromosomes at chromosome 13.
- Firstly, It is estimated that only 5 percent of infants with Patau syndrome survive their first year and most of the pregnancies end in miscarriage.
- Secondly, Children who have trisomy 13 have lots of breathing problems especially when they sleep and many children have seizures.
Edward syndrome shows an extra copy (3 copies) of chromosomes at chromosome 18.
- Firstly, It is estimated that only 10% of infants survive their first year and most of the survivors of Edward syndrome are female.
- Secondly, Children who have trisomy 18 have breathing problems and many have seizures and serious heart conditions.
Chromosome Structural Abnormality: –
- Change in chromosome structure
- Removal, addition, or alternation in chromosome structure or in its part
- It results in loss of genetic information
Types of Chromosome Structural Abnormality: –
Primarily, Deletion is a structural chromosomal disorder that occurs when the chromosome is removed or missing. E.g., DiGeorge syndrome which is a structural chromosomal abnormality also states as 22q11 is actually a deletion syndrome that is mainly caused by the deletion of a small portion of chromosome 22.
- The portion of a chromosome is removed and is missing
a b c d => a c d
- Example: Wolf-Hirschhorn syndrome
Duplications are also a structural chromosomal disorder that arises from the copying of a portion of a chromosome. E.g. MECP2 duplication syndrome is an unusual disease that is described by impaired motor function and severe intellectual disability. It is an X-linked genetic disorder affected by overexpression of MeCP2 protein.
- A portion of chromosome duplicates due to unequal recombination
a b c d => a b b c d
- Example: Charcot Marie Tooth disease type 1A
Firstly, Translocation is a structural chromosomal abnormality where one chromosome is transferred to another chromosome. Secondly, DNA segments from two different chromosomes are exchanged in reciprocal translocation.
- The portion from one chromosome is transferred to the other chromosome
Chromosome 1: a b c d + Chromosome 2: e f g h =>
Chromosome 1: a b c f Chromosome 2: d e g h
Example: XX male syndrome
Types of translocations: –
- Reciprocal translocation:
- Segments from two different chromosomes have been exchanged
- Robertsonian translocation:
- The entire chromosome attaches to the other chromosome at the centromere
- Firstly, A portion of the chromosome is deleted from its origin and place or insert into the other chromosome
Chromosome 1: a b c d + Chromosome 2: e f g h =>
Chromosome 1: a b c Chromosome 2: d e f g h
See also Immunological techniques