Chromosomes are thread-like structure present in the nucleus that carries hereditary information in the form of genes which is passed from parents to offspring. Every species has a characteristic structure and number of chromosome present. Due to certain irregularities at the time of cell division, alteration in the structure or number of chromosomes may happen. Even the slightest alteration can lead to various abnormalities.
Changes in one chromosome parts, whole chromosome or chromosomal sets are known as “chromosomal aberrations”
Chromosomal Disorders in Humans
Each human cell contains 46 (2n) chromosomes present as 23 pairs (n pairs), out of which 22 are autosomes and 1 pair of sex chromosomes.
Chromosomal disorders result from structural changes or numerical changes in chromosomes.
A. Chromosomal disorders due to numerical abnormalities
Chromosomal disorders are due to the change in the number of chromosomes present. This can be categorised into various types:
- Aneuploidy: loss or gain of a chromosome. This happens due to non-disjunction of chromatids when chromatids fail to separate during cell division. This results in one gamete having two copies of one chromosome and the other having no chromosome.
- Trisomy: The cell has one extra chromosome (2n+1)
- Monosomy: The cell has one chromosome less (2n-1)
Aneuploidy can be due to nondisjunction of autosomes i.e. chromosomes 1-22 or sex chromosomes.
Chromosomal disorders due to aneuploidy: This is the cause of most of the genetically inherited disorders and abortion during pregnancy
|Genetic Disorder||Defect in||Genotype||No. of Chromosomes||Phenotypic effect|
|Down’s syndrome||Autosomes||Trisomy of 21st chromosome||47||
|Patau syndrome||Trisomy of 13th chromosome||47||
|Edward’s syndrome||Trisomy of 18th chromosome||47||
|Klinefelter’s syndrome||Sex chromosome||XXY||47||
|Turner’s syndrome||Monosomy- XO||45||
|Triple X syndrome||XXX||47||
- Euploidy: Loss or gain of the whole set of chromosome. Mostly occurs in plants.
- Haploid: Loss of one set of the chromosomes, i.e. ‘n’ number of chromosomes
- Polyploid: Addition of one or more set of chromosomes, e.g. ‘3n (triploid)’, ‘6n (hexaploid)’ etc.
B. Chromosomal disorders due to structural abnormalities
This happens when a large set of genes are deleted, duplicated or rearranged causing structural changes in the chromosome. Structural abnormalities can be due to:
1) Deletion: A portion of the chromosome is lost during cell division. A portion of chromosome without the centromere lags during anaphase movement and are lost from reorganising nuclei or digested by nucleases. The resulting chromosome lacks certain genes, that gets inherited to offspring. This condition is usually lethal due to missing genes.
- Deletion can be terminal, where a terminal portion of a chromosome breaks resulting in one break
- Intercalary deletion, where an intermediate portion is lost resulting from two breaks, which results in 3 pieces. The middle piece is lost and the other two parts rejoin
⇒ Example of disorder due to deletion:
Cri du chat (cry of the cat): Deletion of a small portion of 5th chromosome. Children with this disease have a small head with unusual facial features, severe mental retardation and make a sound like a cat while crying.
2) Duplication: The presence of part of a chromosome in excess is known as duplication. If the duplication is present only in one of the homologous pair of a chromosome, the duplicated part makes a loop to maximise juxtaposition of homologous regions during pairing. The extra segment can be arranged in many ways:
- Tandem duplication, where the duplicated region is present side by side (ABCDEF→ABCDEDEF)
- Reverse tandem, here duplicated region is just reverse of the normal sequence (ABCDEF→ABCDEEDF)
- Displaced duplication, here duplicated region is not situated adjacent to the normal sequence
- Transposed duplication, duplicated part becomes attached to a non-homologous chromosome
- Extra-chromosomal duplication, here duplicated part acts as an independent chromosome in the presence of centromere
⇒ Example of disorder due to duplication:
Fragile X: Affects 1:1500 males and 1:2500 females. This is the most common form of mental retardation, where the CGS segment is repeated more than 200 times.
3. Inversion: inversion results from breakage and reunion of a part of the chromosome rotating by 180° on its own axis. So there occurs a rearrangement of genes. Its effects are not as severe as in other structural defects
4. Translocation: The shifting or transfer of a set of genes or part of a chromosome to a non-homologous one is known as translocation. There is no addition or loss of genes, only the rearrangement occurs. This rearrangement may lead to phenotype changes pertaining to the new environment. It can cause difficulties in the development of egg, sperm or zygote. These often result in miscarriages and children born with disabilities.
- Reciprocal translocation, here segment of two chromosomes gets interchanged
- Robertsonian translocation, here an entire chromosome attaches to another chromosome
⇒ Example of disorder due to translocation:
Acute Myelogenous Leukemia: In this type of cancer, bone marrow and cells derived from it show the presence of a short chromosome named as “Philadelphia (Ph1) chromosome”. 22nd chromosome loses a part of its arm which gets translocated to the distal end of the 9th chromosome. It is not transmitted to the offspring.